The rate of many chemical reactions increases with the increase of the concentration of substrates. For a single substrate irreversible enzyme reaction, when the substrate concentration increases, the rate of enzyme reaction increases and approaches a maximum value.

Michaelis–Menten Equation

Based on the saturation of substrate to enzymatic reaction, the theory of “intermediate product” is put forward by German biochemist Leonor Michaelis and Canadian physician Maud Menten: the enzyme (E) binds to the substrate (S) to form an unstable intermediate product or complex (ES), and then produces the product (P) and free enzyme (E).

The relationship between reaction rate and substrate concentration was derived by the theory, that is, Michaelis–Menten equation:
ν = d[P]/dt = V_max[S]/(Km+[S])

It is an equation to study the basic kinetics of various factors affecting the rate of enzymatic reaction. In the equation, v indicates the reaction rate and the activity of the enzyme. [S] represents substrate concentration, V_max indicates maximum reaction rate, and Km is Michaelis constant, Km= (k_-1+k₂) / k₁. The value of Michaelis constant is equal to the substrate concentration when the enzyme reaction rate is half of the maximum reaction rate. Km is a characteristic constant of the enzyme. When the rate constant k_-1 is much larger than that of k₂, Km is close to the dissociation constant of complex (ES). Therefore, Km represents the affinity of the enzyme to the substrate. A high Km value indicates a weak affinity between E and S, and a low km value indicates a strong affinity.

Saturation of Substrate to Enzymatic Reaction

Figure 1. Michaelis–Menten saturation curve for an enzyme reaction showing the relation between the substrate concentration and reaction rate.

When enzyme concentration is constant, the relationship between different substrate concentration and reaction rate is a rectangular hyperbolic. When the substrate concentration is low, the increase of reaction rate is proportional to the increase of substrate concentration. After that, with the increase of substrate concentration, the increase of reaction rate decreased gradually. Finally, when the substrate concentration increased to a certain amount, the reaction rate reached a maximum, and no longer increased with the increase of substrate concentration.

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