title: Steady-state kinetic studies of binding and catalysis by ribonuclease T2: a microenvironmental survey of the active site by using a series of adenosine-3'- and -5'-alkylphosphates.
abstract: The effects of a series of adenosine-3'-alkylphosphates, Ap(CH2)n-1CH3 with n = 1-8, on the binding and catalytic activities of RNase T2 were investigated. The inhibition of the RNase T2-catalyzed reaction by a series of adenosine-5'-alkylphosphates, CH3(CH2)n-1pA with n = 1-7, was also studied. Multiple regression analyses of the observed kinetic constants were carried out to determine the contribution of polar and hydrophobic effects to kcat, kcat/Kmi, and cosubstrate and inhibitor bindings (Kmi and Ki). The data show that the pKmi for Ap(CH2)n-1CH3 increases uniformly with n up to n = 4, then remains constant. By contrast pKi for CH3(CH2)n-1pA is independent of n. The data for log kcat correlated well with polar and hydrophobic variables. On the other hand, log kcat/Kmi is independent of a hydrophobic effect, and the data were fitted by a single polar variable equation. The contribution of hydrophobic regions at or near the active site of RNase T2 is discussed. The present results also offer evidence for the existence of an isomerization step of the first formed enzyme-substrate complex.
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism* / Q000494:pharmacology; D001665:Binding Sites; D002384:Catalysis; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004722:Endoribonucleases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004734:Energy Metabolism; D007700:Kinetics; D010766:Phosphorylation
chemical_list: D000227:Adenine Nucleotides; D004722:Endoribonucleases; C022543:ribonuclease T(2)