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Rate of Reaction

For a reaction of stoichiometry

E + A E + Z

at a given time the rate of formation of Z is equal to the rate of consumption of A, and can be called the rate of reaction, v:

........ (3)

A similar set of relationships applies when there are several reactants in one-to-one stoichiometry. Complications arise if a one-to-one stoichiometry between all substrates and all products does not exist. This can arise in two ways:

a) The overall stoichiometry may be, for example,

A + 3B 2Z.

The numbers -1, -3 and 2 are the stoichiometric coefficients of A, B and Z respectively in this reaction; by convention they are positive for products and negative for reactants. For a reaction of this kind the rate of reaction is the rate of consumption or formation of any reactant divided by the appropriate stoichiometric coefficient, i.e. (in this example):

........ (4)

This situation rarely arises in enzyme kinetics.

b) Intermediates may be formed in amounts comparable with those of reactants, for example

E + A + B L E + Z + Y.

This gives rise to time-dependent stoichiometry and there is not a one-to-one relationship between the amounts of reactants. It is not then permissible to speak of rate of reaction for the complete reaction, though it may be meaningful to define rates of reaction for the individual steps. Time-dependent stoichiometry occurs in the pre-steady-state phase of enzyme-catalysed reactions (Section 9), during which the amount of intermediate formed is comparable with the amounts of product.

The rate of reaction as defined here is an intensive quantity with dimensions of concentration divided by time. It conforms with almost universal usage in kinetics and with the current (1980) recommendations of IUPAC [3], but differs from earlier IUPAC recommendations, in which the term rate of reaction referred to the corresponding extensive quantity with dimensions of amount of substance divided by time. There is little evidence that this latter recommendation has ever been widely adopted and in the context of enzyme kinetics it is recommended that the term rate of reaction be used exclusively for the intensive quantity as defined above. If the extensive quantity is required it may be termed the rate of conversion and symbolized [3, 5].

Note An intensive quantity is one like temperature or density that does not depend on the amount of material being considered and may be contrasted with an extensive quantity, such as heat or mass which does. (See also Addendum 1.)

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