Chapter 29:  Protein Synthesis

Genetic information is most important because of the proteins that it encodes, in that proteins play most of the functional roles in cells. In Chapters 27 and 28, we examined how DNA is replicated and how DNA is transcribed into RNA. We now turn to the mechanism of protein synthesis, a process called translation because the four-letter alphabet of nucleic acids is translated into the entirely different twenty-letter alphabet of proteins. Translation is a conceptually more complex process than either replication or transcription, both of which take place within the framework of a common base-pairing language. Befitting its position linking the nucleic acid and protein languages, the process of protein synthesis depends critically on both nucleic acid and protein factors. Protein synthesis takes place on ribosomes—enormous complexes containing three large RNA molecules and more than 50 proteins. One of the great triumphs in biochemistry in recent years has been the determination of the structure of the ribosome and its components so that its function can be examined in atomic detail (Figure 29.1). Perhaps the most significant conclusion from these studies is that the ribosome is a ribozyme; that is, the RNA components play the most fundamental roles. These observations strongly support the notion that the ribosome is a surviving inhabitant of the RNA world. As such, the ribosome is rich in information regarding very early steps in evolution.

Transfer RNA molecules (tRNAs), messenger RNA, and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyl-tRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes implement the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is relatively well understood. Some distinctive features of protein synthesis in eukaryotes also are presented.