Fatty acid is a kind of compound composed of carbon, hydrogen and oxygen, which is the main component of neutral fat, phospholipid and glycolipid.Fatty acid metabolism Fatty acid can be divided into:Short chain fatty acids, the number of carbon atoms on the carbon chain is less than 6, also known asVolatile fatty acids；Medium chain fatty acids refer to the fatty acids with 6-12 carbon atoms on the carbon chain, and the main components are octanoic acid (C8) and decanoic acid (C10);The number of carbon atoms on the carbon chain of long-chain fatty acids is greater than 12.General food contains mostly long-chain fatty acids.Fatty acids can be divided into three categories according to the difference between saturated and unsaturated hydrocarbon chains, namely, saturated fatty acids, with no unsaturated bond on the hydrocarbon;Monounsaturated fatty acid, its hydrocarbon chain has an unsaturated bond;Polyunsaturated fatty acidsIts hydrocarbon chain has two or more unsaturated bonds.
Fatty acids are a class of compounds composed of carbon, hydrogen and oxygen, and are the main components of neutral fats, phospholipids and glycolipids.[1]
fatty acid
Fatty acids can be divided into two categories: one is those without carbon carbon double bonds in the moleculeSaturated fatty acid, such as stearic acid and palmitic acid;The other is unsaturated fatty acids with one or several carbon carbon double bonds in the molecule. The most common one is oleic acid. There is only one carbon carbon double bond in the carbon chain of oleic acid, so it is also calledMonounsaturated fatty acid。Generally, the carbon chain of fatty acid compounds is short, with the length of 18-36 carbon atoms, and the minimum is 12 carbon atoms, such as lauric acid.No matter saturated or unsaturated, the number of carbon atoms of fatty acids in organisms is mostly even, and there are few odd carbon atoms, especially in higher animals and plants, there are mainly more than 12 carbon atomsHigher fatty acids, generally 14-24 carbon, 16 and 18 carbon fatty acids are the most common.Odd carbon atom fatty acids only exist in some plants, ruminants, marine organisms, petroleum yeast and other parts of the body.[2]
Contains a lot ofSaturated fatty acidAt normal temperature, triglyceride of is often a solid, such as butter, lanolin, etc. Most of them are animal fats.Contains moreUnsaturated fatty acidOftriglycerideIt is usually liquid at room temperature, such asCorn oil, vegetable oil, etc.Most vegetable and fish oils are unsaturated fatty acidsGlyceride。[2][15]
Unsaturated fatty acids with 2-4 double bonds cannot be synthesized in animals and must be obtained from food, so these fatty acids are calledEssential fatty acidsSome people call it vitamin F.Although it has been thought that they can reduce the cholesterol in the blood, there is no evidence that people can cause diseases due to the lack of these fatty acids in food.Microbes also contain unsaturated fatty acids. The most unique feature of cyanobacteria is that they contain unsaturated fatty acids composed of two or more double bonds, while bacteria usually only contain saturated fatty acids and unsaturated fatty acids with one double bond.[2]
In 2022, Chinese scientists will successfully synthesize fatty acids from carbon dioxide and water through the combination of electrocatalysis and biosynthesis.[14]
Physical and chemical properties
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(1) Color and odor
Pure fatty acids are colorless, and some fatty acids have their own unique smell.[3]
(2) Density
The relative density of fatty acids is generally less than 1relative molecular massIn inverse proportion, it decreases with the increase of temperature, and decreases with the increase of carbon chain. The more unsaturated bonds, the greater the density.[3]
(3) Melting point
The melting point of fatty acids increases irregularly with the growth of the carbon chain. The melting point of the odd carbon chain fatty acids is lower than its adjacent even carbon fatty acids. The melting point of unsaturated fatty acids is usually lower than that of saturated fatty acids. The more double bonds, the lower the melting point. The closer the double bonds are to the ends of the carbon chain, the higher the melting point.[3]
The introduction of a double bond into the carbon chain will reduce the melting point of fatty acids. The more the position of the double bond moves towards the middle of the carbon chain, the greater the melting point reduction. The effect of cis double bonds is greater than that of trans bonds.The melting point decreases with the increase of double bond, but the conjugated double bond is not in this case.The melting point of conjugated enoic acid can be increased by hydrogenation, reaction or non conjugated double bond isomerization.The melting point of each odd carbon atom fatty acid is lower than that of the even carbon atom fatty acid closest to it, for example, the melting point of heptadecanoic acid (61.3 ℃) is lower than that of octadecanoic acid (69.6 ℃) and hexadecanoic acid (62.7 ℃).This phenomenon exists not only in fatty acids, but also in other long-chain compounds.[4]
(4) Boiling point
The boiling point of fatty acids increases with the increase of carbon chain, and the boiling points of fatty acids with different saturations but the same carbon chain length are similar.[3]
(5) Solubility
Low fatty acids are easily soluble in water, but with the increase of relative molecular weight, their solubility in water decreases, so that they are soluble or insoluble in water, but soluble in organic solvents.Generally, the lower the level of fatty acid and the higher the degree of unsaturation, the greater the solubility in organic solvents. The higher the temperature, the greater the solubility, and the longer the carbon chain, the smaller the solubility.[3]
