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The observation is explained in terms ofenzyme-catalysed quantum tunnelling.

Under this mechanism, rather than overcoming the potential energy barrier, the

reaction proceeds through the barrier (hence ‘tunnelling’) at an energy level near that

of the ground state of the reactants. Concerted enzyme and substrate vibrations are

coupled in such a way as to reduce the width and height of the potential energy barrier

and facilitate the cleavage of the C–H bond by the process of quantum mechanical

tunnelling. This phenomenon is known to occur with some chemical reactions but

only at low temperatures. The fine detail of precisely how enzymes promote this

process remains to be elucidated.

Effect of pH

The state of ionisation of amino acid residues in the catalytic site of an enzyme is

pH dependent. Since catalytic activity relies on a specific state of ionisation of these

residues, enzyme activity is also pH dependent. As a consequence, plots of logKmand

logVmax(or better,kcat) against pH are either bell-shaped (indicating two important

ionisable amino acid residues in the active site), giving a narrowpH optimum, or they

have a plateau (one important ionisable amino acid residue in the active site). In either

case, the enzyme is generally studied at a pH at which its activity is maximal.

By studying the variation of logKmand logVmaxwith pH, it is possible to identify

the pKavalues of key amino acid residues involved in the binding and catalytic

processes (Fig. 15.8).

5.5 6.5 7.5 8.5

pH

log

Vmax

pKa^1 pKa^2

Fig. 15.8The effect of pH onVmaxof an enzyme-catalysed reaction involving two ionisable groups in the

active site of the enzyme. The construction of tangents to the experimental line allows the pKavalues of the

ionisable groups to be estimated.

598 Enzymes