Active site glucose-6-phosphatase

Phosphatases specific for such substrates as glucose-6-phosphate, fructose-l,6-bisphosphate, and phospho-glycolate help to drive metabolic cycles (Chapter 17). The 335-residue fructose-1,6-bisphosphatase associates to form a tetramer with D2 symmetry. ° The allosteric enzyme exists in two conformational states (see Chapter 11). Activity is dependent upon Mg + or other suitable divalent cation, e.g., Mn or Zn, and is further enhanced by K+ or NH3. While the dimetal sites depicted in Figs. 12-23 and 12-24 are quite rigid and undergo little change upon formation of complexes with substrates or products, the active site of fructose-1,6-bisphosphatase is more flexible. There are three metal-binding sites but they contain no histidine side chains and have been seen clearly only in a product complex. Perhaps because of the need for... 

As with peptide hydrolysis, several enzyme systems exist that catalyze carboxylic and phosphoric ester hydrolysis without the need for a metal ion. They generally involve a serine residue as the nucleophile in turn, serine may be activated by hydrogen-bond formation—or even proton abstraction—by other acid-base groups in the active site. The reaction proceeds to form an acyl- or phosphory 1-enzyme intermediate, which is then hydrolyzed with readdition of a proton to the serine oxygen. Mechanisms of this type have been proposed for chymotrypsin. In glucose-6-phosphatase the nucleophile has been proposed to be a histidine residue. ... 

It was Fujita and his colleagues (1969) who established the unique substrate specificity of -dependent p-nitrophenylphosphata e. For example, the enzyme did not hydrolyze -glycerophosphate or phenylphos-phate, which are the standard substrates for acid and alkaline phosphatases, nor did it hydrolyze ATP, AMP, 2 -(3 )-AMP, CMP, IMP, GMP, glucose-6-phosphate, or glucose-l-phosphate. From the noncompetitive nature of inhibition of both enzyme activities in the presence of ATP and p-nitrophenylphosphate, Fujita suggests diat there are two different active sites which may or may not be located on the same protein molecule. 

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