Abstract
It generally is agreed that initiation of a glycogen molecule occurs through the addition of a linear array of glucose molecules to a protein referred to as glycogenin. The addition of glucosyl units is catalyzed by glycogenin itself. The product is then branched and expanded by other enzymes. This process has been studied best in skeletal muscle. In skeletal muscle, all of the glycogenin is incorporated into glycogen molecules (1) The glycogen molecules also are all presumed to be proteoglycans.
In liver, glycogenin is free, i.e., not incorporated into a proteoglycogen molecule in the fasted state and it becomes incorporated into a glycogen molecule only later in the feeding phase. Furthermore, preliminary data suggest that most of the glycogen molecules are not proteoglycans but rather are independent of glycogenin. Thus, the character and biosynthesis of glycogen in liver appears to be much more complex than in skeletal muscle. Regulation of the various forms of glycogen also is likely to be complex.
