Molecular Biology: Protein Structure and Functions - Stepanov V.M. 2005

Peptides
Natural peptides

Peptides in natural sources do not merely appear as intermediates of protein degradation; they frequently play an independent physiological role.

Peptide Antibiotics are predominantly synthesized by microorganisms via a specialized non-ribosomal mechanism and contain A number of non-protein Amino Acids as well as D-isomers. Many of them are cyclopeptides. Among such antibiotics, the cyclodecadepsipeptide gramicidin S deserves mention as a broad-spectrum antimicrobial agent:

Attention should be drawn to the presence of the non-protein amino acid Ornithine and the D-isomer of phenylalanine within the molecule. The arrows indicate the direction of the peptide bonds in the cycle.

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Another effective immunosuppressant is cyclosporin, a cyclic peptide antibiotic containing N-methylated and other non-protein amino acids (an unsaturated hydroxy amino acid, $\alpha$-aminobutyric acid, and a D-Alanine residue)

Recently, peptide antibiotics produced by animal Cells, particularly lymphocytes, have been discovered. They are synthesized via the standard pathway of METABOLISM/35.html">Protein Biosynthesis.

Peptide Hormones, and regulatory peptides in a broader sense, play a vital role in controlling metabolic processes during Organism development and signal Transduction. These include a number of short peptides, such as oxytocin, which stimulates uterine contractions and Lactation:

Vasopressin is structured similarly; this nonapeptide suppresses diuresis and elevates Blood pressure. Adrenocorticotropic hormone (ACTH), whose peptide chain consists of 39 amino acid residues, regulates Adrenal gland function and can influence a wide range of processes, including motivation, learning, and behavior. a-Melanocyte-stimulating hormone CH₃CO—Ser—Tyr—Ser-Met—Glu—His—Phe—Arg—Trp—Gly—Lys—Pro—Val—NH₂ controls melanin production in vertebrate pigment cells, but also affects Nervous system function, behavioral responses, and fetal development.

It is worth noting that the functions of peptide hormones are generally pleiotropic, and specific sequence regions responsible for distinct types of biological activity can often be identified within them. Peptide hormones are characterized by post-translational modification processes: in the case of a-melanocyte-stimulating hormone, these include Acetylation of the a-NН₂ group and amidation of the a-carboxyl.

The biosynthesis of peptide hormones and other regulatory peptides proceeds similarly to Protein Synthesis. Many physiologically active peptides, especially short ones, are synthesized as much longer polypeptide precursors. These precursors then undergo Limited proteolysis by specific Enzymes which, targeting strictly defined bonds, release the active peptides. For instance, enkephalins—peptides that interact with the same receptors as morphine and act as natural analgesics—are synthesized as a precursor whose peptide chain contains the enkephalin sequence Tyr—Gly—Gly—Phe—Leu or Tyr—Gly—Gly—Phe—Met repeated multiple times. These sequences are preceded and followed by Arg—Arg pairs, which serve as a signal for a specific proteinase that cleaves the peptide bond following such a pair of Arginine residues:

Both arginine residues are subsequently cleaved off by a specific carboxypeptidase, releasing the enkephalin.

It often turns out that peptides can mimic The behavior of corresponding protein fragments in their interactions with other biological molecules, particularly other Proteins such as receptors or structural molecules. For example, the pentapeptide fragment of the Connective Tissue protein Laminin, with the sequence Tyr—Ile—Gly—Ser—Arg, is responsible for Cell Adhesion to this protein. A pentapeptide with the same Structure is capable of inhibiting this process, apparently by blocking cell surface receptors involved in the interaction with laminin.