The physical properties of a substance are the expression of its chemical composition and structure.stayHigher fatty acidsThere are non-polar long carbon chains and polar COOH and COOR groups.There are differences in the length of the carbon chain and the number of unsaturated bonds, leading to small, large and sometimes significant differences in the physical and chemical properties of fatty acids.[4]
Fatty acids can be expressed in the following way: their name, number of carbon atoms, number and position of unsaturated double bonds.When expressing their names, first write down the number of carbon atoms, then write down the number of double bonds, finally use △ and the number at the upper right corner to indicate the position of double bonds, and add c (cis, cis) or t (trans, trans) after the number of double bond positions to indicate the configuration of double bonds.For example, the chemical name of linoleic acid is cis, cis - 9, 12-Octadecenoic acid。[4]
Structural characteristics
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There are some common rules in the molecular structure of natural fatty acids[4]：
(1) Generally, they are long-chain fatty acids with an even number of carbon atoms. The majority are 14-20 carbon atoms, and the most common are 16 or 18 carbon atoms, such as palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1 △nine）。[4]
(2) Higher plant and animalUnsaturated fatty acidGenerally, cis structure (cis), trans structure (trans) is rare.[4]
(3) The double bond position of unsaturated fatty acids has certain rules: one double bond is located between 9 and 10 carbon atoms, multiple double bonds also often have double bonds at position 9, and other double bonds are located at C.There is a methylene interval between the two double bonds and the methyl end of the carbon chain, such as oleic acid (18:1 △nine）Linoleic acid (18:2 △9，12）Linolenic acid (18:3 △9，12，14）、Arachidonic acid（20：4△5，8，11，14）。[4]
(4) General animal fat containsSaturated fatty acidMultiple;In the fat of higher plants and animals growing at low temperature, the content of unsaturated fatty acids is higher.[4]
The saturated fatty acid of natural triacylglycerol is mostly straight chain with even carbon number, and very few chain with odd carbon number.[4]
Saturated fatty acidIt is a very flexible molecule. Theoretically, it can rotate relatively freely around each C-C bond, so some configurations have a wide range.However, its fully extended conformation has the least energy and is the most stable;Because this conformation has the smallest steric hindrance between adjacent methylene groups.Like most substances, the melting point of saturated fatty acids increases with the increase of molecular weight.[5]
More than half of the fatty acids of animal and vegetable lipids containdouble bondOfUnsaturated fatty acidAnd is often a polydouble bond unsaturated fatty acid.Bacterial fatty acids rarely have double bonds but are often hydroxylated or containBranched chain, or ring structure containing cyclopropane.Some vegetable oils and waxes contain uncommon fatty acids.[6]
Mammals and humans cannot synthesize linoleic acid and linolenic acid, which are necessary for growth and need to be supplied by food, so they are called essential fatty acids.These two fatty acids are very abundant in plantsArachidonic acidIt is synthesized from linoleic acid, and arachidonic acid is rarely found in plants.[4]
Classification basis
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There are more than 40 different fatty acids in the natural world, which are the key components of lipids.The physical properties of many lipids depend on the saturation degree of fatty acids and the length of carbon chains, of which only an even number can be absorbed and utilized by the human bodycarbon atomFatty acids.Fatty acids can be classified according to their different structures, as well as their nutritional value from the perspective of nutrition.It is classified according to the length of carbon chain.It can be divided into three categories: short chain (containing 2-4 carbon atoms) fatty acids, medium chain (containing 6-12 carbon atoms) fatty acids and long chain (containing more than 14 carbon atoms) fatty acids.The human body mainly containsLong chain fatty acidsConstituent lipids.[1]
Fatty acids are composed of three elements, C, H and O. They are aliphatic hydrocarbon chains containing a carboxyl group at one end, and are components of many complex esters.Low fatty acids are colorless liquid with pungent smell,Higher fatty acidsIt is a waxy solid without obvious odor.With sufficient oxygen supply, fatty acids can be oxidized and decomposed into COtwoAnd HtwoO，It releases a lot of energy, so it is one of the main energy sources of organisms.[3]
Fatty acids can be classified in many ways, as follows.[3]
Classification according to different carbon chain lengths
It can be divided into short chain fatty acids, medium chain fatty acids and long chain fatty acids.[3]
Fatty acid metabolism
Fatty acids according toCarbon chainDifferent lengths can be divided into[6]：
Short chain fatty acids(short chain fat acids, SCFA), whose number of carbon atoms on the carbon chain is less than 6, is also called volatile fatty acids (VFA);[6]
Mid chain fatty acids (MCFA) refer to fatty acids with 6-12 carbon atoms on the carbon chain. The main components arebitter（Ceight）AndDecanoic acid（Cten）；[6]
Long chain fatty acids(Longchain fat acids, LCFA), the number of carbon atoms on the carbon chain is greater than 12.General food contains mostly long-chain fatty acids.[6]
Classification according to saturated and unsaturated hydrocarbon chains
Fatty acids can be divided into three categories according to the difference between saturated and unsaturated hydrocarbon chains[6], i.e.:
Saturated fatty acid(Saturated fat acids, SFA), there is no unsaturated bond on hydrocarbon;[6]
Monounsaturated fatty acid(Monounsaturated fatty acids, MUFA), whose hydrocarbon chain has an unsaturated bond;[6]
