Covalently linked phosphate monoesters on alpha-polyglucans reduce substrate affinity of branching enzymes

Starch and glycogen are α-polyglucans which represent important sources of long- and short-term cellular carbohydrate storage synthesized in living cells. Both polyglucans contain variable, but significant, levels of covalently bound phosphate monoesters whose biological role is likely connected to the regulation of turnover of these storage polymers by promoting water solubility. The amount of α-glucan-bound phosphate found in plant starch appears to be closely related to the average chain length of α-1,4-glucans, and inversely related to the frequency of α-1,6-branch linkages. The enzymes responsible for adding branches to linear α-1,4-glucan chains in starch and glycogen are 1,4-α-glucan: 1,4-α-glucan 6-glucosyl transferases (branching enzymes). In this study, glucan bound phosphate was shown to reduce the affinity of branching enzymes for α-glucan substrates. Plant starch branching enzymes and glycogen branching enzymes from various prokaryotic and eukaryotic sources showed reduced substrate affinities in native gels as the α-glucan phosphate content was increased. The substrate affinities of all branching enzymes tested showed an inverse linear relationship with α-glucan phosphate content. The possible biological significance of this phenomenon is discussed in relation to known models of starch structure in plants and specific glycogen storage diseases in mammals.