The Regulation of Glycogen Metabolism by Multivalent Phosphorylation

Abstract

In the last symposium of this series, we described a novel form of enzyme regulation by covalent modification which appeared to be involved in the hormonal control of muscle phosphorylase kinase (Cohen et al., 1974). When phosphorylase kinase, subunit structure (αβγ) ₄, was incubated with cyclic AMP-dependent protein kinase, cyclic AMP, and ATP-Mg, two moles of phosphate were incorporated per mole of enzyme (αβγ). The first mole rapidly entered the β-subunit and paralleled the 20-4-fold rise in activity, while the second mole was incorporated into the α-subunit five-fold slower after a short lag period, without any effect on activity (Cohen, 1973; Hayakawa et al., 1973). However, the secondary phosphorylation of the α-subunit appeared to alter the conformation of the enzyme in such a way that the rate of dephosphorylation of the β-subunit catalysed by phosphorylase kinase phosphatase was enhanced at least 50-fold. The results showed that the reversible phosphorylation of the β-subunit correlated with the reversible activation of the enzyme, and suggested that phosphoylation of the α-subunit played a role in controlling the reversal of phosphorylase kinase activation (Cohen and Antoniw, 1973; Cohen et al., 1974).