To understant the relationship between substrate concentration, product concentration and enzyme concentration consider the following situation.

Keq is a small number

• You have a reaction in which you have 1.0 M of substrate and 0.01 M enzyme .

• Let's suppose that most of the enzyme forms a complex with substrate so that at equilibrium 90% of the enzyme molecules have bound substrate and 10% enzyme molecule is free. This gives

(90% of 0.01M) is

0.9 X 0.01 = .009 M (enz-sub complex)

.001 M free enzyme and .009 M enz-sub complex

Keq = (.001)(.999)/.099 = .001M

At equilibrium, if most of the enzyme is in complex with substrate the the Keq is a reatively small number.

Keq is a large number

• If, on the other hand very little of the enzyme is bound to substrate so that 10% of the enzyme was bound to substrate and 90% was free the equation would be

10% of 0.01M is

0.1 X 0.01 = .001 (enz-sub complex)

Keq = (.009)(.999)/.001 = .8991M

Very little of the enzyme is in complex with substrate and the Keq is a relatively large number.

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Thus one can extrapolate and conclude, under very simplifying conditions

• If the Keq is a small number then the enzyme has a high affinity for substrate.

• If the Keq is a large number then the enzyme has a relatively low affinity for substrate.

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