title: Further evidence for an allosteric model for ribonuclease.
abstract: Evidence is presented from three experimental systems to support the allosteric model of Walker et al. (1975) (Biochem. J. 147, 425-433) which explains the substrate-concentration-dependent transition observed in the RNAase (ribonuclease)-catalysed hydrolysis of 2':3'-cyclic CMP (cytidine 2':3'-cyclic monophosphate). 1. Kinetic studies of the initial rate of hydrolysis of 2':3'-cyclic CMP show that the midpoint of the transition shifts to lower concentrations of 2':3'-cyclic CMP in the presence of the substrate analogues 3'-CMP, 5'-CMP, 3'-AMP, 3'-UMP and Pi; 2'-CMP and 2'-UMP do not cause such a shift. 2. Trypsin-digestion studies show that a conformational change in RNAase to a form less susceptible to tryptic inactivation is induced in the presence of the substrate analogues 3'-CMP, 5'-CMP, 3'-AMP, and 3'-UMP. 2'-CMP, 2'-AMP and 2'-UMP do not induce this conformational change. 3. Equilibrium-dialysis experiments demonstrate the multiple binding of molecules of 3'-CMP, 3'-AMP and 5'-AMP to a molecule of RNAase. 2'-CMP binds the ratio 1:1 over the analogue concentration range studied.
mesh_terms: D000249:Adenosine Monophosphate; D000494:Allosteric Regulation; D003597:Cytosine Nucleotides / Q000378:metabolism; D003956:Dialysis; D007700:Kinetics; D008954:Models, Biological; D012260:Ribonucleases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014357:Trypsin; D014500:Uracil Nucleotides
chemical_list: D003597:Cytosine Nucleotides; D014500:Uracil Nucleotides; D000249:Adenosine Monophosphate; D012260:Ribonucleases; D014357:Trypsin