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Elementary and Composite Reactions

An elementary reaction is one in which no reaction intermediates have been detected or need be postulated to describe the chemical reaction on a molecular scale. Such reactions are said to occur in a single step.

The term molecularity, which applies only to an elementary reaction, refers to the number of molecular particles involved in the microscopic chemical event. With reactions in solution, solvent molecules are counted in the molecularity if they enter into the overall process, but not if they merely exert an environmental or solvent effect. For example. the formation of an enzyme-substrate complex in aqueous solution.

E + A EA

has a molecularity of two and is said to be bimolecular. The reverse process in solution,

EA E + A

has a molecularity of unity and is said to be unimolecular. The hydrolysis of an acyl-enzyme, in which water enters into the process,

R-CO-E + H₂O E-H + R-COOH

is bimolecular.

Virtually all enzyme-catalysed overall reactions occur in more than one elementary step and are described as composite reactions. The terms complex reaction may also be used with the same meaning. Simple examples in enzyme kinetics are as follows:

E + A EA EZ E + Z

E + A + B EA + B EAB EYZ EY + Z E + Y + Z

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