A Missing Step in Glycogen Synthesis

INTRODUCTION. There is strong evidence for the existence of a discrete intermediate in glycogen (macroglycogen) synthesis. This has been termed proglycogen[1]. It is narrowly disperse and has a mass of about 400kDa. Containing 10% by weight of the glycogen primer, the self-glucosylating glycogenin, proglycogen is separable from macroglycogen by the insolubility of the former in 10% trichloracetic acid (TCA). The synthesis of glycogen from glucose in cultured astrocytes can be arrested at proglycogen, as can the degradation of macroglycogen in cultured quail embryo muscle. This suggests the possibility of distinct enzyme species operating between glucose and proglycogen that are different from the classical glycogen synthase and phosphorylase, which until now have been considered the only enzymes involved in extending and pruning the chains of glycogen. The putative proglycogen synthase differs from the already known glycogen synthase by having a much greater affinity for UDPglucose. Rather than postulating that proglycogen synthase is a different gene product, it has been suggested that changes in the pattern of multisite phosphorylation of glycogen synthase might modulate its affinity for UDPglucose[1].