This chapter describes the molecular and catalytic properties of aldose–ketose isomerases. Aldose–ketose isomerases catalyze the interconversion of isomeric aldo- and keto sugars by causing the migration of carbon-bound hydrogen between carbons 1 and 2. These enzymes may be classified in two groups according to their action on free or on phosphorylated monosaccharide substrates. Those acting on free sugars appear to be confined mainly to microorganisms, whereas some of those acting on phosphorylated substrates are common to all living organisms. Prominent among the latter are glucose-6-phosphate isomerase, triosephosphate isomerase, and ribose- 5-phosphate isomerase. The basic function of glucose-6-phosphate isomerase is catalyzing an obligatory step in glycolysis. Its potential for exerting a regulatory influence on carbohydrate metabolism, however, must be considered uncertain. Despite its strategic location near the branching point of several pathways utilizing glucose 6-phosphate, three properties would appear to make this enzyme difficult to control: (1) its equilibrium constant being close to unity, (2) its ubiquitous presence in relatively high concentration, and (3) its high catalytic efficiency. © 1972, Academic Press Inc.
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