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.