Stabilization of a phosphorylase b active conformation by hydrophobic solvents

Abstract

It is often supposed that glycogen phosphorylase b from rabbit skeletal muscle (EC 2.4.1.1) shows an absolute nucleotide requirement for catalytic activity (reviewed [l] ). The molecular mechanism by which the enzyme is allosterically activated by nucleotides is still essentially unknown. However, the direct participation of AMP in the catalytic process seems unlikely, since recent crystallographic work has shown that the nucleotide binding site is quite distant from the active site [2,3] . If the action of 5’-AMP simply consists of shifting an allosteric equilibrium from a T to an R state, as assumed in the concerted model for allosteric transitions [4] , it is conceivable that structurally unrelated compounds could produce a similar shift, resulting eventually in a bypass of the nucleotide requirement for activity. We have found that, upon addition to the buffer of a variety of hydrophobic compounds, the enzyme activity loses its requirement for AMP. We present here a detailed analysis of this unexpected phenomenon, and we briefly discuss its possible significance with respect to the mechanism of the allosteric activation of phosphorylase.