An allosteric model for ribonuclease activity

Abstract

An allosteric model for ribonuclease activity is proposed to explain the non-conventional kinetic patterns observed when the hydrolysis of cytidine 2':3'-cyclic monophosphate is examined. The model proposes that the enzyme exists as an equilibrium between three conformations, E, F and G. E and F are thought to be active conformations and the position of the equilibrium between these forms is determined by the binding of a large number of nucleotide molecules to a set of allosteric sites on the enzyme.

Evidence for a ligand-induced conformational change from form E to form F is presented from product inhibition, equilibrium dialysis and trypsin susceptibility studies. Evidence is presented from product inhibition and protein modification studies that factors which favour the binding of nucleotides to the aliosteric sites, favour form F of the enzyme, while factors which hinder binding favour form E.

It is suggested that the binding of ligands to the allosteric sites involves an electrostatic interaction between the positively charged lysine residues of ribonuclease and either RNA, the natura1 substrate or a poly nucieotide comp1ex formed by aggregation of mononucleotides at high concentrations.

A possib1e ro1e for such a mechanism in the contro1 of the relative rates of the two reactions cataiyzed by ribonuclease is suggested.