Electron microscopy of glycogen degrading enzymes

Single molecules of glycogen phosphorylase b exhibit images in the electron microscope which are similar in shape and dimension to those derived from X-ray crystallography. Phosphorylase alpha exhibits tetramers but shows dimers in the presence of glucose. Glycogen debranching enzyme appears as a monomer with an unusual crescent or shrimp-like shape, with occasional isologous aggregation to circular dimers. The longest dimension of the monomer is very similar to that of the phosphorylase dimer, 11.5 nm. Strong binding of the debranching enzyme to glycogen is readily visualized in the electron microscope. It is suggested that the distinctive shape of the debranching enzyme may be related to its catalytic function.