The study of the rates of chemical reactions is called kinetics, and the study of the rates of enzyme-catalyzed reactions is called enzyme kinetics. We begin by briefly examining some of the basic principles of reaction kinetics.

What do we mean when we say the “velocity” or “rate” of a chemical reaction? Consider a simple reaction:

The velocity of the reaction, V (for velocity), is the quantity of reactant A that disappears in a specified unit of time t. It is equal to the velocity of the appearance of product P, or the quantity of P that appears in a specified unit of time:

where d is the decrease in substrate concentration or the increase in product concentration. If A is yellow and P is colorless, we can follow the decrease in the concentration of A by measuring the decrease in the intensity of yellow color with time. Consider only the change in the concentration of A for now. The velocity of the reaction is directly related to the concentration of A by a proportionality constant k, called the rate constant:

Reactions in which the velocity is directly proportional to the reactant concentration are called first-order reactions. First-order rate constants have the unit of s⁻¹ (per second).

Many important biochemical reactions are biomolecular; that is, they include two reactants. Such reactions are called second-order reactions. For example,

or

The corresponding rate equations often take the form

and

The rate constants, called second-order rate constants, have the units M⁻¹ s⁻¹ (per mole per second).

Sometimes, second-order reactions can appear to be first-order reactions. For instance, in reaction 4, if the concentration of B greatly exceeds that of A and if A is present at low concentrations, the reaction rate will be first order with respect to A and will not appear to depend on the concentration of B. These reactions are called pseudo-first-order reactions, and we will see them a number of times in our study of biochemistry.

Interestingly enough, under some conditions, a reaction can be zero order. In these cases, the rate is independent of reactant concentrations. As we will see shortly, enzyme-catalyzed reactions can approximate zero-order reactions under some circumstances.

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