2009_Book_FoodChemistry

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 microorganisms
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 an α-keto acid (pyruvic acid or α-keto butyric
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 decarboxylation
is initiated by the formation of a Schiff
base by the reaction mechanism in Fig. 2.20.
2.4.3 Closing Remarks
The hypotheses discussed here allow some understanding
of the fundamentals involved in the
action of enzymes. However, the knowledge is
far from the point where the individual or combined
effects which regulate the rates of enzymecatalyzed
reactions can be calculated.
Fig. 2.20. A proposed mechanism for the reaction of
histidine decarboxylase
analyze the parameters which influence or determine
the rate of an enzyme-catalyzed reaction.
The reaction rate is dependent on the concentrations
of the components involved in the reaction.
Here we mean primarily the substrate and the enzyme.
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 Kinetics of Enzyme-Catalyzed
Reactions
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
2.5.1 Effect of Substrate Concentration
2.5.1.1 Single-Substrate Reactions
2.5.1.1.1 Michaelis–Menten Equation
Let us consider a single-substrate reaction. Enzyme
E reacts with substrate A to form an in-