Oxytocin cyclic structure

Beta structures are found in many small peptides. Tire hormone oxytocin (Fig. 2-4), the antibiotics gramicidin S (Fig. 2-4) and valinomycin (Fig. 8-22), and the mushroom peptide antamanide (Box 28-B) are among these. The cyclic structures of these compounds favor formation of antiparallel (3 strands with sharp turns at the ends. Polypeptide antibiotics that have alternating... 

Some important peptides have cyclic structures. Two well-known examples with many structural features in common are oxytocin and vasopressin (Figure 3.13). In each, there is an —S—S— bond similar to that in the oxidized form... 

You can recall that oxytocin is a neurotransmitter that is made in the brain in response to a hug, caress, baby nursing, and so on and so forth (go back to Lecture 1). It contains nine NAAs, and it is not a straight-chain peptide, but rather it forms a cyclic structure due to an S—S bond that is made between two cysteine (Cys/C) residues (see Scheme 5.12b). It is said that S—S bonds in our hair proteins cause curly hair. 

The structures of oxytocin and vasopressin also illustrate the importance of the disulfide linkage between cysteine residues (Section 24.2A) in the overall primary structure of a polypeptide. In these two molecules this disulfide linkage leads to a cyclic structure. 

Syntheses on polymer support are of invaluable merit in the preparation of linear structures of about decapeptide size — e.g., the bradykinins — and of precursors for cyclic structures as in the case of gramicidin S, oxytocin, vasopressin, and the antamanides. 

Structurally unique, potent, and selective oxytocin antagonistic agents characterized by a cyclic hexapeptide structure have recently been isolated from the broth filtrate of a novel Streptomyces silvensis strain [434,435]. As shown in formula (106), these compounds contain two hexahydro-3-pyrid-azinecarboxylic acid moieties. A study has been made of their structural modifications (including oxidation of the reduced pyridazine nuclei) [436]. 

Polarography has been successfully applied to the investigation of structural problems involving sulphur compounds. The presence of a disulphide bond has been established by means of the polarographic reduction waves of cytochrome C (156) and lipoic acid (757), and in cyclic disulphides of the oxytocine and vasopressine type (158). The elucidation of the process responsible for the reduction wave of lipoic acid was carried out by comparison with reduction waves of cyclic disulphides, where the disulphide bond was incorporated into rings of various size. The similarity indicated that in lipoic acid an S—S bond which is a part of a larger cyclic system is reduced. 

The structure of both peptides includes a disulfide bond, a form of covalent bonding in which the - SH groups from two cysteine residues are oxidized to form a sulfur-sulfur bond. In oxytocin and vasopressin, the disulfide bonds make the peptides cyclic. Three-dimensional structures of oxytocin and vasopressin are shown in Figure 28.7. 

The Secondary Structure of the Cyclic Moiety of Deamino Oxytocin... 

D. W. Urry, M. Danishi, R. Walter, Secondary structure of the cyclic moiety of the peptide hormone oxytocin and its deamino analog. Proc. Natl. Acad. Sci. US 66 111-116 (1970) also D.W. Urry, R. Walter, Proposed conformation of oxytocin in solution, ibid. 68 956-958 (1971)... 

Oxytocin and vasopressin are two cyclic nonapeptide hormones produced by the posterior pituitary gland. Oxytocin regulates uterine contraction and lactation and may be administered when it is necessary to induce labor at childbirth. Note that its structure includes two cysteine units Joined by a disulfide bond, and, once again, the C-terminal amino acid is present as the amide. Vasopressin... 

If a rough characterisation of the shape of oxytocin is to be given, this molecule may be said to consist of a large cyclic moiety (6 amino acid residues) and a main peptide side-chain (3 amino acid residues). This general structure has a number of... 

In the uterus, the production of 3, S -cyclic AMP is induced by the activation of the /7-adrenergic receptors and followed by inhibition of the electrical and mechanical activities. The action of oxytocin may be partly due to its ability to prevent the formation of cyclic AMP. The role of the reactive disulfide groups in the peptide receptors has already been discussed. The complexity of the receptor structure is pointed out by the observation that metals potentiate specifically the action of S-S polypeptides on the rat uterus, in such a way that it becomes very sensitive to vasopressin when the perfusion fluid contains a metal The order of metal potency is... 

Oxytocin, which is produced in the pituitary gland, causes the contraction of smooth muscle, including the uterus. It is used clinically to induce labor or to increase the strength of uterine contractions. Vasopressin, another pituitary hormone, regulates the secretion of water by the kidneys and affects blood pressure. The structures of vasopressin and oxytocin differ by only two amino acids. They are cyclic peptides that are hnked by a disulfide bond between two cysteine residues. 

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