Metabolism is also called metabolism, which generally refers to a series of orderlychemical reactionThe general name of.These reaction processes enable organisms to grow andreproduction, maintain their structure and respond to the external environment.Metabolism is generally divided into two categories: catabolism can decompose large molecules to obtain energy (e.gCellular respiration）；AnabolismEnergy can be used to synthesize various components in cells, such as proteins and nucleic acids.Metabolism can be considered asorganismContinuous material andenergy exchangeOnce the exchange of material and energy stops, the structure of organism will disintegrate[3]。
The chemical reactions in the metabolic process can be summarized as metabolic pathways, that is, organisms transform one chemical substance into another through the action of enzymes.The basic metabolic pathways of all living organisms are similar, which may be the result of the high efficiency of related metabolic pathways and the emergence of these pathways early in the evolutionary history[4]。
The general term for various chemical reactions occurring in cells, mainly including catabolism and anabolism.
Concept of metabolism
Metabolism is all orderly in the organismChemical changeThe general name of.It includes material metabolism and energy metabolism.
Material metabolismIt refers to the exchange of substances between organisms and the external environment and the transformation process of substances in organisms.
energy metabolismIt refers to the process of energy exchange between organisms and the external environment and the transformation of energy in organisms.
Assimilation: also called anabolism, refers to the process in which an organism converts nutrients obtained from the external environment into its own components and stores energy.
Alienation: also called catabolism, it refers to the change process in which an organism can decompose some of its original components, release its energy, and expel the end products of decomposition out of the body.
The relationship between assimilation, dissimilation, material metabolism and energy metabolism in metabolism can be summarized by the expression on the left:
Basic types of metabolism
In the long-term evolution process, organisms constantly interact with their environment, and gradually form different types of metabolism.According to the organism in natureassimilationandAlienationDifferent ways, the basic types of metabolism can be divided into the following.
metabolize
Three types of assimilation
Metabolism can be divided intoAutotrophic typeAnd heterotrophic and facultative.
Autotrophic green plants absorb inorganic substances directly from the external environment, produce inorganic substances into complex organic substances through photosynthesis, and store energy to maintain their own life activities. This type of metabolism belongs to autotrophic type.A few kinds of bacteria, unable to carry out photosynthesis, can use the energy released by the oxidation of some inorganic substances in the external environment to produce organic substances, and rely on these organic substancesOxidative decompositionThe energy released by the time to maintain their own life activities, this synthesis is calledChemosynthesis。For example,nitrifying bacteriaIt can convert ammonia (NH3) in soil intoNitrite(HNO2) and nitric acid (HNO3), and use the energy released by this oxidation process to synthesize organic compounds.In short, the organismassimilationIn the process of metabolism, it can transform the inorganic substances taken from the external environment into its own constituent substances and store energy. This type of metabolism is called autotrophic.
Heterotrophic people and animals can not conduct photosynthesis like green plants, nor can they conduct chemosynthesis like nitrifying bacteria. They can only rely on the ingestion of readily available organic substances in the external environment to maintain their own life activities. This type of metabolism belongs to heterotrophic type.In addition, the metabolic type of saprophytic or parasitic fungi and most kinds of bacteria also belongs to heterotrophic type.In short, the organismassimilationIn the process of metabolism, the ready-made organic substances taken from the external environment are transformed into their own constituent substances and stored energy. This type of metabolism is called heterotrophic.
Facultative trophic organisms (such asrhodospirillum ）It can be fixed by light without organic mattercarbon dioxideAnd synthesize organics to meet their own growth and development needs;When there is ready organic matter, these organisms will use the ready organic matter to meet their growth and development needs.
Three types of alienation
According to the oxygen demand of organisms in the process of alienation, the basic types of metabolism can be divided into aerobicAnaerobic typeAnd facultative anaerobic type.
AerobicAnimals and plantsBoth need to live in an oxygen rich environment.They areAlienationDuring the process ofOxidative decompositionThe organic matter in the body releases its energy in order to maintain its various life activities.This type of metabolism is called aerobic, also calledAerobic respirationType.
Anaerobic organisms includelactobacillusAnd parasitic in animalsparasiteA few animals can still oxidize organic substances in their bodies under the condition of hypoxia, and obtain the energy needed to maintain their own life activities.This type of metabolism is calledAnaerobic type, also calledAnaerobic respirationType.
Facultative anaerobic organisms carry out aerobic respiration under the condition of sufficient oxygen, completely decomposing organic matter into carbon dioxide and water, and incompletely decomposing organic matter into lactic acid or alcohol and water under the condition of hypoxia.The typical facultative anaerobic organism isYeast。Now let's introduce yeast.
Facultative anaerobic organism yeast
Yeasts are single celled fungi, usually distributed in high sugar content and acidic environment, such as the surface of vegetables and fruits, and the soil of vegetable gardens and orchards.Yeast isFacultative anaerobe, in the presence of oxygen, decomposes sugars into carbon dioxide and water;Under the condition of hypoxia, the carbohydrate is decomposed into carbon dioxide and alcohol.Yeast is widely used in production. In addition to the well-known wine making and dough making, it can also be used in productionOrganic acid, extracting various enzymes, etc.Any living creature must constantly eat and accumulate energy;We must also constantly discharge waste and consume energy.This process of material and energy exchange between the organism and the outside world is called metabolism.Metabolism isLife phenomenonIt is composed of two opposite and unified processes. One isassimilationThe other isAlienationProcess.