Polyunsaturated fatty acids(Polyunsaturated fatty acids, PUFA), whose hydrocarbon chain has two or more unsaturated bonds.[6]
（1）Saturated fatty acidThere is no unsaturated bond on the hydrocarbon chain, generally from CfourTo Cthirty-eight。Fatty acids from 4 to 24 carbon atoms are often found in oils, while those with more than 24 carbon atoms are found in waxes.According to the number of carbon atoms in the molecule, it can be divided into low-grade saturated fatty acids (carbon atoms ≤ 10, liquid at normal temperature) and high-grade saturated fatty acids (carbon atoms>10, solid at normal temperature).The most common saturated fatty acids in animal and vegetable oils are butyric acid, caproic acid, caprylic acid, capric acid and higher saturated fatty acids such as hexadecanoic acid（Palmitic acid）And octadecanoic acid（stearic acid）, followed by dodecanoic acid（Lauric acid）, myristic acid（Myristic acid）And eicosanoic acid（Arachidic acid）Etc.[3]
（2）Unsaturated fatty acidFatty acids with one or more unsaturated bonds in their molecules are called unsaturated fatty acids.[3]
Unsaturated fatty acids are usually liquid, mostly vegetable oils, such as peanut oil, corn oil, soybean oil, nut oil (namely glycerin), rapeseed oil, etc.According to the number of unsaturated bondsMonounsaturated fatty acid(There is an unsaturated bond, such as myristic acidPalmitoleic acid, rapeseed oleic acid) and polyunsaturated fatty acids (with two or more unsaturated bonds, such as linoleic acid and linolenic acid).Unsaturated fatty acids include linolenic acidlinoleic acid、oleic acidMost common.[3]
Natural fatty acids
Some have been foundPolyunsaturated fatty acids(From the number of methyl ends, the last unsaturated double bond is located between the third and fourth carbon atoms.) It has special functions for human body.The most important fatty acids are C22:6 (4, 7, 10, 13, 16, 19 docosahexaenoic acid, i.e. DHA) and C20, 5 (5, 8, 11, 14, 17 eicosapentaenoic acid, i.e. EPA), which are important functional substances.The research shows that DHA has a good brain function, and has curative effect on Alzheimer's disease, atopic dermatitis, hyperlipidemia;EPA can reduce platelet aggregation, prolong blood coagulation time after bleeding, and reduce the incidence of myocardial infarction.In addition to the above functions, EPA can also reduceBlood viscosity. Increase the concentration of high-density cholesterol (high-quality cholesterol) and reduceLow density cholesterol(poor cholesterol), so EPA is considered to have a good preventive effect on cardiovascular disease.The main source of DHA and EPA is deep-sea fish oil, such as sardines, cuttlefish, cod, etc.[3]
be rich inMonounsaturated fatty acidandPolyunsaturated fatty acidsThe fat is liquid at room temperature, mostly vegetable oil, such as peanut oilCorn oil, soybean oilNut oil(i.e. glycerin), rapeseed oil, etc.withSaturated fatty acidThe fat mainly composed of is solid at room temperature, mostly animal fat, such as butter, sheep fat, lard, etc.There are exceptions, for example, although deep-sea fish oil is animal fat, it is rich in polyunsaturated fatty acids, such as EPA and DHA, so it is liquid at room temperature.[6]
Classification according to the degree of meeting the needs of the body
Animals can synthesize saturated fatty acids andoleic acidThis kind of double bondUnsaturated fatty acidMultiple double bond fatty acids containing two or more double bonds must be obtained from plants, so the latter is called essential fatty acids, in which linolenic acid andlinoleic acidMost important.Arachidonic acidIt is formed from linoleic acid.Arachidonic acid is the majorityprostaglandinProstaglandins are hormone like substances that can regulate cell function.[6]
Essential fatty acidsThe best source of oil is vegetable oil, but it is less than other vegetable oils in rapeseed oil and tea oil.The content of animal fat is generally lower than vegetable oil, but relatively speaking, lard is more than beef and sheep fat, and poultry fat is more than lard.Chicken and duck meat are more abundant than pig, beef and mutton.The content of animal heart, liver, kidney and intestine is higher than that of muscle, while the content of lean meat is higher than that of fat meat.In addition, the content of egg yolk is also high.[3]
Most of the fatty acids can be synthesized by the body itself and can be taken directly without food. These fatty acids are called nonessential fatty acids.Non essential fatty acids are mainlySaturated fatty acid。Although saturated fatty acids are nonessential fatty acids, excessive intake will increase the content of blood lipids in the body, but because it plays an irreplaceable role in human body, especially in the development of human brain, if long-term intake is insufficient, it is bound to affect brain development.Therefore, the intake of various animal fats and vegetable oils should be determined according to the actual situation.[3]
biochemical reaction
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Oxidative decomposition
Beta oxidation
Fatty acids are insoluble in water and are transported to the whole body after being combined with albumin in the blood (10:1)HistiocyteIt is oxidized and decomposed in the mitochondria of cells, releasing a lot of energy, and the liver and muscle are the most active.In 1904, Knoop rigid benzene ring was used as a marker to track the transformation of fatty acids in animals. It was found that hippuric acid was detected in urine when odd carbon fatty acid derivatives were degraded. If even carbon, hippuric acid was detected in urinePhenylethyluric acid。It is speculated that the degradation of fatty acid acyl chain occurs on the β - carbon atom, that is, a two carbon unit is cut off from the fatty acid chain each time.Later experiments proved that the β - oxidation theory was correct. The cut carbon dioxide unit was acetyl CoA, and fatty acids had to be activated before entering mitochondria.
