Reactive Sulfur Species:  Kinetics and Mechanisms of the Oxidation of Cysteine by Hypohalous Acid to Give Cysteine Sulfenic Acid

Abstract

Cysteine sulfenic acid has been generated in alkaline aqueous solution by oxidation of cysteine with hypohalous acid (HOX, X = Cl or Br). The kinetics and mechanisms of the oxidation reaction and the subsequent reactions of cysteine sulfenic acid have been studied by stopped-flow spectrophotometry between pH 10 and 14. Two reaction pathways were observed: (1) below pH 12, the condensation of two sulfenic acids to give cysteine thiosulfinate ester followed by the nucleophilic attack of cysteinate on cysteine thiosulfinate ester and (2) above pH 10, a pH-dependent fast equilibrium protonation of cysteine sulfenate that is followed by rate-limiting comproportionation of cysteine sulfenic acid with cysteinate to give cystine. The observation of the first reaction suggests that the condensation of cysteine sulfenic acid to give cysteine thiosulfinate ester can be competitive with the reaction of cysteine sulfenic acid with cysteine.

Abiotic methanogenesis from organosulphur compounds under ambient conditions

“…Two additional products having accurate mass values and adducts consistent with empirical formulae C 4 H 9 NO 3 S and C 4 H 8 O 4 S were identified by LC-TOFMS. Although these formulae are compatible with structures such as the previously unreported homocysteine sulphenic acid and 2-hydroxy-4-sulphenobutanoic acid; sulphenic acids are normally found to be highly reactive transient compounds that are difficult to detect and isolate unless stabilized by steric hindrance 44 or intramolecular H-bonding involving a seven-membered ring 45 . However, in view of the known instability of most sulphenic acids 44 , much stronger evidence will be required before the structures for the minor products can be confirmed and thus they are not shown as decay products in Fig.…”

Model for the Exceptional Reactivity of Peroxiredoxins 2 and 3 with Hydrogen Peroxide

“…Both methods gave similar rate constants, suggesting that Reaction 1 (between H 2 O 2 and C p ) was being measured, and not Reaction 2 (which is expected to be rapid) (19,20). A potential complication is that the Prx dimer bands on the gels represent both the single and double disulfide-bonded isoforms of the protein.…”

How does the protein environment optimize the thermodynamics of thiol sulfenylation? Insights from model systems to QM/MM calculations on human 2-Cys peroxiredoxin

“…The RSOH pKa is expected to be lower than the pK a of the corresponding thiol [4] . In aqueous solution, the pKa for the sulfenyl group of CysOH is found to be between 6 and 10 [29] . Since the sulfenic acid pK a is determined to be 6.1 in Salmonella typhimurium AhpC [30] , and 6.6 in Mycobacterium tuberculosis AhpE [31] , and given the hydrogen bond network around Cys51-SO -, the Cys51…”