Food Chemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 121

Fig. 2.23.Determination of Kmand V (according to
LineweaverandBurk)

υ 0 =−Km

υ 0

(A 0 )

·V (c) (2.48)

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 inter-
cepts correspond to V and V/Km, 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. Hydroly-
sis by hydrolase enzymes can also be considered
a single-substrate reaction when the water con-
tent 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 Kmand V is import-

Fig. 2.24.Determination of Kmand V (according to
Hofstee)

ant in enzymatic food analysis (cf. 2.6.4) and for

assessment of enzymatic reactions occurring in

food (e. g. enzymatic browning of sliced pota-

toes, 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-SubstrateReactions

For many enzymes, for examples, oxidoreductase

and ligase-catalyzed reactions, two or more sub-

strates or cosubstrates are involved.

2.5.1.2.1 OrderofSubstrateBinding

In the reaction of an enzyme with two substrates,

the binding of the substrates can occur sequen-

tially in a specific order. Thus, the binding mech-

anism can be divided into catalysis which pro-

ceeds through a ternary adsorption complex (en-

zyme+two substrates) or through a binary com-

plex (enzyme+one substrate), i. e. when the en-

zyme binds only one of the two available sub-

strates 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 enzyme-

substrate complex, EAB, in a random fashion and

the P and Q products dissociate randomly from

the ternary enzyme-product complex, EPQ:

(2.50)

Creatine kinase from muscle (cf. 12.3.6) is an ex-

ample of an enzyme which reacts by a random