Food Biochemistry and Food Processing (2 edition)

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BLBS102-c07 BLBS102-Simpson March 21, 2012 11:12 Trim: 276mm X 219mm Printer Name: Yet to Come


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).
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