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PRE-STEADY-STATE KINETICS

The pre-steady-state or transient phase in enzyme-catalysed reactions occupies very short periods (usually fractions of a second) and very low product concentrations. Special techniques therefore have to be used. For reactions that are not too fast the stopped-flow technique, in which the enzyme and reactants are rapidly mixed and the flow stopped. is commonly used.

For reactions that have to be studied over periods of less than 1 ms relaxation techniques are used. In these techniques the system is disturbed, usually but not necessarily from a state of equilibrium, after which it relaxes to equilibrium or a new steady state. In the temperature-jump (T jump) technique the temperature is increased rapidly and the system relaxes to a new state of equilibrium or a new steady state at the final temperature. In the pressure-jump technique the pressure is rapidly changed.

The relaxation time of a reaction is the time it takes for the extent of reaction to change by a proportion (1 - e^-1) of the total change during the relaxation process (e = 2.71828... is the base of natural logarithms). For composite mechanisms, such as those that occur with enzyme-catalysed reactions, relaxation experiments usually reveal more than one relaxation time. These relaxation times can be related to the rate constants of the elementary steps in the mechanism, but the relationships are usually complicated.

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