Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Protein synthesis proceeds in five main stages

Today we know that The process of Protein Synthesis proceeds in five main stages, each requiring a specific set of components. Table 29-1 lists the components required for Protein synthesis in E. coli and other prokaryotes. Protein synthesis in Eukaryotic Cells follows essentially the same scheme, differing only in minor details from that in prokaryotes. A brief Description of the stages of METABOLISM/35.html">Protein Biosynthesis is given below.

Class="center">Table 29-1. Components Required for the Five Principal Stages of Polypeptide Synthesis in E. coli

a. Stage 1: amino acid activation

At this stage, which takes place in the Cytosol rather than on the ribosome, each of the 20 amino acids is covalently attached to a specific tRNA at the expense of ATP energy. These reactions are catalyzed by a group of Mg²⁺-dependent activating enzymes, each specific for a single Amino Acid and its corresponding tRNA.

b. Stage 2: polypeptide chain initiation

At this stage, the mRNA encoding a given polypeptide binds to the small ribosomal subunit, followed by the binding of The initiating amino acid attached to its cognate tRNA, thereby forming the initiation complex. The tRNA carrying the initiating amino acid interacts via complementary base pairing with a specific triplet, or codon, within the mRNA that signals THE START OF the polypeptide chain. This process, which requires guanosine triphosphate (GTP), is facilitated by three cytosolic Proteins known as initiation factors.

c. Stage 3: elongation

The polypeptide chain is subsequently extended by the sequential covalent addition of amino acids, each delivered to the ribosome and incorporated into its designated position by a specific tRNA that forms complementary Base Pairs with the corresponding codon in the mRNA. Elongation is mediated by cytosolic proteins called elongation factors. The energy derived from the Hydrolysis of two GTP molecules is consumed for each incoming aminoacyl-tRNA binding and for the translocation of the ribosome along the mRNA by one codon, i.e., for the extension of the growing polypeptide by a single residue.

d. Stage 4: termination and release

Upon completion of the Polypeptide chain synthesis, signaled by a termination codon in the mRNA, the polypeptide is released from the ribosome with the participation of specialized release factors, or termination factors.

e. Stage 5: polypeptide chain folding and processing

To assume its native biologically active conformation (Section 8.1), the polypeptide must fold into a specific three-dimensional Structure. Before or after folding, the newly synthesized polypeptide may undergo enzymatic processing, which involves the removal of initiating amino acids, the Cleavage of extra amino acid residues, the Introduction of phosphate, methyl, carboxyl, or other groups into specific amino acid residues, as well as the attachment of Oligosaccharides or prosthetic groups.

Let us now examine each of these stages in greater detail.