PARALLEL EVOLUTION IN 2 HOMOLOGS OF PHOSPHORYLASE

The structure of the unphosphorylated, inactive form of yeast glycogen phosphorylase has been determined to a resolution of 2.6 Angstrom. Th e structure is similar to the phosphorylated, active form of muscle ph osphorylase in the orientations of the subunits and catalytic residues , but resembles the inactive muscle enzyme in the closed, or substrate excluding, orientation of the two domains. Part of the unique yeast a mino-terminal extension of 40 residues binds near the catalytic site o f the second subunit in the homodimer, preventing the domain movement required for substrate access, Phosphorylation may displace the amino terminus from the active site, allowing the domains to separate.