27.7 Peptides 4

Peptides Peptides are compounds in which an amide bond links the amino group of one a-amino acid and the carboxyl group of another. An amide bond of this type is often referred to as a peptide bond.

Alanine and Glycine CH3 O C + H – H3N O C H H3N + –

Alanylglycine CH3 O C H3N + H N – Two a-amino acids are joined by a peptide bond in alanylglycine. It is a dipeptide.

Alanylglycine CH3 O C H3N + H N – N-terminus C-terminus Ala—Gly AG

Alanylglycine and glycylalanine are constitutional isomers CH3 O C H3N + H N – Alanylglycine Ala—Gly AG H O C H3N + N CH3 – Glycylalanine Gly—Ala GA

Alanylglycine CH3 O C H3N + H N – The peptide bond is characterized by a planar geometry.

Higher Peptides Peptides are classified according to the number of amino acids linked together. dipeptides, tripeptides, tetrapeptides, etc. Leucine enkephalin is an example of a pentapeptide.

Tyr—Gly—Gly—Phe—Leu YGGFL Leucine Enkephalin Tyr—Gly—Gly—Phe—Leu YGGFL

Oxytocin is a cyclic nonapeptide. Ile—Gln—Asn Tyr Cys S Cys—Pro—Leu—GlyNH2 1 2 3 4 5 6 7 8 9 C-terminus N-terminus Oxytocin is a cyclic nonapeptide. Instead of having its amino acids linked in an extended chain, two cysteine residues are joined by an S—S bond.

Oxytocin S—S bond An S—S bond between two cysteines is often referred to as a disulfide bridge.