Question

In Michaelis-Menten kinetics, what values would make an enzyme “good” at catalysis?

Answer 1

Let us consider an enzyme substrate reaction given below:

.............Eqn 1

Say rate constant for forward reaction is K_f and for backward step is K_r and rate constant for formation of the product is K_cat

Following the Michaelis-Menten kinetics the rate of the reaction will be given by the following equation:

.............Eqn 2

Where v is the rate of the reaction, v_max is the maximum rate of the reaction, [S] is the concentration of the substrate, [E]₀ is the initial enzyme concentration, K_M is the Michealis-Menten constant which is given by the following equation:

.............Eqn 3

So the values that makes the enzyme "good" at catalysis is:

1. K_M: if the value of K_M is lower then the rate of the reaction will be higher (See Eqn 2) which indicates high affinity of the substrate towards the enzyme which makes it a good catalyst. Actually K_M is the substrate concentration at which the rate of the reaction is half of its maximum velocity (Consider K_M=[S] and put it in Eqn 2). So lower K_M value means at lower concentration of the substrate the reaction reaches high velocity. Now K_M is independent the concentration or purity of an enzyme but dependent on both the enzyme and the substrate, as well as conditions such as temperature and pH.

2. K_cat: Higher K_cat means higher rate of product formation or high ternover number means higher number of substrate molecules converted to product per enzyme molecule per second.

3. Catalytic efficiency (K_cat/K_M): this is a more sofisticated term as K_cat and K_M both are interrelated (see Eqn 3). This is a measure of how efficient an enzyme is in converting a substrate into product.

4. If K_f is higher than the K_r then the reaction is favoured in the forward direction which facilates the formation of enzyme-substrate complex (ES).