Food Chemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 117

the first product, glyceraldehyde-3-phosphate,
is released. An enamine rearrangement into
a ketimine structure is followed by release of
dihydroxyacetone phosphate.
This is the mechanism of catalysis by aldolases
which occur in plant and animal tissues (lysine
aldolases or class I aldolases). A second group
of these enzymes often produced by microorgan-
isms contains a metal ion (metallo-aldolases).
This group is involved in accelerating retroaldol
condensations through electrophilic reactions
with carbonyl groups:

(2.29)

Other examples of electrophilic metal catalysis
are given under section 2.3.3.1. Electrophilic
reactions are also carried out by enzymes which
have anα-keto acid (pyruvic acid orα-keto bu-
tyric acid) at the transforming locus of the active
site. One example of such an enzyme is histidine
decarboxylase in which the N-terminal amino
acid residue is bound to pyruvate. Histidine decar-
boxylation is initiated by the formation of aSchiff
base by the reaction mechanism in Fig. 2.20.

2.4.3 ClosingRemarks........................................

The hypotheses discussed here allow some un-
derstanding of the fundamentals involved in the
action of enzymes. However, the knowledge is
far from the point where the individual or com-
bined effects which regulate the rates of enzyme-
catalyzed reactions can be calculated.

2.5 KineticsofEnzyme-CatalyzedReactions

Enzymes in food can be detected only indirectly
by measuring their catalytic activity and, in this
way, differentiated from other enzymes. This is
the rationale for acquiring knowledge needed to

Fig. 2.20.A proposed mechanism for the reaction of

histidine decarboxylase

analyze the parameters which influence or deter-

mine the rate of an enzyme-catalyzed reaction.

The reaction rate is dependent on the concentra-

tions of the components involved in the reaction.

Here we mean primarily the substrate and the en-

zyme. Also, the reaction can be influenced by the

presence of activators and inhibitors. Finally, the

pH, the ionic strength of the reaction medium, the

dielectric constant of the solvent (usually water)

and the temperature exert an effect.

2.5.1 EffectofSubstrateConcentration

2.5.1.1 Single-SubstrateReactions

2.5.1.1.1 Michaelis–MentenEquation

Let us consider a single-substrate reaction. En-

zyme E reacts with substrate A to form an in-