Subunit structure and amino-acid composition of crystallized human-muscle glycogen phosphorylase.

Abstract

The subunit structure and molecular weight of the a and b forms of human muscle glycogen phosphorylase were investigated. Electrophoresis in the presence of sodium dodecylsulfate gave a molecular weight of 94 500 for the monomer. When studied at 1 mg/ml and 20 °C by high‐speed sedimentation equilibrium, sucrose density gradient ultracentrifugation and specific activity‐concentration dependence, both a and b forms of the human enzyme were found to exist as dimers. A slight association of human phosphorylase a was observed in the high‐speed sedimentation equilibrium values of the z‐average molecular weight (262000 for Mmacr;zvs 188000 daltons for Mmacr;w). Some association of the a enzyme was also detected by schlieren sedimentation velocity when the enzyme was studied at 5 mg/ml. Human phosphorylase b could be fully associated to tetramer at 20 °C when Pi, Mg2+ and AMP were added but not in the presence of NaF and AMP which were found to be effective in tetramerizing rabbit phosphorylase b. Another important difference from rabbit phosphorylase was the significantly lower proline content of the enzyme from human muscle.