2009_Book_FoodChemistry

2.4 Theory of Enzyme Catalysis 113
Fig. 2.14. Influence of the steric effect on deacylation of two acyl-chymotrypsins (according to Bender
et al., 1964). a N-acetyl-L-tyrosyl-chymotrypsin, b acetyl-chymotrypsin
In case a) deacylation is faster by a factor of 3540
since the carbonyl group is immobilized by insertion
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 nucleophilic
OH ⊖ ion 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 difference
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 entropy
of the transition state, S # ; a fact that as
mentioned before can lead to a considerable increase
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 General Acid–Base Catalysis
When the reaction rate is affected by the concentration
of hydronium (H 3 O ⊕ )orOH ⊖ ions from
water, the reaction is considered to be specifically
Table 2.7. Thermodynamic data for transition states of
twoacyl-chymotrypsins
Acyl- ΔG # ΔH # ΔS #
enzyme (kJ · mol −1 )(kJ· mol −1 )(J· K −1· mol −1 )
N-Acetyl-
L-tyrosyl 59.6 43.0 −55.9
Acetyl 85.1 40.5 −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 proton
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-