LOW-RESOLUTION STRUCTURE OF GLYCOGEN PHOSPHORYLASE-A MONOMER AND COMPARISON WITH PHOSPHORYLASE-B

Crystals of rabbit muscle glycogen phosphorylase [EC 2.4.1.1] a with the same space group and lattice constants as the b enzyme were obtained. An electron density map of phosphorylase a was calculated to 6 .ANG. resolution using 4 heavy-atom derivatives. With the exception of the monomer-monomer interface region within the dimer, the molecular boundary is clearly defined in all regions of the map. This interface region is notable for significant contacts and interpenetration of polypeptide chains. A difference Fourier at 6 .ANG. resolution was calculated for the a and b enzymes. Extensive structural changes are evident, though these differences are essentially confined to 2 regions. One of these regions is the interface, where an apparent shift of a 30 .ANG. length of main-chain is visible.