After people and animals eat the external substances (food), they can transform and synthesize the available substances into their own substances through digestion and absorption;At the same time, the energy released during food transformation is stored, which is called assimilation.Green plants use photosynthesis to convert water, carbon dioxide and other substances absorbed from the outside into starch, cellulose and other substances, and store energy, which is also assimilation.The role of alienation isassimilationAt the same time, the material of the organism itself is constantly decomposed and changed, and the stored energy is released for use in life activities, while the unnecessary and unusable material is discharged from the body.
The balance between assimilation and dissimilation The metabolism of various organisms is different in the stages of growth, development and aging.In the process of growing up, infants and adolescents need more materials to build their own bodies, so metabolism is strong and assimilation plays a leading role.In old age and old age, the human body functions are gradually degraded, and the metabolism is gradually slowAlienationBoth of them have declined, but they are always in balance (provided they are healthy).WhenConsumptive diseaseThe alienation effect will be greater than the assimilation effect, such as: tumor, tuberculosis, severe trauma, burns, fluid drainage after major surgery, chronic suppurative infection, chronic blood loss, etc.
Although animals do not eat or drink during hibernation, their metabolism does not stop, but it becomes very slow.
Metabolism isLiving bodyThe process of continuous self renewal is also a judgmentBiological and abioticIf metabolism stops, life will end[1]。
history
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The scientific research on metabolism has spanned several centuries, from the early research on the overall metabolism of animals to the exploration of the single metabolic reaction mechanism in modern biochemistry.The concept of metabolism dates back to the 13th century,Arab medicinehomeIbn al-Nafis （Ibn al-Nafis）"The body and its various parts are in a continuous state of decomposition and nutrition acceptance, so they are inevitably changing all the time".The first experiment on human metabolism was conducted by ItalianSantorio Santorio（Santorio Santorio）Completed in 1614 and published in his book《Medical statistical methods》(Ars de statica medecina).In the book, he described how he weighed himself before and after various activities such as eating, sleeping, working, sex life, fasting, drinking and excretion;He found that most of the food he ate was eventually consumed through what he called "unconscious sweating".In these early studies, the mechanism of metabolic process has not been revealed, and it is generally believed that there is a“vitality”It can activate organs.In the 19th century, the saccharomyces cerevisiaeZymolysisbyalcoholFrench scientistslouis pasteur It was concluded that the process of fermentation was catalyzed by what he called "enzyme" in yeast cells.He wrote: "Alcoholysis is a behavior related to life and yeast cell tissue, but not to cell death and decay." This discovery is related toFriedrich Wohler The chemical synthesis of urea published in 1828 proved that the chemical reactions and organic substances found in cells are no different from other chemistry and follow the basic principles of chemistry.
At the beginning of the 20th century, enzymes were firstEduard Buchner It is found that this discovery makes the research on chemical reactions in metabolism independent from the research on cell biology, and it also marks the beginning of biochemical research.Since the beginning of the 20th century, people's understanding of biochemistry has increased rapidly.Among modern biochemists,Hans KrebsHe is one of the most productive researchers and has made great contributions to the study of metabolism: he discovered the urea cycle, and thenHans Kornberg （Hans Kornberg）The tricarboxylic acid cycle and glyoxylic acid cycle were discovered jointly.Modern biochemical research benefits from the application of a large number of new technologies, such aschromatographic analysis 、X-ray crystallography、nuclear magnetic resonance、Electron microscopy、Isotope labeling、Mass Spectrometryandmolecular dynamicsSimulation, etc.These techniques allow researchers to discover and analyze specifically molecules related to metabolic pathways in cells.
Structure of coenzyme adenosine triphosphate
metabolizeIt is generated in the organism for maintenancelifeA series of orderedchemical reactionThe general name of.These reaction processes enable organisms to grow and reproduce, maintain their structure and respond to the external environment.Metabolism is generally divided into two categories:CatabolismLarge molecules can be decomposed to obtain energy (e.gCellular respiration）；AnabolismEnergy can be used to synthesize various components in cells, such as proteins and nucleic acids.Metabolism can be considered as the process of continuous exchange of material and energy. Once the exchange of material and energy stops, the structure and system of the organism will disintegrate.
The chemical reactions in metabolism can be summarized asMetabolic pathwayThrough the action of a series of enzymes, one chemical substance is transformed into another.Enzymes are crucial for metabolism, because their catalysis enables organisms tothermodynamicsA reaction that is difficult to occur on.When the external environment changes or receives signals from other cells, cells also need to regulate metabolic pathways through enzymes to respond to these changes and signals.
The metabolic mechanism of an organism determines which substances areNutritive, and which aredeleterious。For example, someprokaryoteutilizehydrogen sulfideAs a nutrient, this gas is fatal to animals.Metabolic rate, ormetabolic rateIt also affects the demand of an organism for food.
One of the great characteristics of metabolism is that the basic metabolic pathways between different species are similar, even though they are very different.For example,carboxylic acid, asCitric acid cycle(also known as“Tricarboxylic acid cycle”）The most well known intermediate products in theunicellularThe bacteria are still hugemulticellular Biology such aselephant。Such similarity in metabolism is likely due to related metabolismchannelHigh efficiency and the result of these pathways appearing early in the evolutionary history.
influence factor
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Metabolism refers to the body activities that are carried out continuously without consciousness, includingheartKeep your body temperature and breath.Metabolism is affected by the following factors:
Age
The younger a person is, the faster his metabolism will be.This is due to the growth of the body, especially in infancy and adolescence.
Body epidermis
The larger the skin area of the body, the faster the metabolism.Two people with the same weight and different height will have a slower metabolism than a shorter one.
High people need to speed up metabolism to generate heat because of their large skin area and fast body heat dissipation.
