Starch Biosynthesis. 3: The Glycogen Precursor Mechanism Using Phosphorylase in the Production of the Precursor Glycogen

Based on the previously proposed glycogen precursor mechanism (S. R. Erlander, Enzymologia 19 (1958), 273–283), it is now proposed that phosphorylase is the primary enzyme for the production of linear chains in the precursor glycogen. The mechanism involves the translocation of ADPglucose (ADPGlu), and is suppressed by ATP because of a reverse of the ADPglucose pyrophosphorylase (ADPG pp) mechanism. Soluble starch synthase II (SSS II) is a back-up system, involves the translocation of glucose-6-phosphate (Glu-6-P), and is activated, not suppressed, by ATP, and is used if phosphorylase or the ADPGlu translocator is destroyed. Each system is independent and produces products which suppress the other. Hence, only one system works at a time. Both mechanisms produce a glycogen precursor and both are dependent upon ADPGlu pp. The initial higher radioactivity of amylose and the constant yield of amylose can be explained by a three or four day biosynthesis of this glycogen, followed by the removal of the glycogen's exterior branches by debranching enzymes to produce amylose and amylopectin. These removed branches are first degraded (using SSS I and II) to ADPGlu, which is the only source of ADPGlu for amylose synthesis. The retention of the polymodal behavior of debranched amylopectin in going from Bomi to shx barley amylopectins is most likely due to a change from phosphorylase to SSS II since both debranched amylopectins produce Poisson distributions.