Enzyme catalysis, also known as enzyme catalysis or enzyme catalysis, refers to chemical reactions catalyzed by enzymes as catalysts.
Intrabiologicalchemical reactionMost of them are enzymatic reactions.enzymeAs aBiocatalystWhen catalyzing a chemical reaction, it not only has the characteristics of the general catalyst, but also has the particularity different from the general catalyst.
Enzyme is a specific and efficientBiocatalystMost enzymes are proteins produced by living cells.Enzymes can be catalyzed under mild conditions at normal temperature and pressure.Enzymatic reactionNon catalytic reactionFast 10three-10sevenTimes.Enzymatic reaction kineticsIt is called enzyme kinetics for short. It mainly studies the relationship between the speed of enzymatic reaction and the concentration of substrate (i.e. reactant) as well as other factors.When the substrate concentration is very low, the enzymatic reaction is a first order reaction;When the substrate concentration is in the middle range, it is a mixed order reaction;Transition to zero order reaction when substrate concentration increases[1]。
Main characteristics
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universality
1. Like general catalysts, enzymes only catalyze chemical reactions (i.e. reverse reactions) allowed by thermodynamics.
2. It can accelerate the rate of chemical reaction without changing the equilibrium point of the reaction, that is, without changing the reactionequilibrium constant。
4. Before and after the reaction, there is no change in the quality and quantity of the enzyme, and a small amount of enzyme can play a huge role inCatalysis。
specific characteristics
Enzymatic reaction
However, enzyme also has its own particularity different from other catalysts.
In enzymatic reaction, the enzyme acts as a highly efficientcatalyzerSo that the reaction can be carried out at an extremely fast speed or under conditions that cannot be reacted in general.
Enzymes are the most important substances for various chemical reactions in organisms.
characteristic
1、 Enzymatic reaction has high efficiency
Enzymatic reaction
2、 Enzymatic reaction has high specificity
The specificity of enzyme refers to thesubstrateThere are three types of selectivity:
1. The absolutely specific enzyme only acts on the substrate of a specific structure to generate a product of a specific structure.If amylase only functionsstarch。
2. Relatively specific enzymes can act on a class of compounds or a chemical bond.for examplephosphataseIt can act on all compounds containing phosphate ester bond.
Effect of substrate concentration on enzyme reaction rate
Figure 1
The effect of substrate concentration on enzyme reaction rate is complex.At a certain enzyme concentration, when the substrate concentration is low (the substrate concentration gradually increases from 0), the relationship between the reaction rate and the substrate concentration isProportionalRelationship (as shown in Figure 1);With the increase of substrate concentration, the reaction rate no longer increases proportionally;If we continue to increase the substrate concentration, the reaction rate will no longer rise, approaching a limit[2]。
influence
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At a certain temperature andpHWhen the substrate concentration is sufficient to saturate the enzyme
Figure 2
In this case, the concentration of enzyme is in direct proportion to the rate of enzymatic reaction.Figure 2:
Effect of pH on enzyme reaction rate
The influence of pH on the speed of enzymatic reaction The pH of the enzyme reaction medium can affect the enzyme molecules, especially the degree of dissociation of the essential groups on the active center and the required ionization state of the proton donor or proton acceptor in the catalytic group, and can also affect the degree of dissociation of the substrate and coenzyme, thus affecting the binding of the enzyme to the substrate.Only under specific pH conditions, the dissociation of enzyme, substrate and coenzyme is most suitable for their combination and occurrenceCatalysis, so as to maximize the speed of enzymatic reaction. This pH value is called the optimal pH of the enzyme.
Figure 3
TrypsinThe activity of most enzymes is affected by the pH of their environment. At a certain pH, the enzymatic reaction has the maximum speed. If it is higher or lower than this value, the reaction will decline. Usually, this pH is called the optimal pH of the enzyme.The optimum pH of different enzymes is different.Figure 3: For example:PepsinThe optimum pH of trypsin is 8.0~9.0,Salivary amylaseThe optimum pH of is 6.8, etc.Animal enzymes are mostly in the range of pH 6.5~8.0, and plants and microorganisms are mostly in the range of pH 4.5~6.5, but there are exceptions.For example, the optimal pH of fungi is 5.0~6.0, most bacteria is 6.5~7.5, and actinomycetes is 7.5~8.5.
The optimal pH of most enzymes in the body is close to neutral, but there are exceptions, such asPepsinThe optimal pH of is about 1.8ArgininaseThe optimum pH is about 9.8.When the pH value of the solution is higher or lower than the optimal pH value, the activity of the enzyme will be reduced, and when the solution is far away from the optimal pH value, it will even cause denaturation and inactivation of the enzyme (Figure 3).Therefore, appropriate buffer solution should be selected when determining the enzyme activity to keep the enzyme activity relatively constant.According to the optimal pH of pepsinMetaacidFor this feature, a certain amount of dilute hydrochloric acid is added when preparing pepsin mixture to help digestion, so that it can play a better curative effect.
