Allostery through Covalent Modification

Glycogen phosphorylase catalyzes the release of a glucose from glycogen, while transfering a phosphate group to generate Glucose-1-phosphate. The enzyme is a homodimer, colored red and blue. The phosphorylation (green) of Ser14, colored yellow, determines whether the enzyme is active or inactive. Phosphorylation causes a change in how the two monomers interact and opens up the catalytic site. The active sites are at a distance, and are marked by a cofactor PLP colored cyan.

9gpb (PDB)