Gender
Men usually metabolize faster than women.It is generally believed that this is due to theMuscle tissueThe proportion is larger.Muscle tissue is active even when people are at rest, andadipose tissueBut not active.
motion
The metabolism of the body will be accelerated in the process of intense physical exercise and within a few hours after the activity.
Network meaning: as a neologism, metabolism expression is not a memory of innocent years, it is very yellow and violentnude picture scandalevent.Chen is Edison Chen, Xie isNicholas Tse。The main line isCecilia CheungThe word means that Chen replaces Xie and has a good time with Zhang.
Alcohol affects metabolism
Effect on glucose metabolism
Glucose is the only energy supply that can be used by the brain.Once glucose is lacking, the brain will show symptoms of different degrees, such as disturbance of consciousness, coma and even death.
Ethanol can significantly affect the metabolism of glucose.After drinking, the anaerobic degradation of glucose (called glycolysis) increases while gluconeogenesis decreases. Finally, even the synthesis of glycogen is inhibited.For alcoholics, there are two common reasons for hypoglycemia: ① poor diet quality or eating conditions lead to insufficient sugar intake, which will reduce glycogen reserves in the body over time; ②Ethanol can inhibit gluconeogenesis and reduce available glucose.Both may cause hypoglycemia, and even coma in severe cases.
The hypoglycemia after drinking mostly occurs in the case of empty stomach, usually 6 to 36 hours after drinking.Early diagnosis and timely treatment of alcohol abuse and alcohol dependent hypoglycemia are very important clinically.When hypoglycemia occurs, the patient may show trembling, sweating, agitation, andAbstinence syndromeSimilar performance.It is easy to be misdiagnosed.Therefore, it is suggested that the blood glucose level should be routinely checked before the preparation of the treatment for each case of alcohol abuse and alcohol dependence to exclude the possibility of hypoglycemia.Similarly, for anyone who has a long history of drinking and goes to the hospital with consciousness disorder, check the blood sugar level in an emergency and supplement glucose intravenously if necessary to avoid the occurrence ofHypoglycemic coma。
In addition to hypoglycemia, alcoholics may also have hyperglycemia, which mostly occurs in those who still have good eating conditions when drinking.Because ethanol can block the conversion of glucose to glycogen and increase blood sugar, drinkers often have transient hyperglycemia, which generally does not need treatment.If hyperglycemia persists, the possibility of diabetes should be considered, and corresponding examination and treatment should be carried out.
Effect on water and electrolyte balance
Almost everyone with drinking experience knows that alcohol can promote diuresis.
However, this phenomenon only occurs at the beginning of drinking and when the blood alcohol concentration is on the rise.For alcohol abusers and alcohol addicts, the situation is often the opposite. Water retention often occurs in patients, and the water content in the body increases. Some patients often have edema of varying degrees.
Long term alcoholics may also appearElectrolyte disorderWhen monitoring the electrolyte of alcohol abusers and alcohol addicts, in addition to monitoring the conventional indicators such as sodium, potassium, chlorine and carbonate, attention should also be paid to the level of phosphorus and magnesium.
Biochemical substance
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Most of the structures of animals, plants and microorganisms are composed of three basic biological molecules, which areamino acid, sugars and lipids (often referred to as fats).Since these molecules are necessary to maintain life, metabolism not only manufactures these molecules for building cells and tissues, but also digests and degrades these molecules in food after ingesting food to provide energy needed to maintain life.Many important biochemical substances can be aggregated to formPOLYMER, such as DNA and proteins.these ones hereBiomacromoleculeIt is an essential component for all organisms.The following table lists some of the most common biomacromolecules.
The proteins are arranged in a linear wayamino acidIt is composed of amino acidsPeptide bondInterconnection.Enzymes are the most common proteins that catalyze various chemical reactions in metabolism.Some proteins have structural or mechanical functions, such as participating in the formation ofCytoskeletonTo maintain cell morphology.There are many proteins inCell signal transduction、immune reaction、cell adhesion andcell cycle regulationPlay an important role in.
Lipids are the most diverse biomolecules.Their main structural purpose is to form biological membranes, such as cell membranes;In addition, they can also serve as a source of energy for the body.Lipids are generally defined asHydrophobicityOr amphoteric biomolecules, soluble inbenzeneorchloroformAnd other organic solvents.Fat is composed of fatty acidsGroupandglycerolA large class of lipid compounds composed of groups;Its structure is a glycerol molecule withesterThe bond connects three fatty acid molecules to formtriglyceride。On the basis of this basic structure, there are also many variants, including hydrophobic skeletons of different sizes and lengths (such asSphingolipidsInSphingosineGroup) and different types ofHydrophilicGroup (such as phosphate group in phospholipid).steroid(such as cholesterol) is another major lipid molecule synthesized by cells.
Glucose can exist in both linear and circular forms.
Sugar
More sugarhydroxylOfaldehydeorketone, can exist in the form of straight chain or ring.Carbohydrates are the most abundant biological molecules, which have many functions, such as storing and transporting energy (such as starchGlycogen）And as structural components (cellulose in plants andchitin）。The basic constituent units of sugars aremonosaccharide, includingGalactose、fructoseAnd very important glucose.Monosaccharides can be linked together to form polysaccharides through glycosidic bonds, and the way of linking is extremely diverse, which results in the diversity of polysaccharide types.
