Both the binding of the substrate to the enzyme and catalysis of the reaction depend on interactions between the substrates and reactive groups in the amino acid side-chains which make up the active site. They have to be in the appropriate ionization state for binding and reaction to occur — this depends on the pH of the medium. Any enzyme will have maximum activity at a specific pH — the optimum pH for that enzyme. As the pH rises or falls away from the optimum, so the activity of the enzyme will decrease. Most enzymes have little or no activity 2—3 pH units away from their pH optimum. Figure 2.6 shows the activity of two enzymes that are found in plasma and which catalyse the same reaction, hydrolysis of a phosphate ester; enzyme A is acid phosphatase (released from the prostate gland, with a pH optimum around 3.5) and enzyme B is alkaline phosphatase (released from bone, with a pH optimum around 9.0). Neither has any significant activity at pH 7.35—7.45, which is the normal range of plasma pH. However, alkaline phosphatase is significantly active in the alkaline microenvironment at cell surfaces, and is important, for example, in the hydrolysis of pyridoxal phosphate (the main form of vitamin B6 in plasma; section 11.9) to free pyridoxal for uptake into tissues.
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