Food Chemistry

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):

(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 prod-
ucts (acetaldehyde and NADH) is, however, not
altered.
Polyphenol oxidase from potato tubers also re-
actsbyan“ordered mechanism”. Oxygen is ab-
sorbed first, followed by phenolic substrates. The
main substrates are chlorogenic acid and tyro-
sine. Enzyme affinity for tyrosine is greater and
the reaction velocity is higher than for chloro-
genic acid. The ratio of chlorogenic acid to tyro-
sine affects enzymatic browning to such an ex-
tent that it is considered to be the major prob-
lem in potato processing. The deep-brown col-
ored melanoidins are formed quickly from tyro-
sine but not from chlorogenic acid. In assessing
the processing quality of potato cultivars, the dif-
ferences in phenol oxidase activity and the con-
tent 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 prod-
uct of enzymatic oxidation (cf. 18.1.2.5.8).

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 re-

acts during alteration of the enzyme (a change in

the oxidation state of the prosthetic group, a con-

formational change, or only a change in covalent

binding of a functional group). The modified en-

zyme, which is denoted F, binds the second sub-

strate and the second reaction occurs, which re-

generates the initial enzyme, E, and releases the

second product:

(2.54)

The glycolytic enzyme hexokinase reacts by

a “ping pong mechanism”:

(2.55)

2.5.1.2.2 RateEquationsforaTwo-SubstrateReaction

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 +

Ka

(A 0 )

+

Kb

(B 0 )

+

Kja·kb

(A 0 )(B 0 )

(2.56)

When compared to the rate equation for a single-

substrate reaction (Equation 2.41), the difference

becomes obvious when the equation for a single-