Phosphoglucomutase

Phosphoglucomutase is an enzyme that transfers a phosphoryl group on a glucose monomer from the 1' to the 6' position in the forward direction or the 6' to the 1' position in the reverse.

More specifically, it facilitates the interconversion of glucose 1-phosphate and glucose 6-phosphate.

Function in glycogenolysis
After glycogen phosphorylase has broken off a single glucose molecule from the greater glycogen structure, the free glucose has a phosphate group on its 1-carbon. This glucose-1-phosphate isomer cannot be metabolized easily. The enzyme phosphoglucomutase phosphorylates the 6-carbon, while subsequently dephosphorylating the 1-carbon. The result is glucose-6-phosphate, which can now theoretically travel down the glycolysis or pentose phosphate pathway.

Function in glycogenesis
Phosphoglucomutase also acts in the opposite fashion when a large concentration of glucose-6-phosphate is present. In this case, it is the 1-carbon that is phosphorylated and the 6-carbon that is dephosphorylated. The resulting glucose-1-phosphate is then changed into UDP-glucose in a number of intermediate steps. If activated by insulin, glycogen synthase will proceed to clip the glucose from the UDP-glucose complex and on to the glycogen molecule.