2009_Book_FoodChemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 121
ant in enzymatic food analysis (cf. 2.6.4) and for
assessment of enzymatic reactions occurring in
food (e. g. enzymatic browning of sliced potatoes,
cf. 2.5.1.2.1) and for utilization of enzymes
in food processing, e. g., aldehyde dehydrogenase
(cf. 2.7.2.1.4).
2.5.1.2 Two-Substrate Reactions
Fig. 2.23. Determination of K m and V (according to
Lineweaver and Burk)
For many enzymes, for examples, oxidoreductase
and ligase-catalyzed reactions, two or more substrates
or cosubstrates are involved.
υ 0
υ 0 = −K m · V (c) (2.48)
(A 0 )
When Equation 2.48c is plotted using the
substrate-reaction velocity data, a straight line
with a negative slope is obtained (Fig. 2.24)
where y is v 0 and x is v 0 /[A 0 ]. The y and x intercepts
correspond to V and V/K m , respectively.
Single-substrate reactions, for which the kinetics
outlined above (with some exceptions, cf. 2.5.1.3)
are particularly pertinent, are those catalyzed by
lyase enzymes and certain isomerases. Hydrolysis
by hydrolase enzymes can also be considered
a single-substrate reaction when the water content
remains unchanged, i. e., when it is present
in high concentration (55.6mol/μ). Thus, water,
as a reactant, can be disregarded.
Characterization of an enzyme-substrate system
by determining values for K m and V is import-
2.5.1.2.1 Order of Substrate Binding
In the reaction of an enzyme with two substrates,
the binding of the substrates can occur sequentially
in a specific order. Thus, the binding mechanism
can be divided into catalysis which proceeds
through a ternary adsorption complex (enzyme
+ two substrates) or through a binary complex
(enzyme + one substrate), i. e. when the enzyme
binds only one of the two available substrates
at a time.
A ternary enzyme-substrate complex can be
formed in two ways. The substrates are bound
to the enzyme in a random fashion (“random
mechanism”) or they are bound in a well-defined
order (“ordered mechanism”).
Let us consider the reaction
(2.49)
If the enzyme reacts by a “random mechanism”,
substrates A and B form the ternary enzymesubstrate
complex, EAB, in a random fashion and
the P and Q products dissociate randomly from
the ternary enzyme-product complex, EPQ:
(2.50)
Fig. 2.24. Determination of K m and V (according to
Hofstee)
Creatine kinase from muscle (cf. 12.3.6) is an example
of an enzyme which reacts by a random