Food Biochemistry and Food Processing (2 edition)

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7 Biocatalysis, Enzyme Engineering and Biotechnology 131

(A) (B)

(C) (D)

Figure 7.4.The four-class classification system of domains. (A)Theα+βclass (structure of glycyl-tRNA synthetaseα-chain). (B) the allα

class (structure of the hypothetical protein (Tm0613) fromThermotoga maritima). (C)Theα/βclass (structure of glycerophosphodiester

phosphodiesterase). (D) The allβclass (structure of allantoicase fromSaccharomyces cerevisiae).

Despite the impression that the enzyme’s structure is static and

locked into a single conformation, several motions and confor-

mational changes of the various regions always occur (Hammes

2002). The extent of these motions depends on many factors, in-

cluding temperature, the properties of the solvating medium, the

presence or absence of substrate and product (Hammes 2002).

The conformational changes undergone by the enzyme play an

important role in controlling the catalytic cycle. In some en-

zymes, there are significant movements of the binding residues,

usually on surface loops, and in other cases, there are larger con-

formational changes. Catalysis takes place in the closed form and

the enzyme opens again to release the product. This favoured

model that explains enzyme catalysis and substrate interaction is

the so-calledinduced fit hypothesis(Anderson et al. 1979, Joseph

et al. 1990). In this hypothesis, the initial interaction between

enzyme and substrate rapidly induces conformational changes

in the shape of the active site, which results in a new shape of

the active site that brings catalytic residues close to substrate

bonds to be altered (Fig. 7.8). When binding of the substrate

to the enzyme takes place, the shape adjustment triggers catal-

ysis by generating transition-state complexes. This hypothesis

helps to explain why enzymes only catalyse specific reactions

(Anderson et al. 1979, Joseph et al. 1990). This basic cycle has

been seen in many different enzymes, including triosephosphate

isomerase, which uses a small hinged loop to close the active

site (Joseph et al. 1990) and kinases, which use two large lobes

moving towards each other when the substrate binds (Anderson

et al. 1979).