Question

Km is sometimes described as the dissociation constant of the ES complex. Is this description accurate, and if so when? Justify your answer with expressions of Km and KD under specific situations.

Answer 1

The Michaelis-Menten model is the one of the simplest and best-known approaches to enzyme kinetics. An enzyme E combines with substrate S to form an ES complex, with a rate constant k₁. The ES complex has two possible fates. It can dissociate to E and S, with a rate constant k_-1, or it can proceed to form product P, with a rate constant k₂. Again, we assume that almost none of the product reverts to the initial substrate, a condition that holds in the initial stage of a reaction before the concentration of product is appreciable. This can be represented as

E+S ES E+P

The Michaelis-Menten equation for this system is:

v = V_max [S] / K_M + [S]

Here, Vmax represents the maximum velocity achieved by the system, at maximum (saturating) substrate concentrations. K_M (the Michaelis constant; sometimes represented as KS instead) is the substrate concentration at which the reaction velocity is 50% of the Vmax. [S] is the concentration of the substrate S.

K_M is known as the Michaelis constant with a value typically in the range 10^-1 - 10^-5 M. When the conditions met, k₂<<k_-1, K_M equals the dissociation constant of the ES complex. A high K_M indicates weak binding; a low K_M indicates strong binding. And K_M indicates the affinity of the ES complex only when k_-1 is much greater than k2.