Question

What assumption concerning substrate concentration is made in the discussion of enzyme kinetics? Why is this important?

Answer 1

Enzyme kinetics are generally studied by making certain assumptions regarding the concentration levels of substrate. We know that in an enzyme substrate system, the reaction taking place is described as below:

E + S ---> [ES] ---> E + P

Here, [ES] is the intermediate complex state.

One of the main assumption is termed as the steady state assumption, which states that:

d[ES]/dt = 0

This means that we assume that conc. of ES does not change with time. This means that ES rapidly reaches a constant value when the reaction starts. After that it remains appreciably constant until a fair amount of S is consumed.

This is important because we usually measure initial velocities of the reaction, and seldom do we concern ourselves with reaction velocities during later time intervals. In order to be able to make such initial velocity measurements, we definitely have to go with this assumtion that [ES] remains constant. This signifies that product is formed very less, so that the backward reaction from E + P ---> [ES] does not occur.

This is also the reason that we assume that substrate concentration [S] is much greater than enzyme concentration [E]. This ensures that all enzyme is either present as free state or as [ES], and none of it reacts with P in the initial stages.