title: Kinetic studies on ribosomal peptidyltransferase. The behaviour of the inhibitor blasticidin S.
abstract: In a cell-free system derived from Escherichia coli, the reaction between Ac[3H]Phe-tRNA and puromycin (S) is inhibited by blasticidin S (I). In this reaction Ac[3H]Phe-tRNA is part of the Ac[3H]Phe-tRNA--poly(U)--ribosome complex (C). After preincubating the complex C with I and then adding S, the degree of inhibition is greater than that observed when C reacts with a mixture of S and I. Without preincubation, the inhibition is competitive giving a Ki of 2 X 10(-7) M. After preincubation the inhibition becomes of the mixed non-competitive type. A first-order kinetic analysis of the reaction between C and excess S, in the presence or in the absence of I, with or without preincubation, suggests that I acts as a modifier decreasing the catalytic rate constant of ribosomal peptidyltransferase (the putative enzyme that catalyzes the reaction between C and S). The effectiveness of I cannot be expressed by an equilibrium constant such as the above-mentioned Ki. A model is proposed which explains the results obtained. In this model, in the presence of I, C is converted to a modified species C, which is still able to react with S but with a lower catalytic rate constant. This is a novel concept, in which the ribosome can be subjected to modulation of its activity by small ligands. It can be useful in studies on translational control of protein synthesis.
mesh_terms: D000217:Acyltransferases / Q000037:antagonists & inhibitors*; D001667:Binding, Competitive; D002384:Catalysis; D004926:Escherichia coli / Q000201:enzymology; D006146:Guanidines / Q000494:pharmacology; D007700:Kinetics; D008956:Models, Chemical; D009705:Nucleosides; D010458:Peptidyl Transferases / Q000037:antagonists & inhibitors*; D011072:Poly U; D011485:Protein Binding; D014176:Protein Biosynthesis; D011691:Puromycin / Q000378:metabolism; D012346:RNA, Transfer, Amino Acyl; D012270:Ribosomes / Q000201:enzymology*
chemical_list: D006146:Guanidines; D009705:Nucleosides; D012346:RNA, Transfer, Amino Acyl; C006383:tRNA, N-acetylphenylalanine-; D011072:Poly U; D011691:Puromycin; C004500:blasticidin S; D000217:Acyltransferases; D010458:Peptidyl Transferases