DNA andRNAAre two main types of nucleic acids, both of which are composed ofnucleotideA straight chain molecule formed by joining.Nucleic acid molecule forgenetic informationThe storage and utilization ofTranscriptionAnd translation to complete the process from genetic information to protein.This genetic information is generated byDNA repairMechanism, and throughDNA replicationTo amplify.Some viruses (such asHIV）Containing RNAgenome, they can useReverse transcriptionTo synthesize DNA templates from viral RNA.ribozyme(e.gShear bodyandribosome）RNA inCharacteristics of enzyme, which can catalyze chemical reactions.A single nucleotide is composed of aRiboseMolecule connected to previousBaseTo form.The base is nitrogenousHeterocycle, can be divided into two categories:purineandpyrimidine。Nucleotides can also be used ascoenzymeParticipate in the transfer reaction of metabolic groups.
Acetyl CoAStructure of.TransferableAcetylBound to the leftmost sulfur atom.
coenzyme
Metabolism includes a wide range of chemical reactions, but most of them belong to several basic types that contain the transfer of functional groupsReaction type。In these reactions, cells use a series ofSmall moleculeMetabolize intermediates to carry chemical groups between different reactions.The intermediates of these group transfers are called coenzymes.Each type of group transfer reaction is performed by a specific coenzyme, which synthesizes it and consumes a series of enzymessubstrate。These coenzymes are constantly generated, consumed and recycled.
Adenosine triphosphate(ATP) YesLife formOne of the most important coenzymes in the cellEnergy flowThe universal form of.ATP is used between different chemical reactionschemical energyDelivery of.Although there is only a small amount of ATP in cells, it is constantly synthesized, and the amount of ATP consumed by the human body in a day can be accumulated to reach its own weight.ATP is connectedAnabolismandCatabolismBridges:Catabolic reactionATP is generated, andAnabolic reactionConsume ATP.It can also be used as phosphoric acidGroupCarriers ofPhosphorylationReaction.
Vitamin is a kind of trace amount needed by lifeOrganic compound, but the cell itself cannot synthesize.In humansNutritionMost vitamins can play the role of coenzyme after being modified;For example, all theWater-soluble vitaminAre phosphorylated orcouplingTo nucleotides.Nicotinamide adenine dinucleotide(NAD, reduced as NADH) YesVitamin B3(commonly known as nicotinic acid), which is also an important coenzyme, can be used as a hydrogen receptor.Hundreds of different types ofdehydrogenaseElectrons can be removed from their substrates and NAD+can be reduced to NADH at the same time.Then, this reduction form can be used as anyreductaseA coenzyme that provides electrons for the reduction of enzyme substrates.Nicotinamide adenine dinucleotide exists in two different forms in cells: NADH and NADPH.NAD+/NADHCatabolic reactionWhile NADP+/NADPH is mostly used forAnabolic reactionMedium.
inorganic elements It also plays an important role in metabolism;Some of them are rich in the body (such as sodium and potassium), while others are trace elements.About 99% of mammals have carbon, nitrogen, calcium, sodiumchlorine, potassiumhydrogen, phosphorusoxygenAnd sulfur.Most of carbon and nitrogen exist in organic matter (such as protein, lipid and sugar), while hydrogen and oxygen mainly exist in water.
The abundant inorganic elements are all ions used as electrolytes.The most important ions in the body are metal ions such as sodium, potassium, calcium, magnesium, and chloride ionsPhosphate radicalIons andBicarbonateIons.Maintain accurateIonic gradient, can be maintainedosmotic pressureAnd pH stability.Ions are also indispensable for nerve and muscle tissues, because theaction potential(can cause nerve signals andMuscle contraction）Is created byExtracellular fluidAnd cellsProtoplasmProduced by the exchange of electrolytes between them.Electrolytes enter and leave cells through the cell membraneion channelProtein.For example, muscle contraction depends onTransverse tubule（T-tubule）The ion channel on thecalcium ion, potassium ion and sodium ion flow control.
transition metalIt usually exists in organism as trace elements, in which zinc andironIs the most abundant.These metal elements are used by some proteinsCofactorOr it plays a key role in the play of enzyme activity, such as oxygen carrying hemoglobin andcatalase。These cofactors can tightly bind to specific proteins;Although the cofactors of the enzyme will be modified during the catalytic process, these cofactors can always return to the initial state after the completion of catalysis.
Catabolism
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Catabolism (also called dissimilation) is the general term for a series of reaction processes that break down macromolecules, including breaking down and oxidizing food molecules.Catabolic reactionThe purpose ofAnabolic reactionProvide the required energy and reactants.The mechanism of catabolism varies in organisms, such asOrganotrophic bacteriadecomposeOrganic moleculeTo gain energy, andInorganic nutrient bacteriautilizeInorganicAs a source of energy,Light energy utilization bacteriaCan absorb sunlight and transform it into usablechemical energy。However, all these metabolic forms requireRedoxThe participation of the reaction is mainly to transfer electrons fromReducibilityDonor molecules (such as organic moleculeswater、ammonia、hydrogen sulfide、Ferrous ionEtc.) toReceptor molecule(e.goxygen、Nitrate、sulfateEtc.).In animals, these reactions also involve breaking down complex organic molecules into simple molecules (such as carbon dioxide and water).stayPhotosynthesisBiological (such as plants andblue-green algae）These electron transfer reactions do not release energy, but are used as a way to store absorbed light energy.
Most common among animalsCatabolic reactionIt can be divided into three main steps: first,macromoleculeOrganic compounds, such as proteins, polysaccharides or lipids, are digested and decomposed into small molecular components;These small molecules are then ingested by cells and transformed into smaller molecules, usuallyAcetyl CoA, some energy will be released in this process;Finally, theacetylGroup passingCitric acid cycleandElectron transfer chainIt is oxidized to water and carbon dioxide and releases energy, which canNicotinamide adenine dinucleotide(NAD+) is reduced to NADH and stored in the form of chemical energy.
