phosphorylase

Meaning: Glycogen phosphorylase removes glucose units from the nonreducing ends of glycogen by phosphorolysis. Inorganic phosphate attacks the α-1,4 bond, so glucose-1-phosphate is released without spending ATP and without water. The enzyme stops a few residues short of an α-1,6 branch point, where debranching enzyme is needed. Phosphoglucomutase then converts glucose-1-phosphate to glucose-6-phosphate. Liver has glucose-6-phosphatase and can release free glucose to the blood; muscle keeps the glucose-6-phosphate for its own glycolysis.

Equation: glycogen (n residues) + Pi → glycogen (n − 1 residues) + glucose-1-phosphate, where Pi is inorganic phosphate and n is the number of glucose residues (quantities in mol).

On the MCAT: Passages show glucagon (liver) or epinephrine (liver and muscle) raising cAMP, which activates protein kinase A. That kinase phosphorylates phosphorylase kinase, which phosphorylates phosphorylase b to the more active a form. Insulin favors dephosphorylation by a phosphatase and glycogen synthesis. In muscle, AMP activates the enzyme allosterically, while ATP and glucose-6-phosphate inhibit it. Typical question: if the enzyme is deficient, predict glycogen buildup and lower glucose release during fasting.

Common confusion: Phosphorylase is not a kinase. A kinase transfers a phosphate from ATP to a substrate, whereas phosphorylase uses free Pi to cleave a bond. It is also not a hydrolase, since no water is used. Phosphorylase itself is regulated by being phosphorylated.