2009_Book_FoodChemistry

124 2 Enzymes
Fig. 2.26. Plotting slopes (a) and ordinate intercepts (b)
from Fig. 2.25 versus 1/[A 0 ]
Thus, enzymes which do not obey the Michaelis–
Menten model of kinetics are allosterically
regulated. These enzymes have a site which
reversibly binds the allosteric regulator (substrate,
cosubstrate or low molecular weight
compound) in addition to an active site with
a binding and transforming locus. Allosteric
enzymes are, as a rule, engaged at control sites of
metabolism. An example is tetrameric phosphofructokinase,
the key enzyme in glycolysis. In
glycolysis and alcoholic fermentation it catalyzes
the phosphorylation of fructose-6-phosphate
to fructose-1,6-diphosphate. The enzyme is
activated by its substrate in the presence of
ATP. The prior binding of a substrate molecule
which enhances the binding of each succeeding
substrate molecule is called positive cooperation.
The two enzyme-catalyzed reactions, one which
obeys Michaelis–Menten kinetics and the other
which is regulated by allosteric effects, can be
reliably distinguished experimentally by comparing
the ratio of the substrate concentration
needed to obtain the observed value of 0.9V to
that needed to obtain 0.1 V. This ratio, denoted
as R s , is a measure of the cooperativity of the
interaction.
R S = (A 0) 0.9 V
(2.63)
(A 0 ) 0.1 V
For all enzymes which obey Michaelis–Menten
kinetics, R s = 81 regardless of the value of K m
or V. The value of R s is either lower or higher
than 81 for allosteric enzymes. R s < 81 is
Fig. 2.27. Evaluation of a two-substrate reaction, proceeding
through a binary enzyme-substrate complex
(according to Lineweaver and Burk). [A 0 ] 4 > [A 0 ] 3 >
[A 0 ] 2 > [A 0 ] 1
of each other in catalysis, the Michaelis–Menten
kinetics, as outlined under sections 2.5.1.1
and 2.5.1.2, are valid. However, when the
subunits cooperate, the enzymes deviate from
these kinetics. This is particularly true in the
case of positive cooperation when the enzyme is
activated by the substrate. In this kind of plot, v 0
versus [A 0 ] yields not a hyperbolic curve but a saturation
curve with a sigmoidal shape (Fig. 2.28).
Fig. 2.28. The effect of substrate concentration on the
catalytic reaction rate. a Enzyme obeying Michaelis–
Menten kinetics; b allosterically regulated enzyme with
positive cooperativity; c allosterically regulated enzyme
with negative cooperativity