The factors that affect enzyme catalysis (that is, enzyme activity) will definitely affect the rate of enzymatic reactions, but the factors that affect the rate of enzymatic reactions do not necessarily affect the catalysis. pH, temperature, ultraviolet light, heavy metal salts, inhibitors, activators, etc. will affect the catalysis and the rate of enzymatic reactions. The concentration of the enzyme, the concentration of the substrate, etc. will not affect the enzyme catalysis, but can affect the rate of the enzymatic reaction. Factors affecting enzyme catalysis are:

1. Effect of temperature on enzyme activity

Temperature has a great influence on the speed of enzymatic reaction, and it has a dual effect:

• As with non-enzymatic chemical reactions, when the temperature rises, the number of activated molecules increases, and the enzymatic reaction speeds up. For many enzymes, the temperature coefficient Q10 is mostly 1 to 2, which means that every time the reaction temperature is increased by 10°C, the enzyme reaction speed increases by 1 to 2 times.

• Since the enzyme is a protein, the enzyme gradually denatures as the temperature rises, that is, the reaction speed of the enzyme is reduced by the decrease of the enzyme activity. Use temperature (T) as the abscissa and enzymatic reaction speed (V) as the ordinate to draw the graph, and the resulting curve is a slightly inclined bell jar shape.

The temperature corresponding to the peak of the curve is called the optimum temperature. The optimum temperature is the result of the double influence of the above temperature on the enzyme reaction. When the temperature is lower than the optimum temperature, the former effect is dominant; when the temperature is higher than the optimum temperature, the latter effect is dominant, so the enzyme activity is quickly lost and the reaction speed drops quickly.

1. Effect of pH on enzyme activity

The reason why pH affects the speed of enzymatic reaction:

• Over-acid or over-alkali environments will affect the conformation of the enzyme protein and denature and inactivate the enzyme itself.

• pH affects the dissociation of the polar groups on the side chain of the enzyme molecule, changing their charged state, and changing the structure of the active center of the enzyme. At the optimum pH, the dissociation state of the related groups on the active center of the enzyme molecule is most suitable for binding to the substrate. When the pH is higher or lower than the optimum pH, the dissociation state of the related groups on the active center changes, and the binding force between the enzyme and the substrate decreases, and the enzyme reaction speed decreases.

• pH can affect the dissociation of substrate molecules. It is conceivable that certain groups on the substrate molecule are only suitable for binding and reacting with the enzyme under a certain dissociation state. If the change of pH affects the dissociation of these groups, making them unsuitable for binding with enzymes, the reaction speed will also slow down.

1. Enzyme concentration

In the case of sufficient substrate and not affected by other factors, the enzymatic reaction rate is directly proportional to the enzyme concentration. When the concentration of substrate molecules is sufficient, the more enzyme molecules, the faster the substrate conversion rate. But in fact, when the enzyme concentration is high, this relationship is not maintained, and the curve gradually tends to be flat. According to analysis, this may be caused by the high concentration of substrate entrained with many inhibitors.

When the temperature and pH of the enzymatic reaction system remain unchanged, and the substrate concentration is large enough to saturate the enzyme, the reaction speed is proportional to the enzyme concentration. Because in the enzymatic reaction, the enzyme molecule first interacts with the substrate molecule to generate an activated intermediate product (or activation complex), which is then transformed into the final product. In the case of sufficient substrate excess, it is conceivable that the greater the number of enzymes, the more intermediate products will be produced and the faster the reaction rate will be. On the contrary, if the substrate is insufficient in the reaction system and the enzyme molecules are excessive, the existing enzyme molecules have not yet played a role, and the number of intermediate products is less than the number of free enzyme molecules. In this case, increasing the enzyme concentration will not increase the speed of the enzymatic reaction.

1. Substrate concentration

In the biochemical reaction, if the concentration of the enzyme is a constant value and the initial concentration of the substrate is low, the enzymatic reaction speed is proportional to the substrate concentration, that is, it increases with the increase of the substrate concentration. When all the enzymes are combined with the substrate to form an intermediate product, even if the substrate concentration is increased, the intermediate product concentration will not increase, and the enzymatic reaction speed will not increase.

It can also be concluded that under the same conditions of the substrate concentration, the enzymatic reaction rate is directly proportional to the initial concentration of the enzyme. The greater the initial concentration of the enzyme, the greater the enzymatic reaction rate.