Beta oxidation
1) Activation of fatty acids;[7]
2) Lipoyl CoA enters mitochondria;[7]
3) β - oxidation of acyl CoA;[7]
The oxidation of fatty acyl CoA to acetyl CoA involves four reactions - dehydrogenation, water addition, re dehydrogenation and thiolysis.One molecule of acetyl CoA and two C less aliphatic acyl CoA are produced each time.Then carry out the next round of β - oxidation, and repeat the cycle.[7]
4) Energy calculation of fatty acid oxidation
1 molecule palmitic acid (Csixteen）After 7 times of β - oxidation, 8 acetyl CoA, 7 NADH and 7 FADH can be generatedtwo。Each acetyl CoA enters the TCA cycle to generate three NADHs, one FADH and one GTP, and releases two molecules of COtwo。[7]
When fat is used as energy, organisms can also obtain a lot of water.The hump of a camel is a "warehouse" for storing fat, which can provide both energy and water.[7]
Other pathways of fatty acid oxidation
(1) Oxidation of fatty acids with odd carbon atoms.The human body contains trace amounts of odd carbon fatty acids, and many plants, marine organisms and petroleum yeasts contain a certain amount of odd carbon fatty acids.In addition to acetyl CoA, its β - oxidation also generates 1 molecule propionyl CoA, which generates succinyl CoA under the action of β - carboxylase and isomeraseTCAThe pathway is completely oxidized.[7]
（2）Unsaturated fatty acidOxidation of.About half of the fatty acids in the body are unsaturated fatty acids, in which the double bonds are cis configuration, which cannot be catalyzed by the alkenyl CoA hydratase, which catalyzes the water addition process of trans configuration double bonds, so the participation of isomerase and reductase is required to enable the oxidation of general unsaturated fatty acids.For example, oleic acid is octadecenoic acid (cis - △nine）The oleic acid in the cytoplasm is also activated to generate oleoyl CoA, which is generated by the transport systemMitochondrial matrixOleoyl CoA in, through three rounds of β - oxidation to produce three molecules of acetyl CoA and cis - △three-DodeceneFatty acyl CoA, which is converted to trans - △ by isomerasetwoDodecanoyl CoA is produced by the action of alkenyl CoA hydratase to generate L - β - hydroxyacyl CoA, and then through five rounds of β - oxidation to generate 6 molecules of acetyl CoA, and a total of 9 molecules of acetyl CoA are generated.[7]
Acetone bodies are special intermediates generated by normal catabolism of fatty acids in the liver, includingAcetoacetic acid(30% of the total is acid),β - hydroxybutyric acid(β - hydroxybutyric acid accounts for about 70%) and a very small amount of acetone.In normal human bloodKetone bodyThe content is very low, which is a normal phenomenon that the human body uses fat oxidation to provide energy.But in some physiological conditions (hunger, fasting) or pathological conditions (such asdiabetes）The source of sugar or oxidative energy supply is blocked, and lipid mobilization is enhanced, so fatty acids become the main energy supply substance of the human body.If the amount of ketone bodies synthesized in the liver exceeds the ability of extrahepatic tissues to use ketone bodies, the balance between the two will be lost, and the concentration in the blood will be too high, leading toKetosis(acetone mia) andKetonuria（acetonuria）。Acetoacetic acidandβ - hydroxybutyric acidThey are all acidic substances, so the accumulation of ketones in the body can also cause acidosis.[8]
biosynthesis
Fatty acid synthesis site
There are fatty acid synthetase systems in the cytoplasm of liver, kidney, brain, lung, breast, fat and other tissues in the body, so these tissues can synthesize fatty acidsFatty acid synthase systemThe activity is the highest, so hepatocytes are the main part of the body to synthesize fatty acids.[9]
Although adipose tissue can also use the intermediate products of glucose metabolism as raw materialsSynthetic fatty acidsIts main source is exogenous fatty acids absorbed by the small intestine and endogenous fatty acids synthesized by the liver.[9]
Fatty acid synthesis
The synthesis of palmitic acid is completed in cytoplasm, but the elongation of fatty acid chain is completed in mitochondria and endoplasmic reticulum.[9]
Raw material for fatty acid synthesis
Raw materials for fatty acid synthesis includeAcetyl CoA、HCOthree-(C02), NADPH and ATP, Mn2+It can be used as an activator of enzyme.[9]
Fatty acid synthesis pathway
Acetyl CoA in organismSynthetic fatty acidsThere are: ① Non mitochondrial enzyme system synthesis pathway: that is, cytosolic enzyme system synthesis saturated fatty acid pathway.The end product of this pathway is palmitic acid, so it is also called palmitic acid synthesis pathway, which is the main pathway of fatty acid synthesis. ②Mitochondrial enzyme system synthesis pathway: also called saturated fatty acid carbon chain extension pathway.[10]
Palmitic acid
1. Transfer of acetyl CoA
Acetyl CoA can be oxidized and decomposed by sugar or by fatty acidKetone bodyAnd protein decomposition to produce acetyl CoA, and the reaction to produce acetyl CoA occurs in mitochondria, while the synthesis site of fatty acids is cytoplasm, so acetyl CoA must be transported from mitochondria toCytoplasm。howeverAcetyl CoAIt cannot pass through the mitochondrial membrane freely, and needs to complete the transfer of acetyl CoA from mitochondria to cytoplasm through a cycle called citrate pyruvate cycle.[8]