Digestion
Starch, protein, cellulose and other macromolecular polymers cannot be absorbed by cells quickly and need to be decomposed into small molecules firstMonomerThen it can be used for cell metabolism.A variety of digestive enzymes can degrade these polymers, such asproteaseYou can set butProtein degradationIt is a polypeptide fragment or amino acid,Glycoside hydrolasePolysaccharides can be broken down into monosaccharides.
Microbes simply secrete digestive enzymes into the surrounding environment, while animals can onlydigestive systemSpecific cells in the cell to secrete these enzymes.Amino acids or monosaccharides obtained from the decomposition of these extracellular enzymes are then passed throughActive transportationThe protein is transported into the cell.
Energy from organic matter
The catabolism of sugars is to break down sugar chains into smaller units.Usually oncesugar chainIt can be absorbed by cells after being decomposed into monosaccharides.Sugar entering the cell, such as glucose and fructose, will pass throughGlycolysisThe pathway is transformed intoPyruvateAnd generate some ATP.Pyruvate is an intermediate of multiple metabolic pathways, but most of it will be converted intoAcetyl CoAAnd enter the citric acid cycle.Although the citric acid cycle can produce ATP, its most important product is NADH, which is produced by NAD and provides electrons by the oxidation of acetyl coenzyme A, while releasing useless carbon dioxide.Under anaerobic conditions, glycolysis will generateLactate, i.elactate dehydrogenaseIt converts pyruvate into lactate, and oxidizes NADH into NAD+, so that NAD can be recycled for glycolysis.Another way to degrade glucose isPentose phosphate pathway, which can convert coenzymeNicotinamide adenine dinucleotide phosphate(NADP+) is restored to NADPH and generatedPentose, such asRibose(an important component of synthetic nucleotides).
Fat is produced byhydrolysisIt is decomposed into fatty acids andglycerol。Glycerol can enterGlycolysis pathway, viaBeta oxidationIt is decomposed and releases acetyl coenzyme A, which enters the citric acid cycle as described above.Fatty acids are also decomposed by oxidation;Fatty acids can release more energy than sugars in the oxidation process, which is due to the high oxygen content ratio of sugar structure.
amino acidIt can not only be used to synthesize proteins or other biological molecules, but also be oxidized to urea and carbon dioxide to provide energy.The first step of oxidation istransaminasePut theaminoRemoval, the amino group is then fedUrea cycleAnd the remaining carbon skeleton with amino groups removedKeto acidExists in the form of.There are many kinds of ketoacids (such asα - ketoglutaric acid, from deaminatedglutamate) is the intermediate of citric acid cycle.In addition,Raw sugar amino acid（glucogenic amino acid）Able to passGluconeogenesisThe action is converted to glucose (see below for details).
Anabolism
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Anabolism (also known as assimilation) is the general term for a series of anabolic processes (that is, the synthesis of complex molecules using the energy released by catabolism).In general, it is used to formCellular structureThe complex molecules of are all constructed step by step from small and simple precursors.Anabolism consists of three basic stages: first, precursor molecules are generated, such asamino acid、monosaccharide、Isoprenoidandnucleotide；Second, useATP hydrolysisThe energy provided by these molecules is activated to form an active form;Finally, they are assembled into complex molecules, such as proteins, polysaccharides, lipids and nucleic acids.
Different organisms need to synthesize different kinds of complex molecules.AutotrophFor example, plants can use simple small molecules, such as carbon dioxide and water, in their cells to synthesize complex organic molecules, such as polysaccharides and proteins.HeterotrophMore complex material sources, such as monosaccharides and amino acids, are needed to produce corresponding complex molecules.Organisms can also be subdivided into: those that obtain light energyPhotoautotrophandPhotoheterotroph, and energy obtained from inorganic oxidation processChemoautotrophAnd chemoheterotrophs.
Carbon fixation
plant cell(The purple cell wall surrounding it) is full of the "factory" of photosynthesis chloroplast (green).
Photosynthesis is a process that uses sunlight, carbon dioxide (CO2) and water to synthesize sugars and release oxygen.This process usesPhotosynthetic reaction centerThe generated ATP and NADPH convert CO2 into3-phosphoglycerate, and continue to add 3-PhosphoglycerateThis process is calledCarbon fixation。Carbon fixation reaction asCalvin Benson cyclePart of, byRuBisCOEnzymes to catalyze.Photosynthesis occurring in plants can be divided into three types:C3 carbon fixation、C4 carbon fixationandCAM photosynthesis。The difference between these photosynthetic species lies in the different ways when carbon dioxide enters the Calvin cycle: C3 type plants can directly fix CO2;C4 and CAM combine CO2 with other compounds first, which is an adaptation to strong light and arid environment.
In photosynthetic prokaryotes, the mechanism of carbon fixation is only more different.For example, carbon dioxide can pass through the Calvin Benson cycle (aTrans citric acid cycle）Or acetyl coenzyme ACarboxylationAnd is fixed.In addition, prokaryoticChemoautotrophic bacteriaCO2 can also be fixed through the Calvin Benson cycle, but energy from inorganic compounds is used to drive the reaction.
