Food Chemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 129

Table 2.11.pH Optima of various enzymes

Enzyme Source Substrate pH

Optimum

Pepsin Stomach Protein 2

Chymotrypsin Pancreas Protein 7.8

Papain Tropical plants Protein 7–8

Lipase Microorganisms Olive oil 5–8

α-Glucosidase (maltase) Microorganisms Maltose 6.6

β-Amylase Malt Starch 5.2

β-Fructofuranosidase (invertase) Tomato Saccharose 4.5

Pectin lyase Microorganisms Pectic acid 9 .0–9. 2

Xanthine oxidase Milk Xanthine 8.3

Lipoxygenase, type Ia Soybean Linoleic acid 9.0

Lipoxygenase, type IIa Soybean Linoleic acid 6.5

aSee 3.7.2.2.

the reaction rate. However, such effects should be
determined separately. Here, only the influences
mentioned under b) will be considered with some
simplifications.
An enzyme, E, its substrate, A, and the enzyme-
substrate complex formed, EA, depending on pH,
form the following equilibria:

(2.78)

Which of the charged states of E and EA are in-
volved in catalysis can be determined by follow-
ing the effect of pH on V and Km.

a) Plotting Kmversus pH reveals the type of pro-

totropic groups involved in substrate binding

and/or maintaining the conformation of the

enzyme. The results of such a plot, as a rule,

resemble one of the four diagrams shown in

Fig. 2.31.

Figure 2.31a: Km is independent of pH in

the range of 4−9. This means that the

forms En+^1 ,En,andEn−^1 , i. e. enzyme forms

which are neutral, positively or negatively

charged on the active site, can bind substrate.

Figures 2.31b and c: Kmis dependent on one

prototropic group, the pK value of which is

below (Fig. 2.31b) or above (Fig. 2.31c) neu-

trality. In the former case, Enand En−^1 are the

active forms, while in the latter, En+^1 and En

are the active enzyme forms in substrate bind-

ing.

Figure 2.31d: Kmis dependent on two pro-

totropic groups; the active form in substrate

binding is En.

b) The involvement of prototropic groups in the

conversion of an enzyme-substrate complex

into product occurs when the enzyme is satu-

rated with substrate, i. e. when equation 2.40

which defines V is valid ([A 0 ] Km). Thus,

a plot of V versus pH provides essentially the

same four possibilities presented in Fig. 2.31,

the difference being that, here, the prototropic

groups of EA, which are involved in the con-

version to product, are revealed.

Fig. 2.31.The possible effects of pH on theMichaelis

constant, Km