First, in mitochondria, acetyl CoA and oxaloacetateCitrate synthaseCatalysiscitric acid, and then enter the cytoplasm with the help of the corresponding carrier on the mitochondrial inner membranecitric acidCitrate lyase can cleave citric acid to produce acetyl CoA and oxaloacetic acid.The former can be used to generate fatty acids, and the latter can be returned to the consumption of mitochondria to supplement the synthesis of citric acid.But oxaloacetic acid can not be freely permeableMitochondrial intima, so you must firstMalate dehydrogenaseCatalyzed, reduced to malic acid, and then transported into mitochondria through the carrier on the mitochondrial inner membrane, and supplemented after oxidationOxaloacetic acid。Also available atMalaseOxidative decarboxylationpyruvic acid, accompanied by the generation of NADPH.Pyruvate can be transported into mitochondria through the inner membrane carrier, and then pyruvate can be re carboxylated into oxaloacetic acid.Each cycle of citric pyruvate can make one molecule of acetic acid CoA enter the cytosol from mitochondria, consume two molecules of ATP at the same time, and also provide the body with NADPH to supplement the needs of synthetic reaction.[8]
2. Formation of malonyl CoA
Acetyl CoA is composed ofAcetyl CoA carboxylaseAcetyl CoA carboxylase catalyzes the conversion to malonyl CoA (or malonyl CoA). Acetyl CoA carboxylase exists in the cytosol, and its cofactor is biotin, which plays the role of carrying and transferring carboxyl groups in the reaction process.The reaction mechanism is similar to other biotin dependent carboxylation reactions, such as catalysispyruvic acidCarboxylationOxaloacetic acidReaction, etc.[8]
The reaction catalyzed by acetyl CoA carboxylase is a rate limiting step in fatty acid synthesis.This enzyme is aAllosteric enzymeUnder the action of allosteric effector, its inactive monomer and active polymer (linear arrangement of 100 monomers) can be interchangeable.Citric acid andIsocitrateIt can promote monomer polymerization into polymers, enhance enzyme activity, andLong chain fatty acidsIt can accelerate the depolymerization, thus inhibiting the enzyme activity.Acetyl CoA carboxylase can also regulate its activity by phosphorylation and dephosphorylation dependent modification of cAMP.This enzyme loses its activity after phosphorylation, such asGlucagonandadrenalineCan promote this phosphorylation, thereby inhibiting fatty acid synthesis;andinsulinCan promote theDephosphorylationTherefore, it can enhance the activity of acetyl CoA carboxylase and accelerate the synthesis of fatty acids.[8]
At the same time, acetyl CoA carboxylase is also an inducible enzyme. Long term high sugar and low-fat diet can induce the generation of this enzyme and promote fatty acid synthesis;On the contrary, high-fat and low sugar diet can inhibit the synthesis of this enzyme and reduce the production of fatty acids.[8]
In prokaryotes (e.gEscherichia coliMedium) The enzyme that catalyzes the formation of fatty acids is an enzyme with seven different functions and oneAcyl carrier protein(acyl carrier protein, ACP).In eukaryotes, this reaction is catalyzed by an enzyme containing two subunitsSubunitIt has seven structural regions with different catalytic functions and one structural region equivalent to ACP, so it is an enzyme with multiple functions.The structure of this enzyme is different in different organisms.[8]
The synthesis of palmitic acid is actually a repeated cycle process, consisting of 1 moleculeAcetyl CoAAfter repeated processes of transfer, condensation, hydrogenation, dehydration and re hydrogenation with 7 molecules of malonyl CoA, the carbon chain is extended by two carbons each time for a total of 7 times, and finally palmitic acid containing hexadecyl carbon is generated.[8]
Fatty acid synthesis consumes ATP and NADPH+H+, and NADPH mainly comes from pentose phosphate pathway of glucose decomposition.In addition,malic acidOxidative decarboxylation can also produce a small amount of NADPH.[8]
The process of fatty acid synthesis is not the reverse process of β - oxidation. They react with tissues, cell localization, transfer carriers, acyl carriers,Rate limiting enzyme, activator, inhibitor, hydrogen donor and hydrogen acceptor, and reaction substrate and product are different.[8]
Other fatty acids
There are not onlyPalmitic acidAnd other fatty acids with different carbon chainsUnsaturated fatty acid, except nutritionEssential fatty acidsApart from relying on food supply, other fatty acids can be processed and transformed from palmitic acid in cells.[8]
1. Extension and shortening of carbon chain
The shortening of fatty acid carbon chain is completed by β - oxidation in mitochondria, and two carbon atoms can be reduced after one β - oxidation cycle.[8]
The carbon chain extension of fatty acids can be catalyzed by fatty acid elongation enzyme system in smooth endoplasmic reticulum and mitochondria.[8]
In the endoplasmic reticulum,Palmitic acidLengthening is a donor with malonyl CoA as the second carbon unit, which is hydrogen supplied by NADPH+H+, and also extends the carbon chain through condensation, decarboxylation, reduction and other processes, which is basically the same as the process of fatty acid synthesis in the cytosol.However, the enzyme system of catalytic reaction is different. Its fatty acyl group is not carried by ACP, butCoenzyme AJoin in the reaction.In addition to brain tissuestearic acid(18C) mainly, because brain tissue contains other enzymes, it can be extended to 24 carbon fatty acids for lipid metabolism in the brain.[8]
In mitochondria, palmitic acid is condensed with acetyl CoA to gradually extend the carbon chain through the action of mitochondrial fatty acid lengthening enzyme system. Its process is similar to that of fatty acid β oxidation retrograde reaction, only thecoenzymeNADPH+H+is different from β oxidation process.In this way, the carbon chain of fatty acid can be extended to 24 or 26 carbon, but stearic acid is the most.[8]
2. Fatty acid desaturation
