2009_Book_FoodChemistry

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 I a Soybean Linoleic acid 9.0
Lipoxygenase, type II a Soybean Linoleic acid 6.5
a See 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 enzymesubstrate
complex formed, EA, depending on pH,
form the following equilibria:
(2.78)
Which of the charged states of E and EA are involved
in catalysis can be determined by following
the effect of pH on V and K m .
a) Plotting K m versus pH reveals the type of prototropic
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: K m is independent of pH in
the range of 4 − 9. This means that the
forms E n+1 ,E n ,andE n−1 , i. e. enzyme forms
which are neutral, positively or negatively
charged on the active site, can bind substrate.
Figures 2.31b and c: K m is dependent on one
prototropic group, the pK value of which is
below (Fig. 2.31b) or above (Fig. 2.31c) neutrality.
In the former case, E n and E n−1 are the
active forms, while in the latter, E n+1 and E n
are the active enzyme forms in substrate binding.
Figure 2.31d: K m is dependent on two prototropic
groups; the active form in substrate
binding is E n .
b) The involvement of prototropic groups in the
conversion of an enzyme-substrate complex
into product occurs when the enzyme is saturated
with substrate, i. e. when equation 2.40
which defines V is valid ([A 0 ] ≫ K m ). 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 conversion
to product, are revealed.
Fig. 2.31. The possible effects of pH on the Michaelis
constant, K m