Question

A friend of yours (Stan) presents you with steady-state kinetic data for a particular enzyme. He keeps referring to K_M as the dissociation constant for the enzyme-substrate pair. Your other friend (Wendy) says that Stan should not refer to K_M as a dissociation constant. An argument ensues.

a. Is Wendy correct? Explain why or why not.

b. Under what circumstances would Wendy's point not matter?

Answer 1

The Michaelis-Menten Kinetics for Enzyme - Substrate complex is given by-

....1

a. Wendy is correct in mentioning the Michaelis constant a dissociation constant as is expressed as

..........2

and from the expression we can see that depends on different rate constants and on the slowest rate limiting step.If we assume that the step of the Enzyme Substrate complex breakdown is the Rate Limiting step of the reaction with the rate equation 2 will take the form

if   ,then

.....3

The equation 3 is also called the dissociation constant of the ES complex and it is the relative measure of affinity of a substrate for an enzyme.

b. We assumed the rate of the ES complex breakdown of the equation 1 to be the rate determining step.If this assumption is not valid,in equation 2,the condition now would be-

or ,Wendy's point will not matter.