The nature of the allosteric interactions of ribonuclease and its ligands

The allosteric model for ribonuclease activity by Walker, Ralston & Darvey [(1975) Biochem.J. 147, 425--433; (1976) Biochem.J. 153, 329--337] involves the binding of a large number of molecules of substrate or substrate analogue to a series of allosteric sites on the enzyme. In the present paper, the nature of these allosteric interactions is investigated. The effects of ionic strength pH carbamoylation of lysine to homocitrulline and of deamidation of glutamine and asparagine on plots of velocity versus substrate concentration are examined and evidence is presented that the allosteric transition involves an electrostatic interaction between the negatively charged substrate molecules and the cationic groups on the enzyme.