Modification of kinetic parameters of glycogen phosphorylase from mantle tissue of Mytilus galloprovincialis by a phosphorylation mechanism

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Initial rate and affinity studies on mantle Mytilus phosphorylase a were carried out in order to find possible differences in its kinetic properties with respect to phosphorylase b. Phosphorylase a was not stimulated for any AMP concentrations. Michaelis constants ( K m) are 0.05 mg/ml glycogen, 1.15 mM inorganic phosphate and 1.50 mM glucose-1-phosphate. The K ms for the substrates, in the direction of glycogen breakdown, are enhanced by non-saturating concentrations of cosubstrate, without reducing the apparent maximum velocity. First order and hyperbolic kinetics and values of the allosteric constant smaller than 2 were observed. These results suggest a catalytic mechanism different to that shown for mantle Mytilus phosphorylase b.