What is "glycogenin"

Glycogenin is an enzyme involved in converting glucose to glycogen. It acts as a primer, by polymerizing the first few glucose molecules, after which other enzymes take over. It is a homodimer of 37- kDa subunits and is classified as a glycosyltransferase.

It catalyzes the chemical reaction:

UDP-alpha-D-glucose + glycogenin $\rightleftharpoons$ UDP + alpha-D-glucosylglycogenin

Thus, the two substrates of this enzyme are UDP-alpha-D-glucose and glycogenin, whereas its two products are UDP and alpha-D-glucosylglycogenin.