Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Enzymes
Basic Properties of Enzymes
Effect of Substrate and Enzyme Concentrations on the Rate of Enzymatic Reaction

An important Conclusion follows from the preceding material: one of the most critical Factors Determining the rate of an enzymatic reaction is the concentration of the substrate (or substrates) and the product (or products). At a constant Enzyme Concentration, the reaction rate progressively increases until it reaches a certain maximum (see Figs. 4.12, 4.13), beyond which any further increase in Substrate Concentration has virtually no effect on the reaction rate. In such cases, it is generally considered that the substrate is in excess and the enzyme is fully saturated, meaning that all enzyme molecules are bound to the substrate. Under these conditions, the enzyme concentration becomes the rate-limiting factor. It is precisely under these conditions that the maximum velocity (V_max) and the Michaelis constant (K_m) are determined (see Figs. 4.13; 4.14).

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Fig. 4.19. Dependence of the reaction rate on enzyme concentration in the presence of saturating substrate concentrations.

The rate of any enzymatic reaction directly depends on the enzyme concentration (Fig. 4.19). The linear relationship between these variables—where the reaction rate is directly proportional to The amount of enzyme present—holds true only under specific conditions, such as during the initial period of the enzymatic reaction. This is because the reverse reaction is practically negligible during this phase, and the product concentration is insufficient to drive the reaction backward. It is precisely under these conditions that the reaction rate (more precisely, the initial reaction rate v) is proportional to the enzyme concentration. As noted earlier, the enzyme acts as one of the reacting molecules in a chemical reaction; upon interacting with the substrate, it forms an intermediate enzyme-substrate complex, which subsequently breaks down into the product and the free enzyme:

If this equation is simplified by omitting the intermediate ES-complex:

then the enzyme concentration becomes an essential component in the equations for the rates of the forward and reverse reactions:

However, the enzyme concentration no longer appears in the equations for the Equilibrium Constant (K_eq or К_р):

As can be seen, the equilibrium constant (К_р) of an enzymatic reaction is independent of the enzyme concentration. While determining the rate and direction of a chemical reaction, the enzyme nevertheless has no effect on the final (equilibrium) concentrations of the reacting molecules and products that determine the value of the equilibrium constant.