Main reasons for pH affecting enzyme activity
The structure of the enzyme molecule is affected by the excess of acid and alkali, even denaturing and inactivating the enzyme.
It should be noted that the optimal pH of the enzyme in the test tube is not necessarily the same as its physiological pH in normal cells.This is because there may be hundreds of enzymes in a cell, and different enzymes have different sensitivity to the physiological pH in the cell;That is to say, this pH is the optimal pH for some enzymes, but not for others. Different enzymes show different activities.This difference is important for controlling the complexMetabolic pathwayIt may be of great significance.
Effect of temperature on the rate of enzymatic reaction
The speed of chemical reaction increases with the increase of temperature, but the enzyme is a protein and can be denatured with the increase of temperature.At lower temperature, the former has greater influence, and the reaction speed increases with the increase of temperature.However, when the temperature exceeds a certain range, the factor of enzyme denaturation by heating is dominant, and the reaction speed will slow down with the temperature rising.A certain temperature range with the maximum speed of enzymatic reaction is often called enzymaticOptimum temperature
The optimal temperature of enzyme in human body is close to body temperature, generally between 37 ℃ and 40 ℃. If the enzyme is heated to 60 ℃, it will begin to denature. If the temperature exceeds 80 ℃, the denaturation of enzyme is irreversible.
The influence of temperature on the speed of enzymatic reaction has guiding significance in clinical practice.Under low temperature, the activity of the enzyme decreases, but the low temperature generally does not destroy the enzyme. After the temperature rises, the enzyme recovers its activity.Therefore, in the management technology operation, the enzyme preparations and enzyme test samples (such as serum) should be stored at low temperature in the refrigerator, taken out from the refrigerator when necessary, and used or tested after the temperature rises at room temperature.After the temperature exceeds 80 ℃, most of the enzymes are denatured and inactivated. This principle is used in clinical practice for high-temperature sterilization.
EnzymaticOptimum temperatureIt is related to the time required for the reaction. The enzyme can withstand higher temperature in a short time. On the contrary, it can prolongreaction timeThe optimum temperature will be reduced.Therefore, in the biochemical test, the method of properly increasing temperature and shortening time can be adopted for rapid detection of enzymes.
Different temperatures have different effects on the activity, but there is an optimal temperature.On both sides of the optimum temperature,
Figure 4
reaction rate All quite low.Figure 4: The influence of temperature on the enzymatic reaction includes two aspects: on the one hand, when the temperature rises, the reaction speed also accelerates, which is the same as the general chemical reaction.On the other hand, the enzyme gradually denatures with the increase of temperature, that is, the reaction rate of the enzyme is reduced by reducing the active enzyme.BelowOptimum temperatureWhen the temperature is higher than the optimum temperature, the former effect is the main effect, so the enzyme activity is lost,reaction rate Down.
In preparationculture mediumIn the process of sterilization, high temperature can be used to sterilize the culture medium, which mainly destroys the activity of enzymes in microorganisms.High temperature sterilization is widely used in medicine and life practice.
At low temperature, the activity of the enzyme decreases, but the structure of the enzyme molecule will generally change.Therefore, people can choose to preserve the enzyme at low temperature.In daily life, people often choose to store food at low temperature for a long time.
Action of inhibitors
By changing the chemical properties of the essential groups of the enzymeenzyme activityThe reduced or lost effect is called inhibition, and the substances with inhibition are called inhibitors. Inhibitors are usually small molecular compounds, but there are also biological macromolecule type inhibitors in the organism.
Classification of enzyme inhibitors
EnzymaticinhibitorDivided intoIrreversible inhibitorAnd reversible inhibitors.Irreversible inhibitor and essential group of enzymecovalent bondIt will cause permanent inactivation of the enzyme, and its inhibition cannot be removed by dialysis, ultrafiltration isothermal and physical means.Reversible inhibitors bind to enzyme proteins by non covalent bonds, causing temporary loss of enzyme activity. Their inhibition can be relieved by dialysis, ultrafiltration and other means.Reversible inhibitors are divided intoCompetitive inhibitor、Non competitive inhibitorandAnticompetitive inhibitorEtc.
Effect of activator
The activity of enzyme can be improved by some substances, which are called activators. Adding activators in enzymatic reaction can lead toreaction rateIncrease.Generally, the enzyme has certain selectivity to the activator and has certain concentration requirements. The activator of one enzyme may be an inhibitor to another enzyme. When the concentration of the activator exceeds a certain range, it becomes an inhibitor[1]。
