A kinetic study of glycogen phosphorylase b from the mantle tissue of Mytilus galloprovincialis, Lmk

Abstract

1. 1. In order to assign a meaningful role to the phosphorolytic pathway in Mytilus glycogen metabolism the kinetic mechanism of phosphorylase b, and its allosteric control, were studied. 2. 2. The kinetic parameters of phosphorylase b from the mussel Mytilus galloprovincialis were determined. Michaelis constants ( K m or S 0.5) were in the range of 0.32–2.49 mg/ml for glycogen, 7–16 mM for P i and 114–423 μM for AMP. In the direction of glycogen synthesis, the K m value for glucose-1-P was approximately 180 mM. 3. 3. The enzyme displayed homotropic co-operativity towards the binding of co-substrate and AMP (Hill coefficients of 2 and 1.4, respectively) and heterotropic co-operativity between substrates and AMP. 4. 4. The concentration of glycogen in the Mytilus mantle is between 38- and 125-fold higher than the apparent K m of phosphorylase b; the concentration of AMP varies throughout the year from 10 to 175 μM, up to a value close to the apparent K m for the effector. 5. 5. The apparent K m for P i is close to the concentration found in the mantle. This ligand showed more important regulatory effects than the effector AMP.