title: Reactions of mercaptans with cytochrome c oxidase and cytochrome c.
abstract: 1. The steady-state oxidation of ferrocytochrome c by dioxygen catalyzed by cytochrome c oxidase, is inhibited non-competitively towards cytochrome c by methanethiol, ethanethiol, 1-propanethiol and 1-butanethiol with Ki values of 4.5, 91, 200 and 330 microM, respectively. 2. The inhibition constant Ki of ethanethiol is found to be constant between pH 5 and 8, which suggests that only the neutral form of the thiol inhibits the enzyme. 3. The absorption spectrum of oxidized cytochrome c oxidase in the Soret region shows rapid absorbance changes upon addition of ethanethiol to the enzyme. This process is followed by a very slow reduction of the enzyme. The fast reaction, which represents a binding reaction of ethanethiol to cytochrome c oxidase, has a k1 of 33 M-1 . s-1 and a dissociation constant Kd of 3.9 mM. 4. Ethanethiol induces fast spectral changes in the absorption spectrum of cytochrome c, which are followed by a very slow reduction of the heme. The rate constant for the fast ethanethiol reaction representing a bimolecular binding step is 50 M-1 . s-1 and the dissociation constant is about 2 mM. Addition of up to 25 mM ethanethiol to ferrocytochrome c does not cause spectral changes. 5. EPR (electron paramagnetic resonance) spectra of cytochrome c oxidase, incubated with methanethiol or ethanethiol in the presence of cytochrome c and ascorbate, show the formation of low-spin cytochrome alpha 3-mercaptide compounds with g values of 2.39, 2.23, 1.93 and of 2.43, 2.24, 1.91, respectively.
mesh_terms: D000818:Animals; D001205:Ascorbic Acid; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism*; D003576:Electron Transport Complex IV / Q000378:metabolism*; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009206:Myocardium / Q000201:enzymology; D011485:Protein Binding; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D013438:Sulfhydryl Compounds / Q000494:pharmacology
chemical_list: D003574:Cytochrome c Group; D013438:Sulfhydryl Compounds; D003576:Electron Transport Complex IV; D001205:Ascorbic Acid