Ribosomes Are Ribozymes

Ever since the discovery of the role of ribosomes in protein synthesis there has been uncertainty as to the functional roles of the rRNAs and the many ribosomal proteins. Conventional wisdom held that the rRNAs provided the scafolding (structural role) that oriented the ribosomal proteins so that one or more could carry out the enzymatic 

Dissociation of a prokaryotic ribosome. The configuration of two overlapping circles is used throughout this chapter, for the sake of simplicity. 

A crucial feature of the peptidyltransferase reaction is a particular adenine that is conserved in rRNAs extracted from the large ribosomal subunits of thousands of different organisms from all three kingdoms. 8equence analysis shows that this adenine base (2451 A) always is present in the active site of the ribosome and acts as a general acid-base catalyst by deprotonating the nucleophilic amine as shown below. 

The positive charge on 2451A in the tetrahedral intermediate is probably transferred to adjacent tRNA nucleotides that undergo tautomeric shifts. The evidence that ribosomal RNAs perform enzymatic functions critical to protein synthesis lends weight to the evolutionary significance of RNA in primordial chemical reactions that eventually led to the development of cells. 

Every polypeptide has an amino terminus and a carboxyl terminus. In both prokaryotes and eukaryotes, synthesis begins at the amino terminus. For a protein having the sequence HaN-Met-Trp-Asp... Pro-Val-COOH, the f Met (or Met) is the initiating amino acid and Val is the last amino acid added to the chain. Translation of mRNA molecules occurs in the 5 3 direction. 

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