Human and animal tissuesUnsaturated fatty acidMainly soft oleic acid (16:1 △ 9)oleic acid（18：1△9）、linoleic acid（18：2△9，12）、linolenic acid（18：3△9，12，15）、Arachidonic acid(20:4 △ 5, 8, 11, 14), etc.The most commonMonounsaturated fatty acidSoft oleic acid and oleic acid can be activated by corresponding fatty acids and catalyzed by acylCoAdesaturasedehydrogenationBuild.This kind of enzyme exists in the smooth endoplasmic reticulum, belonging toMixed functional oxidase；Because this enzyme only catalyzes the formation of double bond at △ 9, but cannot form double bond between C10 and terminal methyllinoleic acid（linoleate）、linolenic acid(linolenate) andArachidonic acid(arachidonite) cannot be synthesized or is insufficient in vivo.But they are indispensable to the body, so they must be supplied by food. Therefore, they are calledEssential fatty acids（essential fatty acid）。[8]
Acetyl CoA carboxylase catalyzed reaction is the rate limiting step of fatty acid synthesis. Many factors can affect the activity of this enzyme, thus changing the rate of fatty acid synthesis.Other enzymes in fatty acid synthesis, such asFatty acid synthaseCitrate lyase can also be regulated.[11]
1. Regulation of metabolites
After high-fat diet or when fat mobilization is strengthened due to starvation, palmitoyl CoA in cells increases, which can be feedback inhibitedAcetyl CoA carboxylaseTo inhibit fatty acid synthesis in the body.However, when eating sugar and sugar metabolism is strengthened, acetyl CoA and NADPH provided by sugar oxidation and pentose phosphate circulation are increasedSynthetic fatty acidsThe increase of the raw materials is conducive to the synthesis of fatty acids.In addition, as a result of enhanced glucose oxidation, ATP in cells increases, which inhibits isocitrate dehydrogenase, resulting in accumulation of isocitrate and citric acidmitochondrionWith the assistance of the corresponding carrier of the inner membrane, the acetyl CoA carboxylase can be activated allosterically by transferring from mitochondria to the cytosol.At the same time, it can also be cracked to release acetyl CoA, increase the raw materials for fatty acid synthesis, and increase the fatty acid synthesis.[11]
Insulin can induce acetyl CoA carboxylaseFatty acid synthaseAnd the synthesis of citric acid lyase, thereby promoting the synthesis of fatty acids.In addition, it can also promote theDephosphorylationIt increases the enzyme activity and accelerates the fatty acid synthesis.[11]
Glucagon and others can reduce the activity of acetyl CoA carboxylase by increasing cAMP, thus inhibiting the synthesis of fatty acids.In addition, glucagon also inhibitstriglycerideSynthesis, thus increasing the feedback inhibition of long-chain acyl CoA on acetyl CoA carboxylase and inhibiting fatty acid synthesis.[11]
Main role
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Fatty acids often combine with other substances to form esters. Fatty acids in free form are rare in nature.[3]
Hormones are activated when people encounter hunger or stressfat cellsLipase in, will storetriglycerideThey are converted back to fatty acids and glycerol, which are then released into the blood for use.Except for brain cells, all cells in the body adapt themselves to use fatty acids when they are hungry and lack energy. Like glucose, fatty acids can be converted into the energy form of ATP.In fact, hormones that stimulate triglyceride cleavage are ineffective in the brain.Because the human brain does not have the ability to use fatty acids like other parts of the body, it only uses glucose.Glycerol, another product of triglyceride cracking, circulates to the liver, which converts it into glucose through another biochemical pathway to feed the brain.In this way, when nutrients are scarce, other parts of the body can rely on fatty acids, while the brain can only rely on the glucose it needs.[2]
Natural fatty acids[4]
Animals can synthesize saturated fatty acids and linoleic acids that only contain one double bondUnsaturated fatty acidMultiple double bond fatty acids containing two or more double bonds must be obtained from plants, calledEssential fatty acidsLinoleic acid and linoleic acid are the most important.In most fat rich foods, about half of the calories are provided by fat and oil.Natural fats and oils are usually a mixture of various esters formed by more than one fatty acid and glycerol.The functions of these fatty acids are as follows.[3]
① It can provide heat and is a good source of energy.[3]
② Fatty acid storagefat cellsIn case of human emergency.[3]
③ As raw material for synthesis of other compounds.[3]
④ It can maintain the relative fluidity of cell membrane to ensure the normal physiological function of cells.[3]
⑥ Improve brain cell activity, memory and thinking ability.[3]
Fatty acids can be usedbutadiene styrene rubberEmulsifier and others in productionsurface active agent 、lubricantGloss agent;It can also be used to produce the intermediate of high grade soap, transparent soap, stearic acid and various surfactants.[3]
Functionality
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Each type and each kind of fatty acid has its specific use and functional characteristics.Functional fatty acids refer to a group of fatty acids that are derived from human dietary fats and are essential for human nutrition and health, and have positive preventive effects on some corresponding deficiencies and endogenous diseases of the human body that have been found, especially on diseases of modern civilization, such as hypertension, heart disease, cancer, diabetes, etc., which are also of great concern and widely studiedPolyunsaturated fatty acidsMainly.[12]
Polyunsaturated fatty acids
