Stereochemical control of the ribosomal peptidyltransferase reaction: The role of acceptor substrate amino acid side chain orientation

Abstract

The aminoacyl residue of aa-tRNA is bound by an ester linkage to the 2’or 3’-hydroxyl groups of the ribose residue of the 3’-terminal adenosine [l] and is able to migrate rapidly between these two positions [2]. Thus, the determination of an ‘active’ aa-tRNA isomer in various subreactions of protein synthesis has been a topic of considerable interest. Modified aa-tRNAs, or their 3’-terminal fragments, in which the 3%’ transacylation cannot occur, have been recently used for studies of isomer-specificity of the acceptor site of peptidyltransferase [3]. Most of the studies have concluded that 3’-PhetRNA, is the only acceptor in the peptide bondforming step [3], although a significant acceptor activity was also observed [4] with 2’-Phe-tRNA. The comparison of reaction rates of both isomers was not performed, but it was thought that the 3’Phe-tRNA is probably the preferred substrate [5]. From these experiments important questions have evolved: 1. Is the peptidyltransferase really specific for 3’-aatRNA as acceptor substrate and is this specificity