As biocatalysts, enzymes and general catalysts are the same in many respects. For example, the amount is small but the catalytic efficiency is high. Like general catalysts, enzymes can only change the speed of chemical reactions, not the equilibrium point of chemical reactions. The enzyme itself does not change before and after the reaction, so the relatively low content of the enzyme in the cell can catalyze the change of a large number of substrates in a short time, reflecting the high efficiency of enzyme catalysis. Enzymes can reduce the activation energy of the reaction, but do not change the free energy change (△G) during the reaction, thus speeding up the reaction and shortening the time for the reaction to reach equilibrium, but it does not change the equilibrium constant.
Compared with general catalysts, the catalysis of enzymes shows unique characteristics.
(1) High efficiency of enzyme catalysis
The catalytic activity of enzymes is much higher than that of chemical catalysts. For example, catalase (containing Fe2+) and inorganic iron ions both catalyze the decomposition reaction of hydrogen peroxide as follows. 1 mol of catalase can catalyze the decomposition of 5×106 mol of H2O2 in 1 minute. Under the same conditions, 1 mol of the chemical catalyst Fe2+ can only catalyze the decomposition of 6×10-4 mol of H2O2. Compared with the two, the catalytic efficiency of catalase is about 1010 times that of Fe2+.
The level of enzyme catalytic efficiency can be expressed by the concept of turnover number. The conversion number refers to the number of molecules per enzyme molecule that can convert the substrate per minute when the substrate concentration is large enough, that is, the number of molecules that catalyze the chemical change of the substrate. According to the data introduced above, the conversion number of catalase can be calculated as 5×106. The conversion number of most enzymes is around 1,000, and the largest can reach over 106.
(2) High specificity of enzyme catalysis
An enzyme can only act on a certain type or a specific substance. This is the specificity of enzyme action. For example, glycosidic bonds, ester bonds, peptide bonds, etc. can be hydrolyzed by acid-base catalysis, but the enzymes that hydrolyze these chemical bonds are different. They are the corresponding glycosidases, esterases, and peptidases, that is, they are individually specific. It can be hydrolyzed by natural enzymes.
(3) Mild reaction conditions catalyzed by enzymes
Enzymatic reactions generally require mild conditions such as normal temperature, normal pressure, and neutral pH. Because enzymes are proteins, they are prone to lose their activity in environments such as high temperature, strong acid, and alkali. Since enzymes are more sensitive to changes in the external environment and are easily denatured and inactivated, the reaction conditions must be strictly controlled during application.
(4) Adjustability of enzyme activity
Compared with chemical catalysts, another feature of enzyme catalysis is that its catalytic activity can be automatically regulated. Although there are many kinds of chemical reactions in organisms, they are very coordinated and orderly. Changes in substrate concentration, product concentration and environmental conditions may affect enzyme catalytic activity, thereby controlling the coordinated and orderly progress of biochemical reactions. Any disorder and imbalance of the biochemical reaction will inevitably cause the organism to produce disease or even death in severe cases. In order to adapt to changes in the environment and maintain normal life activities, organisms have formed a system that automatically regulates enzyme activities during the long evolutionary process. There are many ways to regulate enzymes, including inhibitor regulation, feedback regulation, covalent modification regulation, zymogen activation, and hormone control.
(5) Enzyme catalytic activity is related to coenzyme, prosthetic group and metal ion
Some enzymes are complex proteins, and the small molecules of coenzymes, cofactor and metal ions are closely related to the catalytic activity of the enzyme. If they are removed, the enzyme loses its activity.
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