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PH EFFECTS

The rates of enzyme-catalysed reactions vary with pH and often pass through a maximum as the pH is varied. If the enzyme obeys Michaelis-Menten kinetics the kinetic parameters k ₀ and k _A often behave similarly. The pH at which the rate or a suitable parameter is a maximum is called the pH optimum and the plot of rate or parameter against pH is called a pH profile. Neither the pH optimum nor the pH profile of an enzyme has any absolute significance and both may vary according to which parameter is plotted and according to the conditions of the measurements.

If the pH is changed and then brought back to its original value, the behaviour is said to be reversible if the original properties of the enzyme are restored; otherwise it is irreversible. Reversible pH behaviour may occur over a narrow range of pH, but effects of large changes in pH are in most cases irreversible.

The diminution in rate as the pH is taken to the acid side of the optimum can be regarded as inhibition by hydrogen ions. The diminution in rate on the alkaline side can be regarded as inhibition by hydroxide ions. The equations describing pH effects are therefore analogous to inhibition equations. For single-substrate reactions the pH behaviour of the parameters k ₀ and k _A can sometimes be represented by an equation of the form

........ (24)

in which k represents k ₀ or k _A, and is the value of the same parameter that would be observed if the enzyme existed entirely in the optimal state of protonation; it may be called the pH-independent value of the parameter. The constants K ₁ and K ₂ can sometimes be identified as acid dissociation constants for the enzyme. substrates or other species in the reaction mixture. The identification is, however, never straight forward and has to be justified by independent evidence. The behaviour is frequently much more complicated than represented by Eqn (24).

It is not accidental that this section has referred exclusively to pH dependences of k ₀ and k _A. The pH dependence of the initial rate or, worse, the extent of reaction after a given time is rarely meaningful; the pH dependence of the Michaelis constant is often too complex to be readily interpretable.

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