Saccharides and glycans
In the anabolism of sugars, simple organic acids can be converted intomonosaccharide(e.g. glucose), and then monosaccharides are polymerized together to form polysaccharides (e.g. starch).Include fromPyruvate、Lactate、glycerol、3-phosphoglycerateandamino acidThe process of producing glucose from compounds within is calledGluconeogenesis。Gluconeogenesis converts pyruvate intoGlucose-6-phosphate, many of which can interact withGlycolysisProcess sharing.However, gluconeogenesis is not a simple glycolysis processReverse reactionMany of these steps are catalyzed by enzymes that do not play a role in glycolysis.In this way, the synthesis and decomposition of glucose can be regulated separately to prevent these two pathways from enteringInvalid loop（futile cycle）。
Although fat is a universal way to store energyvertebrateFor example, in humans, stored fatty acids cannot pass throughGluconeogenesisIt is converted to glucose because these organisms cannot convert acetyl coenzyme A into pyruvate (plants have the necessary enzyme, while animals do not).Therefore, after long-term hunger, vertebrates need to produce fatty acidsKetone bodyTo replace glucose in tissues, because tissues like brain cannot metabolize fatty acids.In other organisms, such as plants and bacteriaGlyoxylic acid cycle, you can skip theDecarboxylation reaction, enabling acetyl coenzyme A to be converted intoOxaloacetate, andOxaloacetic acidSalt can be used for the production of glucose, thus solving this metabolic problem in vertebrates.
Polysaccharides andGlycanIt is synthesized by adding monosaccharides step by step. The process of adding monosaccharides isGlycosyltransferaseConvert the glycosyl group from an activated sugar phosphatedonor(e.gUridine diphosphate glucose）Up transferred tohydroxyl(located in the extended polysaccharide chain).Since any hydroxyl group on the sugar ring can act as a receptor, the polysaccharide chain can be a straight chain structure or contain multiple branched chains.These generated polysaccharides can have structural or metabolic functions themselves, or can be transferred to lipids and proteins under the action of oligosaccharide chain transferase (i.eGlycosylationFunction).
Simplified diagram of steroid metabolic pathway.It includes the intermediate isopenteneinorganpyrophosphate(IPP), dimethyl allyl pyrophosphate (DMAPP), geranyl pyrophosphate (GPP) and squalene.Some intermediates are omitted.The product isLanosterol。
Fatty acid synthesisIt is a process of polymerization and reduction of acetyl coenzyme A.The acetyl chain on the fatty acid is extended through a reaction cycle, including addingacetylBase, reducing it to ethanol and continuing to reduce it toalkaneProcess.Enzymes that play a role in fatty acid biosynthesis can be divided into two categories: all fatty acids in animals and fungiSynthetic reactionBy a singleMultifunctional enzyme, Type IFatty acid synthaseTo complete;But in plantsPlastidAnd bacteria, there are a number of different enzymes that catalyze each reaction, and these enzymes are collectively referred to as type I fatty acid synthase.
TerpeneandIsoprenesCompounds (includingCarotenoids(inside) is a large family of lipids, which constitute plantsNatural compoundsThe largest category of.These compounds are based onisopreneIt is a unit, polymerized and modified;Among them, isoprene is composed of reactive precursors,Isopentene pyrophosphateandDimethylallyl pyrophosphateProvided.These two precursors can be synthesized in different ways.Animal and archaea utilizationMevalonate pathwayTo produce these two compounds from acetyl coenzyme A;And plants and bacteriaNonmevalonate pathwayUsing pyruvate andGlyceraldehyde-3-phosphateThey are produced as substrates.Another uses these activated isoprenedonorThe important response ofBiosynthesis of steroids。Among them, isoprene units are connected together to formSqualene, and then folded up, and obtained through a continuous ring forming reaction initiated by a protonLanolin sterol。Lanolin sterols can be further converted into other steroids, such as cholesterol andErgosterol。
protein
Synthesis of 20 species among organismselementary amino acidsTheir abilities vary.Most bacteria and plants can synthesize all 20 amino acids, while mammals can only synthesize 10Nonessential amino acid。Therefore, for mammals including humansessential amino acid The only way is to eat foods rich in these amino acids.All amino acids can be generated from intermediates in glycolysis, citric acid cycle or pentose phosphate cycle.The nitrogen required for the synthesis process is composed ofglutamateandglutamineTo provide.Amino acid synthesisIt is necessary to form appropriate α - ketoacid first, and then passTransaminationIt acts to form amino acids.
Amino acids are produced byPeptide bondThey join together and further form proteins.Each different protein corresponds to its own unique amino acid sequence (also known asPrimary structure）。Just like more than 20 lettersPermutation and combinationLike tens of thousands of words, different amino acids can be linked together to form a large number of protein types.Amino acids are linked to the correspondingTransport RNA(tRNA) molecular formationAminoacyl tRNAIt is activated before it can be connected together.This aminoacyl tRNA precursor is synthesized through an ATP dependent reaction (linking tRNA to the correct amino acid), which is composed ofAminoacyl tRNA synthetaseCatalysis.Then, useMessenger RNAUnder the guidance of sequence information in, aminoacyl tRNA molecules with correct amino acids can be bound toribosomeUnder the action of ribosome, amino acids are connected to the lengthening protein chain.
nucleotide
nucleotideIt is composed of amino acid, carbon dioxide andformic acidTo synthesize.Because its synthetic route requires a large amount ofMetabolic energyMost organisms have effective systems to carry outNucleotide salvage。purineYesnucleoside(i.eBaseConnectRibose）Synthetic on the basis of.adenineandGuanineIt is composed of precursor nucleoside moleculeInosineMonophosphoric acid (i.eHypoxanthic acid）And hypoxanthic acid is derived fromglycine、glutamineAnd glutamine as well as from coenzymeTetrahydrofolateIt is synthesized from the transferred formic acid group.pyrimidineIt is composed of baseWhey acid saltSynthetic, whey acid salt is converted from glutamine and glutamine.
