Читать книгу Enzyme-Based Organic Synthesis - Cheanyeh Cheng - Страница 16

As a catalyst, the most distinguished property of enzyme is its substrate specificity. This kind of specificity determines its unique biological function. Many enzymes catalyze only one particular biological substrate. Enzymes with this kind of specificity are, therefore, called having absolute substrate specificity. For example, glucose‐6‐phosphatase binds only glucose‐6‐phosphate (G6P) to hydrolyze catalytically the phosphate moiety from the G6P. Therefore, the substrate specificity of glucose‐6‐phosphatase is absolute substrate specificity. Other enzymes possess a broader substrate specificity called relative group specificity that takes a group of biological molecules of similar chemical structure as substrate. As an example, acid and alkaline phosphatase can form phosphate ester bond for a variety of substrates. Therefore, acid and alkaline phosphatase presents wider relative group specificity.

Whether an enzyme has absolute substrate specificity or relative group specificity depends on the number of binding sites of the enzyme–substrate complex. Figure 1.1 illustrates the number of binding sites of an enzyme to the substrate makes the enzyme absolute substrate specificity or relative group substrate specificity. There is only one specific molecule that can attach the enzyme with four binding sites as shown in Figure 1.1a that gives absolute substrate specificity for the enzyme. The number of biding sites for the enzyme with substrate shown in Figure 1.1a–c is three, two, and one, respectively, that makes a variety of different groups available for the unbound group in the substrate to produce a relative group specificity for the enzyme. The substrate specificity of enzyme brings the advantage of reducing the possible side reactions, thus eliminating the laborious purification and separation works.

Figure 1.1 The number of binding sites of a substrate with enzyme determines the type of enzyme specificity: (a) Absolute substrate specificity of enzyme, (b–d) relative group specificity of enzyme.