Polyunsaturated fatty acid is the main body and core of functional fatty acid research and development. According to its structure, it can be divided into two major series, n-6 and n-3.This kind of fatty acid is widely concerned, not only because n-6 series of linoleic acid and n-3 series ofα - linolenic acidIs indispensable to the human bodyEssential fatty acidsMore importantly, it plays an extremely important metabolic role in human physiology, which is closely related to the occurrence and regulation of many modern civilization diseases.At present, it is believed that the outstanding importance of n-6 and n-3 fatty acids is, first of all, that they are precursors of prostaglandins and leukotrienes that have important metabolic functions in the body.Another outstanding importance is that they are absolutely necessary components of biological membranes of human organs and tissues.In addition, these polyunsaturated fatty acid molecules themselves play a special role in many other normal physiological processes of the human body.[12]
1. Linoleic acid
Linoleic acid is functionalPolyunsaturated fatty acidsIt is the first one to be recognized, and accounts for the majority of unsaturated fatty acids in most dietary nutrients worldwide.Linoleic acid has the effect of reducing the serum cholesterol level, andSaturated fatty acidIn contrast, linoleic acid has a strong role in reducing the concentration of LDL cholesterol.A large amount of linoleic acid has obvious effect on patients with hypertriglyceridemia.It is still used in Chinese PharmacopoeiaEthyl linoleatePills and drops are used to prevent and treat hypertension, atherosclerosis and coronary heart disease.[12]
Linoleic acid is defined as an essential fatty acid in part because it is n-6Long chain polyunsaturated fatty acidsArachidonic acid is also the precursor of arachidonic acid. Arachidonic acid is mostly found in nerve tissue and brain. The brain actively metabolizes arachidonic acid, and its metabolites have important effects on the central nervous system, including the regulation of neuronal transmembrane signals, the release of neurotransmitters, and the uptake of glucose.From the third month of pregnancy to the life growth and development of about 2-year-old infants, arachidonic acid rapidly accumulates in the brain and plays an important role in cell division and signal transmission.For adults, it is unclear whether dietary arachidonic acid supply affects the arachidonic acid substrate pool related to brain metabolism.In some anti-tumor animal experiments, it has been proved that arachidonic acid can significantly kill tumor cells in vitro, and has no toxic and side effects on normal cells.Arachidonic acid has been used experimentally in some new dosage forms of anticancer drugs.[12]
γ - linolenic acid was found in evening primrose oil by Heidush Kaand Laft in 1919.At present, evening primrose oil and γ - linolenic acid products rich in γ - linolenic acid have been widely used in nutrition and medical treatment.The clinical test results of γ - linolenic acid show that it has a hypolipidemic effect, and the effect of reducing triacylglycerol, cholesterol and p-lipoprotein is more than 60%. In addition, γ - linolenic acid is transformed into PGI2 with vasodilator effect in vivo, maintaining the balance with thromboxane A2 (TXA2), preventing thrombosis, and thus achieving the effect of preventing cardiovascular disease.γ - linolenic acid can stimulate brown adipose tissue in the body, promote the activation of mitochondria in the tissue, release excessive heat in the body, prevent obesity, and reduce cell membrane in the bodyLipid peroxidationDamage.[12]
The most important physiological function of α - linolenic acid is that it is n-3 seriesPolyunsaturated fatty acidsIt can be metabolized in vivo to produce DHA and EPA.Since DHA is one of the two main polyunsaturated fatty acids in the brain and retina, many animal experiments have shown that α - linolenic acid in the diet, especially in extreme or long-term deficiency, will lead to corresponding deficiency symptoms, visual cycle defects and obstacles.At the same time, the physiological function of α - linolenic acid is also manifested in the prevention and treatment of cardiovascular diseases.In 1987, Berry and Hirsch analyzed the fatty acid composition in adipose tissue of a group of middle-aged men without heart disease or hypertension, and pointed out that every 1% increase in α - linolenic acid in adipose tissue would reduce the arterial systolic and diastolic pressure by 667Pa.After 1988, Salonen et al. observed that the lower blood pressure of Finnish men was significantly related to the level of α - linolenic acid intake, which supported the above research conclusions.The hyperlipidemia test of Chinese Academy of Medical Sciences on mice with perilla oil rich in α - linolenic acid shows that α - linolenic acid can significantly reduce the level of total cholesterol and LDL cholesterol in serum, and increase the HDL cholesterol/LDL cholesterol ratio, which is superior to antomin.[12]Another important function of α - linolenic acid is to enhance the immune effect of the body.[12]
5. DHA and EPA