Heterotypic biomass metabolism and redox metabolism
If all organisms continue to ingest non food substances without corresponding metabolic pathways, these substances will accumulate in cells and cause harm.These substances that may cause damage in the body are calledHeterotypic biomass（xenobiotic）。Heterotypic biomass includesSynthetic drugs、Natural poisonAnd antibiotics, fortunately, they canMetabolic enzymeUnder the action of.In the human body,Cytochrome-P450oxidase、Uridine diphosphateGlucuronic acidTransferases(UDP glucuronosyltransfers) andGlutathione transferase（glutathione S-transferase）All belong to this kind of enzyme.This oneEnzyme systemThere are three stages for the function of "": first, the heterotypic biomass is oxidized, then a water-soluble group is connected to the molecule of the material, and finally the modified heterotypic biomass with water-soluble groups is transported out of the cell (in themulticellular organismIn the body, it can be further metabolized and discharged from the body).In ecology, these responsesContaminantsOfMicrobial degradationAnd contaminated soil (especiallyoil pollution ）The bioremediation of TNF plays a very important role.Many such microbial reactions also exist in multicellular organisms, but due to the diversity of microbial species, they can metabolize more substances than multicellular organisms, and they can even degrade organic chlorinePOPs。
Organisms must also complyThermodynamic law(DescriptionWorkAnd heat).The second law of thermodynamicsPoint out that in anyClosed systemThe entropy always tends to increase.Although the high complexity of organisms seems to be contrary to this rule, they are actuallyopen system, can exchange material and energy with the surrounding environment;Therefore,Life systemNot inbalanceIn the middle, it is shown asdissipative structure To maintain their high complexity and increase the entropy of the surrounding environment.Metabolism in cells is achieved by catabolizingSpontaneous processIt is coupled with the non spontaneous process of anabolism to maintain complexity.To explain with thermodynamics, metabolism is actually to keep order by creating disorder.
Regulation mechanism
Since the external environment of the organism is constantly changing, the metabolic reaction must be accurately regulated to maintain the stability of each component in the cell, that isHomeostasis。Metabolic regulation also enables organisms to generate feedback on external signals and interact with their surroundings.Among them, two closely related concepts are very important for understanding the regulation mechanism of metabolic pathways: first, the regulation of an enzyme in metabolic pathways is how its enzyme activity increases or decreases according to signals;Second, the control imposed by this enzyme is the effect of its activity change on the overall rate of metabolic pathway (pathwayflux）Impact.For example, an enzyme can have great changes in its activity (such as being highly regulated), but if these changes have little effect on the flux of its metabolic pathway, then the enzyme cannot control this pathway.
Metabolic regulation can be divided into multiple levels.In self regulation, metabolic pathways can self regulate to respond to changes in substrate or product levels;For example, a decrease in the amount of product can cause an increase in the pathway flux, thus compensating for the amount of product.This type of regulation involvesAllosteric regulation。multicellular organismIn, cells respond to signals from other cells to change their metabolism, which belongs to external regulation.These signals are usually transmitted through soluble molecules ("messengers"), such as hormones andgrowth factor, they can specifically interact withcell surfaceSpecificrecipientMolecular binding.After binding to the receptor, the signal will pass throughSecond Messenger SystemIs transferred to the cell interior, which usually contains proteinPhosphorylation。
frominsulinRegulated glucose metabolism is a well studied example of external regulation.The body synthesizes insulin forBlood glucose levelRespond to the rise of.Insulin and cell surfaceinsulin receptorCombine and then activate a series ofprotein kinaseCascade reaction, enabling cells to ingest glucose and convert it into energy storage molecules, such as fatty acids andGlycogen。Glycogen metabolism is determined byphosphorylaseandGlycogen synthaseThe former can degrade glycogen, while the latter can synthesize glycogen.These enzymes are mutually regulated: phosphorylation can inhibit the activity of glycogen synthase, but activate the activity of phosphorylase.Insulin via activationProtein phosphataseAnd reduce the phosphorylation of enzyme, so that glycogen can be synthesized.
evolution
The evolutionary tree shows that all organisms from the three biological domains have a common ancestor.Bacteria are blue, eukaryotes are red, and archaea are green.The relative positions of some biota are also marked around the evolutionary tree
As previously mentioned, the central pathways of metabolism, such as glycolysis and tricarboxylic acid cycle, exist inThree fieldsAll organisms in the“Last common ancestor”Medium.Common ancestor cells areprokaryoteAnd is likely to have a wide range of amino acid, sugar and lipid metabolismMethanogen。The reason why these ancient metabolic pathways have not further evolved may be that the reactions in the pathways have been an optimized solution to specific metabolic problems, which can achieve high efficiency with few steps.The first enzyme based metabolic pathway (may have becomePurine nucleotidePart of metabolism) and the previous metabolic pathway is primitiveRNA WorldComponents of.
Researchers have proposed various models to describe how the new metabolic pathway evolved: for example, adding new enzymes to a shorter original pathway, or replicating and then differentiating the entire pathway, and bringing the existing enzymes and their complexes into the new reaction pathway.It is not clear which of these evolutionary mechanisms is more important, but genome research shows that enzymes in the same pathway may have a common "ancestor", which suggests that many pathways use existing reaction steps to obtain new functions through step-by-step evolution.Another more reasonable model comes from theMetabolic networkinprotein structureThe results of the evolutionary study of the enzyme suggest that the same enzyme can be used in different metabolic pathways and play similar roles.These processes of utilization lead to evolution, where enzymes are spliced in a way similar to mosaic arrangement.The third possibility is that some parts of metabolism can exist as "modules", and modules can be used in different ways and perform similar functions on different molecules.
