ABSTRACT
Quantitative analysis of the kinetics of enzyme catalysis in aqueous solution is well developed. Kinetic analysis allows one to formulate a scheme for the reaction, including the formation of intermediates, and, from experimental observation, to assign values of the rate of each step in the process. In addition, from a temperature dependence of the rate, the activation energy of each step can be determined. Kinetics alone cannot determine a reaction path; it can only find the simplest mode that is consistent with the observations. Of course, one of the classic applications of kinetics to enzyme-catalyzed reactions led to the development of the Michaelis-Menten theory, which provided a simple scheme to explain the phenomenon of saturation kinetics and the observation that beyond a certain substrate concentration, the rate of reaction no longer increased. The Michaelis-Menten scheme is
There are some enzyme-catalyzed reactions in aqueous solution that cannot be well described by Michaelis-Menten kinetics because of additional complications such as cooperativity, pre-steady-state phenomena, and reversibility of the reaction, among other reasons. Nevertheless, this simple kinetic scheme still proves extremely valuable in the study of enzyme properties and mechanisms. Analysis of the dependence of the reaction rate on substrate concentration allows one
Enzyme Substrate Enzyme Substrate Complex Enzyme k
k k + ´ ◊ Æ +
to calculate two kinetic constants: V
and K
. Determination of these kinetic constants is useful because these constants have mechanistic implications. V
is the maximal rate of reaction, which is also the rate of conversion of the intermediate enzyme-substrate complex to products. K
is a kinetic constant equal to (k
+ k
)/k
. The significance of K
is that it equals the equilibrium constant for the dissociation of the enzyme-substrate complex when k
>> k
. In addition, the ratio of V
/K
gives the rate at low substrate concentrations and is used as a measure of catalytic efficiency. This ratio is free from complications brought about by the formation of additional intermediates in the path that can affect V
and K
.