From the analysis of the brain, retina and nerve tissue of animals, including humans, we can find that,Docosahexaenoic acid(doco sahexaenoic acid. DHA) is the main fatty acid and is necessary for the normal development and function maintenance of the brain and retina.Its mechanism of action is firstly to form a highly fluid membrane environment due to the high degree of unsaturated. In addition, it also has an irreplaceable special mechanism of action.DHA accounts for2-hydroxyethylamineGlyceride phosphateMore than 30% of medium fatty acids.In brain, DHA combines with synaptosomes, synaptic vesicles, myelin, microsomes and mitochondria.AndArachidonic acidIn contrast, DHA preferentially binds to the retina to form triacylglycerol.Experiments on DHA and visual function in cats, monkeys and other animals have better revealed the importance of DHA in vision.DHA and EPA (eicosapentaenoic acid,Eicosapentaenoic acid）After ingestion, the level of these two fatty acids in the body can be rapidly and significantly increased, which ensures the timely play of their functions.Therefore, in the nervous system, DHA and EPA have been proved to have the physiological effect of maintaining and improving vision, improving memory, learning and other abilities, and inhibiting Alzheimer's disease.[12]
Medium chain fatty acid
Medium chain fatty acids are mainly absorbed in free form in the body.Due to the short carbon chain, medium chain fatty acids and long chain fatty acids have good water solubility and are easy to be absorbed by the gastrointestinal tract. They will not re esterify like long chain fatty acids in intestinal cells.Lipids containing medium chain fatty acids are digested by tongue lipase at the entrance and continue to be hydrolyzed in the stomach. Tongue lipase has specificity for the hydrolysis of triacylglycerol rich in medium chain fatty acids. It takes 05h, 2.5h can reach the peak, which is half of the time consumption of long-chain fatty acids.In addition to a small amount of medium chain fatty acids that exist in the surrounding blood for a short time, most of them are bound to albuminPortal vein systemGet to the liver faster.In the liver, medium chain fatty acids can quickly pass through the mitochondrial bilayer membrane and be acylated under the action of octyl CoA, while almost no fat is synthesized.The excessive acetyl CoA produced by acylation has various metabolic effects in the mitochondrial cytoplasm, most of which tend to synthesize ketone bodies. Its ketogenic effect is stronger than that of long-chain fatty acids, and is not affected by anti ketogenic substances such as glycerol, lactate, glucose insulin, etc. In extrahepatic tissues, the metabolism of medium chain fatty acids is less,However, 15% - 20% of C8:0 or below fatty acids are found in cord blood, which also shows that medium chain fatty acids also have a physiological role in fetal nutrition.[12]
Fatty acid composition and content of main fats
As the biochemical metabolism of medium chain fatty acids is relatively fast, it can be used as a fast energy source, especially for the long chain fatty acids in dietary oils that are difficult to digest orLipid metabolism disorderFor example, patients without biliary disease, pancreatitis, primary biliary cirrhosis, colonic disease, small bowel resection, premature infants lacking lipase, and patients with fibrovesicular disease.Another important role of medium chain fatty acids is the ketone body effect. All extrahepatic tissues can use it to rapidly oxidize to produce a large number of ketones. After surgery, patients can use it to provide heat energy. Pregnant women can supplement the demand of the fetus for consuming more ketones by injecting medium chain fatty acids.It can also save carnitine in muscles of patients with chronic diseases, and improve the inhibition of ketosis related to sepsis or trauma.In addition, ketone bodies generated from medium chain fatty acids have anesthetic and anticonvulsant effects and have been used clinically as non drug resistant epilepsy drugs.[12]
Oil uptake
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Whether vegetable or animal oils, there are 9 calories per gramquantity of heat。However, vegetable oil contains fat decomposing substances. Moderate intake is beneficial, but it does not mean that its calories are low.The common people think that vegetable oil is safe and can be eaten more, which is a wrong idea. Not only people who want to lose weight must limit their intake of vegetable oil to avoid adverse effects on weight loss, but also people who want to live a long and healthy life should do so.[13]
Types and contents of fatty acids in different oils
There are three types of fatty acids people need: polyunsaturated fatty acids, monounsaturated fatty acidsUnsaturated fatty acidandSaturated fatty acid。Our common edible oil usually contains three kinds of fatty acids needed by human body.[13]
The daily oil intake of each person can only account for 20% of the total daily food calories. (The daily oil consumption is controlled at 15 to 30 ml) Each person should eat all three fatty acids every day, and should not prefer any oil. Otherwise, the imbalance of oil intake will lead to disease.The daily intake of unit unsaturated fatty acids should account for 10%, and be diversifiedUnsaturated fatty acid10%, and less than 10% saturated fatty acids.[13]
The main components of animal oil, coconut oil and palm oil are saturated fatty acids, whilePolyunsaturated fatThe acid content is very low.[13]
Olive oil、Nut oil(i.e. glycerin), rapeseed oilCorn oilPeanut oil has a high content of monounsaturated fatty acids. Among the three kinds of fatty acids needed by human body, monounsaturated fatty acids are the largest. Corn oil and olive oil can be important sources of this fatty acid.[13]
Sunflower oil, millet oil, soybean and other vegetable oils andMarine fishMost of the fat contained inPolyunsaturated fatAcids.Polyunsaturated fatty acid is the main component of these edible oils, and the other two fatty acids are not much.Among the three fatty acids, polyunsaturated fatty acids are the most unstable and are most likely to be oxidized into toxic oil under the high temperature of frying, frying or frying.But it is diversifiedUnsaturated fatty acidagainHuman cellsOne of the important raw materials of membrane.[13]