While evolving new metabolic pathways, evolution may also cause the reduction or loss of metabolic functions.For example, someParasitoidLose the metabolic process that is not critical to survival, and replace it with directhostAmino acids, nucleotides and sugars are obtained in the body.Similar phenomena of metabolic capacity degradation are found in someEndosymbiosisIt is also observed in organisms.
Relevant research and analysis
Metabolic network of tricarboxylic acid cycle in Arabidopsis thaliana.Enzymes andmetaboliteThey are represented by red squares, and their interactions are represented by black lines.
Classic of metabolismresearch methodyesreduction methodThat is, to study a single metabolic pathway.Radioactive tracerIt is a very useful research tool. It tracks the metabolic process by locating radiolabeled intermediates and products, so that metabolism can be studied at different levels of the whole organism, tissue or cell.Then, the enzymes that catalyze these chemical reactionspurificationAnd identify their dynamic properties and correspondinginhibitor。Another research method is to identify small molecules related to metabolism in a cell or tissue. All these small molecules are calledMetabolic group（Metabolome）。To sum up, these studies have given the composition, structure and function of a single metabolic pathway;However, these methods cannot be effectively applied to more complex systems, such as all metabolism in a complete cell.
The complexity of metabolic networks (containing thousands of different enzymes) in cells is extremely high.However, genome data can be used to build a complete metabolic chemical reaction network and generate moreintegrationIt has become possible to use mathematical models to explain and predict various metabolic behaviors.In particular, the data of metabolic pathways and metabolites obtained from classical research methods andProteomicsandDNA microarrayThe data obtained in the research can be integrated into these mathematical models, which can greatly improve these models.Using all these technologies, a human metabolic model has been proposed, which will provide guidance for future drug and biochemical research.
A major technical application of metabolic information is metabolic engineering.In metabolic engineering, organisms such as yeast, plants and bacteria aregenetic engineeringTransformed into an efficient tool in biotechnology, used for drugs including antibiotics or industrial chemicals (such as1,3-PropanediolandShikimic acid）Production.These modifications usually help to reduce energy consumption in product synthesis, increase output and reduce waste generation[2]。
energy conversion
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Oxidative phosphorylation
ATP synthaseStructure of.Its proton channel andRotating shaftThe display is blue, the synthase subunit is red, and the fixed subunit is yellow.During oxidative phosphorylation, through metabolic pathways such as citric acid cycle, electrons are transferred from food molecules digested and absorbed to oxygen, and the energy generated is stored in the form of ATP.stayEukaryoteThis process is performed by themitochondrionA series ofMembrane proteinIs calledElectron transfer chain。And inprokaryoteThe corresponding protein is located inCell intimaOn.These proteins utilize electrons fromReducibilityMolecules (e.gNADH）The energy generated by the reaction transferred to oxygen willprotonTransmembrane transportation.Pumping protons out of mitochondria results inmitochondrial membraneThe concentration difference of protons on both sides of the membraneElectrochemical gradient。The driving force generated by the electrochemical gradient makes protons pass through theATP synthaseRe entering mitochondria.Such a proton flow will promote ATPsynthetaseStalk ofSubunitIt rotates and further drives the synthetaseDomainOnActive siteDeformation occurs andAdenosine diphosphate(ADP) phosphorylation, and ultimately ATP.
Energy from inorganic substances
Chemical energy inorganic nutritionIt is a metabolic type found in some prokaryotes, which are oxidizedInorganicTo gain energy.They can use hydrogen,ReducibilityOfSulfurCompounds (such as sulfideshydrogen sulfideandThiosulfate），Ferrous compoundorammoniaAs a source of reducing energy;The oxidation process of these reducing substanceselectron acceptor Often oxygen ornitrite。These processes are important for the overallBiogeochemical cycle, such asAcetic acid formation(acetagenesis) andnitrificationandDenitrificationAre very important, and are critical to soil fertility.
Energy from light
The energy in sunlight can be used by plantsblue-green algae、Purple bacterium、green bacteriumAnd someProtozoaCaptured.This process of obtaining light energy is often associated with the conversion of carbon dioxide into organic matter (i.e“Carbon fixation”）And become part of photosynthesis.Light energy acquisition and carbon fixation systems can be operated separately in prokaryotes, because purple bacteria and green bacteria can use sunlight as energy source no matter when carbon is fixed or organic matter is fermented.
The process of capturing solar energy andOxidative phosphorylationThey are similar in nature, because both include energy and protonconcentration gradientThe form exists and the ATP synthesis driven by this concentration difference.The electrons used to drive the electron transfer chain come fromPhotosynthetic reaction centerLight harvesting protein.According to the containedPhotosynthetic pigmentDifferent types of reaction centrosomes can be divided into two categories:Magnesium free chlorophyll-Quinone type and iron sulfur type;MostPhotosynthetic bacteriaThere is only one type of reaction centrosome, while plants and cyanobacteria contain two types.
In addition,Optical systemIt plays a major role in photosynthesisProtein complex, including optical systems I and II.In plants, photosystem II can use light energy to obtain electrons from water and release oxygen.The electrons then flow inCytochrome b6f complexThe complex uses energy to pump out protonsthylakoid(onchloroplastMedium) membrane.The pumped protons return to the capsule like body through the membrane, thus driving the synthesis of ATP (similar to the synthesis of ATP in oxidative phosphorylation).When electrons continue to flow through photosystem I, they can be used to reduce coenzyme NADP+Calvin cycleOr it can be used to synthesize more ATP after recycling.
