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Mechanistic studies on phosphoglucomutase Percival, Michael David


The mechanism of rabbit skeletal muscle phosphoglucomutase (EC. has been investigated using fluorinated and deoxygenated substrate analogues. Each of the analogues in which the non-acceptor hydroxyls are replaced by fluorine or hydrogen are substrates of the enzyme. The kinetic constants of these substrates are reported. The rate of the mutase reaction of each substrate analogue in the presence of glucose 1,6-diphosphate is the same as that of the half reaction involving production of the fluorinated and deoxygenated glucose 1,6-diphosphate species. The exceptions are 3-fluoro- and 3-deoxy-glucose 1-phosphate, in which cases the rates of the half reactions are 8 times that of the overall mutase reaction. The Km of 3-fluoro-glucose 1,6-diphosphate is approximately 90 times that of glucose 1,6-diphosphate and the other deoxy and fluoro analogues. The inhibition of phosphoglucomutase by fluorinated and deoxygenated substrate analogues has been investigated. The synthesis of a series of novel disubstituted inhibitors (based on glucose 1-phosphate) in which the C-6 hydroxyl is replaced by fluorine and a sugar ring hydroxyl is replaced by either hydrogen or fluorine is described. The inhibition constants show that the hydroxyl distal to the acceptor hydroxyl is most important in the formation of a strong enzyme-inhibitor complex. The synthesis is described of three phosphorofluoridate analogues of glucose phosphate substrates. These analogues were found to only weakly inhibit phosphoglucomutase. No evidence of any phosphoryl transfer between the phosphoenzyme and the phosphorofluoridate analogues could be detected. Thus phosphoglucomutase has a strict requirement for a doubly negatively charged substrate phosphate group. The interaction of phosphoglucomutase with fluorinated substrates and inhibitors has been investigated by ¹⁹Fnmr. Large downfield changes in the chemical shifts of the inhibitors 6-fluoro-glucose 1-phosphate and α-glucosyl fluoride 6-phosphate were found to accompany binding to the phosphoenzyme. The effects of the binding of activating and non-activating metal ions on these spectra were investigated. The different effects observed may be directly related to the chemical basis for the metal induced activation of the enzyme. ¹⁹Fnmr data consistent with a 10² to 10³ fold increase in the tenacity with which phosphoglucomutase binds substrates and inhibitors in the presence of Li⁺ were observed in the spectra of the phosphoenzyme with difluorinated glucose 1-phosphate inhibitors. Two enzyme bound species were detected in the ¹⁹Fnmr spectra of the complexes formed by reaction of the Cd²+ phosphoenzyme with 2- and 3-fluoro-glucose phosphates. These species are tentatively assigned as the fluoro-glucose 1,6-diphosphate species bound in two different modes to the dephosphoenzyme. Only one bound species was observed in the case of 4-fluoro-glucose phosphates. The environment of each substrate glucose hydroxyl in the active site was probed using ¹⁹Fnmr and the fluorinated glucose phosphate substrates. Data inconsistent with a minimal motion type of mechanism (W.J. Ray, A.S. Mildvan & J.W. Long, Biochemistry 1973,12, 3124) were obtained. The results of the nmr and kinetic studies are consistent with an exchange type of mechanism in which the C-3 hydroxyl plays an important role in the reorientation of the glucose 1,6-diphosphate. The data also suggest that there are two distinct glucose binding sites, one for each substrate and glucose 1,6-diphosphate bound in the same mode.

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