BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 27. PROTEIN SYNTHESIS

27.22. Some Short Peptides Are Synthesized without Ribosomes

We now turn to another mechanism of peptide bond formation in biological systems. As an example, let us consider The Biosynthesis of gramicidin S, a cyclic peptide antibiotic consisting of two identical pentapeptides joined HEAD-to-tail (Fig. 27.25). This antibiotic is produced by certain strains of the spore-forming bacterium Bacillus brevis.

Class="center">Fig. 27.25. Amino Acid Sequence of gramicidin S, a cyclic peptide composed of two identical pentapeptide units

The biosynthesis of gramicidin S proceeds in the absence of Ribosomes or mRNA. It requires a much simpler synthetic apparatus consisting of only two Enzymes, E_I and E_II. Enzyme E_II (mass 100 kDa) activates phenylalanine, whereas the remaining four Amino Acids of the pentapeptide moiety are activated by E_I (mass 180 kDa). In addition, both enzymes participate in peptide bond formation.

In this system, Amino acids are activated through The formation of enzyme-bound thioesters. Instead of the 3'-terminal hydroxyl group of tRNA, the activated amino acids are attached to The sulfhydryl groups of E_I and E_II:

When L-Proline, L-valine, L-Ornithine, and L-leucine are incubated with E_I in the presence of ATP, they form thioester bonds with specific sulfhydryl groups of E_I. Similarly, D-phenylalanine in the presence of ATP forms a thioester bond with E_II.

Peptide Synthesis in this system is initiated by the interaction of E_I and E_II. The D-phenylalanine residue attached to E_II is transferred to the imino group of the L-proline residue attached to E_I, yielding a dipeptide. Subsequent reactions involve E_I alone. The activated carbonyl group of the proline residue within the dipeptide reacts with the amino group of the valine residue attached to the same enzyme to yield a tripeptide. This process is repeated with the participation of ornithine and then leucine, ultimately resulting in an enzyme-bound pentapeptide. With the formation of each new peptide bond, the growing peptide is transferred to a fresh sulfhydryl group. Finally, the activated pentapeptides attached to two different E_I molecules react with each other to form cyclic gramicidin S.

Two features of this biosynthetic pathway deserve note.

1. The amino acid sequence of gramicidin S is determined by the spatial Organization and Specificity of enzymes E_I and E_II. At least one protein subunit is required per peptide bond. Consequently, this mode of synthesis is less economical than the ribosomal mechanism. For this reason, Peptides containing more than roughly 15 residues are not synthesized by this pathway.

2. The synthesis of gramicidin S resembles fatty acid synthesis in that thioesters serve as the activated intermediates in both processes. Furthermore, E_I contains a covalently linked phosphopantetheine residue. This thiol presumably transfers the growing peptide chain from one site on E_I to the next. Fritz Lipmann suggested that the synthesis of Polypeptide Antibiotics might represent an evolutionary relic, echoing a primitive Protein Synthesis mechanism utilized at the dawn of evolution. The synthesis of ribosomal Proteins may have evolved from fatty acid synthesis.