BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 27. PROTEIN SYNTHESIS

Conclusion

Protein Synthesis (translation) relies on the coordinated interplay of over 100 macromolecules, which, alongside Ribosomes, include mRNA, tRNA, activating Enzymes, and protein factors. Protein synthesis begins with the activation of Amino Acids by Aminoacyl-tRNA synthetases (activating enzymes) at the expense of ATP energy. Synthetases link the carboxyl group of an amino acid to either the 2'- or 3'-hydroxyl group of the adenosine residue at the 3'-end of the tRNA. There is at least one activating enzyme for each amino acid. Furthermore, there is at least one specific tRNA for each amino acid. All Transfer RNAs, despite their varying specificities, share a common structural blueprint. They consist of single RNA chains approximately 80 NUCLEOTIDES in length, containing several modified (e.g., methylated) derivatives of standard bases. The base sequences of all known tRNAs can be folded into a cloverleaf conformation in which roughly half of the nucleotides are base-paired. X-ray crystallographic studies have demonstrated that the tRNA molecule adopts an L-shaped tertiary Structure. At one end of the L-shaped structure lies the 3'-terminal CCA sequence, which serves as The amino acid attachment site, while at the other end, roughly 80Å away, is the anticodon. Messenger RNA recognizes the tRNA anticodon rather than the amino acid attached to the tRNA. The mRNA codon forms Base Pairs with the tRNA anticodon. Certain tRNAs can recognize more than one codon because base pairing at the third codon position is less stringent than at the other two (The Wobble Hypothesis). Protein synthesis takes place on ribosomes, which are composed of large and small subunits. In each subunit, roughly two-thirds of the mass is attributable to RNA and one-third to protein. The 70S ribosome of E. coli (with a mass of 2500 kDa) is composed of 30S and 50S subparticles.

Protein synthesis proceeds through three distinct stages termed initiation, elongation, and termination, respectively. Messenger RNA, formylmethionyl-tRNA, and the 30S ribosomal subparticle assemble to form the 30S initiation complex. The signal for Translation initiation is an AUG (or GUG) codon preceded by a purine-rich sequence capable of base-pairing with 16S rRNA. Subsequently, the 50S ribosomal subparticle joins this complex, yielding the 70S initiation complex, which is primed for the next stage. The elongation cycle comprises aminoacyl-tRNA binding (codon recognition), peptide bond formation, and translocation. Chain growth proceeds in the N- to C-terminal direction. Protein synthesis is terminated by release factors that recognize the stop codons UAA, UGA, and UAG, triggering the Hydrolysis of the bond between the polypeptide and the tRNA. GTP hydrolysis occurs during The formation of the 70S initiation complex, the binding of aminoacyl-tRNA to the ribosome, and at the translocation step. Various Stages of Protein Synthesis are selectively inhibited by toxins and Antibiotics. Peptide antibiotics and other short Polypeptides are synthesized without the involvement of ribosomes, through a mechanism reminiscent of fatty acid synthesis.