Question

Describe the kinetic differences between an uncatalyzed second order chemical reaction and an enzyme catalyzed reaction.

Answer 1

For enzyme-catalyzed reactions, the velocity of product formation can be described by the equation: v = k[ES]. The reaction is therefore first-order in relation to the concentration of ES complex. However, the reaction is not first-order relative to the directly measurable substrate concentration. Instead, under steady state conditions where [ES] is effectively constant, the velocity of an enzyme-catalyzed reaction is a hyperbolic function of [S]: ! v = Vmax [S] K m + [S] The Michaelis-Menten equation has two parameters, Vmax and Km.

Vmax = kcat[E]total, and therefore is a function of both the total enzyme concentration and of the catalytic rate constant of the enzyme for the reaction; kcat is an intrinsic property of an enzyme, while Vmax is not.

second order More complicated reactions can also occur: S + R → P + Q For these reactions: ! v = d[P] dt = d[Q] dt = –d[S] dt = –d[R] dt and v = k[S][R] Reactions of this type are second-order, and k is a second-order rate constant, because the rate of the reaction depends on the product of [S] and [R]. If the reaction involved the collision of two molecules of S, the velocity equation would be: v = k[S][S] = k[S]2 The order of the reaction comes from the exponent that describes the number of reactants.

Second-order chemical reaction

More complicated reactions can also occur: S + R → P + Q For these reactions: ! v = d[P] dt = d[Q] dt = –d[S] dt = –d[R] dt and v = k[S][R] Reactions of this type are second-order, and k is a second-order rate constant, because the rate of the reaction depends on the product of [S] and [R]. If the reaction involved the collision of two molecules of S, the velocity equation would be: v = k[S][S] = k[S]2 The order of the reaction comes from the exponent that describes the number of reactants. Second-order rate constants have units of M-1•sec-1. Real chemical reactions rarely have more than two molecules interacting at one time, because the simultaneous collision of more than two molecules is unlikely.