Food Chemistry

2.4 Theory of Enzyme Catalysis 113

Fig. 2.14.Influence of the steric effect on deacylation of two acyl-chymotrypsins (according to Ben-
deret al., 1964).aN-acetyl-L-tyrosyl-chymotrypsin,bacetyl-chymotrypsin

In case a) deacylation is faster by a factor of 3540
since the carbonyl group is immobilized by in-
sertion of the bulky N-acetyl-L-tyrosyl group into
a hydrophobic pocket on the enzyme (Fig. 2.14a)
at the correct distance from the attacking nucleo-
philic OHion derived from water (cf. 2.4.2.5).
In case b) the immobilization of the small acetyl
group is not possible (Fig. 2.14b) so that the dif-
ference between the ground and transition states
is very large. The closer the ground state is to the
transition state, the more positive will be the en-
tropy of the transition state,S#; a fact that as
mentioned before can lead to a considerable in-
crease in reaction rate. The thermodynamic data
in Table 2.7 show that the difference in reaction
rates depends, above all, on an entropy effect; the
enthalpies of the transition states scarcely differ.

2.4.2.4 GeneralAcid–BaseCatalysis..............................

When the reaction rate is affected by the concen-
tration of hydronium (H 3 O⊕)orOHions from
water, the reaction is considered to be specifically

Table 2.7.Thermodynamic data fortransition states of

twoacyl-chymotrypsins

Acyl- ΔG# ΔH# ΔS#

enzyme (kJ·mol−^1 )(kJ·mol−^1 )(J·K−^1 ·mol−^1 )

N-Acetyl-

L-tyrosyl 59. 643. 0 − 55. 9

Acetyl 85. 140. 5 − 149. 7

acid or base catalyzed. In the so-called general

acid or base catalysis the reaction rate is affected

by prototropic groups located on the side chains

of the amino acid residues. These groups involve

proton donors (denoted as general acids) and pro-

ton acceptors (general bases). Most of the amino

acids located on the active site of the enzyme

influence the reaction rate by general acid-base

catalysis.

As already mentioned, the amino acid residues in

enzymes have prototropic groups which have the

potential to act as a general acid or as a general

base. Of these, the imidazole ring of histidine is

of special interest since it can perform both func-