2009_Book_FoodChemistry

122 2 Enzymes
mechanism:
(2.51)
In an “ordered mechanism” the binding during
the catalyzed reaction according to equation 2.49
is as follows:
(2.52)
Alcohol dehydrogenase reacts by an “ordered
mechanism”, although the order of the binding of
substrates NAD + and ethanol is decided by the
ethanol concentration. NAD + is absorbed first at
low concentrations (< 4 mmol/l):
In enzymatic reactions where functional group
transfers are involved, as a rule only binary
enzyme-substrate complexes are formed by the
so-called “ping pong mechanism”.
A substrate is adsorbed by enzyme, E, and reacts
during alteration of the enzyme (a change in
the oxidation state of the prosthetic group, a conformational
change, or only a change in covalent
binding of a functional group). The modified enzyme,
which is denoted F, binds the second substrate
and the second reaction occurs, which regenerates
the initial enzyme, E, and releases the
second product:
(2.54)
The glycolytic enzyme hexokinase reacts by
a “ping pong mechanism”:
(2.53)
When the ethanol concentration is increased to 7–
8 mmol/l, ethanol is absorbed first, followed by
the cosubstrate. The order of removal of products
(acetaldehyde and NADH) is, however, not
altered.
Polyphenol oxidase from potato tubers also reactsbyan“ordered
mechanism”. Oxygen is absorbed
first, followed by phenolic substrates. The
main substrates are chlorogenic acid and tyrosine.
Enzyme affinity for tyrosine is greater and
the reaction velocity is higher than for chlorogenic
acid. The ratio of chlorogenic acid to tyrosine
affects enzymatic browning to such an extent
that it is considered to be the major problem
in potato processing. The deep-brown colored
melanoidins are formed quickly from tyrosine
but not from chlorogenic acid. In assessing
the processing quality of potato cultivars, the differences
in phenol oxidase activity and the content
of ascorbic acid in the tubers should also be
considered in relation to “enzymatic browning”.
Ascorbic acid retards formation of melanoidins
by its ability to reduce o-quinone, the initial product
of enzymatic oxidation (cf. 18.1.2.5.8).
2.5.1.2.2 Rate Equations
for a Two-Substrate Reaction
(2.55)
Here the reaction rate is distinguished by
its dependence on two reactants, either two
molecules of the same compound or two different
compounds. The rate equations can be derived by
the same procedures as used for single-substrate
catalysis. Only the final forms of the equations
will be considered.
When the catalysis proceeds through a ternary
enzyme-substrate complex, EAB, the general
equation is:
v 0 =
V
1 + K a
(A 0 ) + K b
(B 0 ) + K ja · k b
(A 0 )(B 0 )
(2.56)
When compared to the rate equation for a singlesubstrate
reaction (Equation 2.41), the difference
becomes obvious when the equation for a single-