Cystine residues ribonuclease

Sokolovsky, M., and A. Patchornik Nonenzymatic cleavage of peptide bonds The hall cystine residues in bovine pancreatic ribonuclease. J. Amer. chem. Soc. 86, 1859—1860 (1964). 

In a classic study on bovine pancreatic ribonuclease A at 90°C and pH conditions relevant for catalysis, irreversible deactivation behavior was found to be a function of pH (Zale, 1986) at pH 4, enzyme inactivation is caused mainly by hydrolysis of peptide bonds at aspartic acid residues as well as deamidation of asparagine and/or glutamine residues, whereas at pH 6-8, enzyme inactivation is caused mainly by thiol-disulfide interchange but also by fi-elimination of cystine residues, and deamidation of asparagine and/or glutamine residues. 

Bovine ribonuclease contains eight half-cystine residues which are... 

Governing the Pairing of Half-cystine Residues in Ribonuclease. 

Luse and M(iLaren (1963) have reviewed published research on the photolysis products and quantum yields tor the destruction of amino acids and have attributed the photochemical inactivation of the enzymes chymo-trypsin, lysozyme, ribonuclease, and trypsin by UV light at 254 m i primarily to destruction of the cystyl and tryptophyl residues. The destruction of these residues in proteins was suggested to be a function of the product of the number of residues present, the molecular extinction coefficient, and the quantum yield for destruction of each residue. Cysteine and tryptamine were identified among the irradiation products from cystine and tryptophan, respectively. Tyrosine, histidine, and phenylalanine were also shown to be degraded by UV, histidine yielding histamine, urocanic acid, and other imidazole derivatives, and phenylalanine yielding tyrosine and dihydroxyphenylalanine. Destruction of these three amino acids was not considered to contribute appreciably to the enzyme inactivation. 

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

The cited evidence for the B-elimination mechanism leading to dehydroalanine formation merits further comment. Nashef et al. (41) report that alkali-treatment of lysozyme ribonuclease and several other proteins resulted in loss of cystine and lysine residues and the appearance of new amino acids lysinoalanine, lanthionine, and B-aminoalanine. Alkali-treatment of the proteins induced an increase in absorbance at 241 nm, presumably from the formation of dehydroalanine residues. The dehydroalanine side chain can participate in nucleophilic addition reactions with the e-NH2 group of lysine to form lysinoalanine, with the SH groups of cysteine to form lanthionine, and with ammonia to form B-aminoalanine. 

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-... 

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