Cross-linking cystine bridges

For the synthesis of heterodimeric cystine peptides where two different peptide chains are cross-linked by a disulfide bridge random co-oxidation of the two chains besides producing the heterodimer leads in the optimal case to the additional two homodimers in statistical distribution. Therefore, chemical control of the disulfide bridging via site-directed disulfide formation techniques is required since a thermodynamic control for generation of heterodimers is extremely difficult to achieve (see Section 6.1.5). 

The absence of half-cystine residues in collagens with chain compositions [ai(I)]2a2 and [ 2(11)13 exclude cystine disulfide bridges from participation in cross-linking in these collagens. However, half-cystine residues have been identified in [ai(III)]3 collagen and disulfide bridges may serve as cross-links in this type of collagen191). 

The second step in thyroid hormone synthesis is the covalent bridging of two different residues of iodinated tyrosine. A dimer is formed, reminiscent of the dimers of cysteine in proteins (the cysteine dimer is called cystine). Only a small fraction of the iodinated tyrosines is bridged in this way. More specifically, only four of the iodinated tyrosines, located at positions 5,2555,2569, and 2748, participate in the reaction. The numbers refer to the amino acid, cormting from the amino terminus of the protein. Thyroglobulin has 2748 amino acids. The first and second steps are catalyzed by thyroperoxidase, a heme protein. It requires hydrogen peroxide for activity. To summarize, thyroperoxidase catalyzes the attachment of iodine atoms to residues of tyrosine as well as the subsequent cross-linking of the iodinated tyrosine residues. 

Prolonged boiling with HCI hydrolyses the peptide linkages (shown as thick bonds) and breaks the -.air down into the constituent amino acids. These crystallize as the zwitterions at about neutral pHs and the mixture of NaOAc and HCI provides a buffer solution. Hair is much cross-linked by disulfide bridges and these do not break down under the hydrolysis conditions so intact cystine emerges. 

For star polymers, an increase in the complexity of the matrix schemes may be required with rectangular matrices for branch points. This is justified if the property of interest is concerned with the conformations of the bridge itself, as for example with the optical activity of the cystine residue disulphide bond, or when interest is directed to the deviations from random flight statistics shown by short-branched stars of homo- and sequential copoly-peptides and by disordered proteins cross-linked by a disulphide bond. Simpler schemes are sufficient for the study of the large effect of helix-coil transition upon the charac-... 

Photochemistry of proteins is derived from diose of its amino acid residues. In addition, important photochemical processes in proteins involve the splitting of the disulfide bridges. Furthermore, DNA-protein cross-linking involves the addition of thymine in DNA with cystine in protein. This addition process has been discussed above. In the case of DNA, UV irradiation can also lead to the breaking of either one or both DNA strand and infra- and inter-molecular DNA cross-linking. The photochemistry of RNA is very similar to that of DNA. 

Crystalline pancreatic ribonuclease has a molecular weight of 14,000 and consists of 124 amino acid residues in a single chain, cross-linked by four cystine disulfide bridges (161, 168). The enzyme can be inactivated by photooxidation, presumably by destruction of histidine residues in ri-... 

Some proteins are built up of two or more polypeptide chains held together by a stable bond other than the peptide link. Such a bond will be referred to as a cross linkage. The only one that is definitely known to exist is the —S—S— bridge of cystine. Here two cysteine residues are joined together through their side chains. 

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