Cysteine sulfur, oxidation

For cysteine sulfur oxidation to taurine, it is curious that we must state that to the best of our knowledge there are no direct experimental facts concerning such an oxidation in vivo. However, the experiments in vitro wdth enzyme preparations make the oxidation of cysteine sulfur to taurine in animals more probable, as we shall see later on. 

After exposure to an oxidant, the potential types of oxidation products in proteins and peptides can be extensive (Stadtman and Fevine, 2000). Cysteine and methionine undergo a variety of sulfur oxidation reactions to yield cysteine disulfides, methionine sulfoxide, methionine... 

NO reacts very efficiently with superoxide to form peroxynitrite (ONOO-), a highly reactive oxidant that leads to DNA damage, nitration of tyrosine, and oxidation of cysteine to disulfides or to various sulfur oxides (SOx). Several cellular... 

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

In oxidized rubredoxin there is a pseudotetrahedral iron(III) coordinated to four cysteine sulfurs. The cysteine protons are broad beyond detection. The PC2H2 signals of 2H labeled cysteines are located far downfield (300-900 ppm) (Figs. 5.14 and 5.15) [30], which correspond to hyperfine coupling constants of about 1-3 MHz for protons. Two of the four 2Ha protons appear downfield (180 and 150 ppm), while the other two appear upfield (—10 ppm, overlapped). Either... 

Iron-sulfur proteins contain non-heme iron and inorganic (acid-labile) sulfur in their active centers as 4Fe-4S or 2Fe-2S or, in the case of rubredoxin, as one iron alone. The iron is always bonded to cysteine sulfur. They catalyze redox reactions between +350 and —600 mV (hydrogen electrode = —420 mV). They are usually of low molecular weight (6000-15,000 Daltons) but can form complex enzymes with molybdenum and flavin. They occur as soluble or membrane-bound proteins and catalyze key reactions in photosynthesis, oxidative phosphorylation, nitrogen fixation, H2 metabolism, steroid hydroxylation, carbon and sulfur metabolism, etc. They occur in all organisms so far investigated and may... 

Recent kinetics studies on protonation of [Ni(SEt)((Ph2 PCH2CH2)2PPh)]+ (14) proposed that the proton interacts with both the nickel and sulfur sites,consistent with the proposal of intramolecular proton transfer between cysteinate sulfur and Ni atom in the Ni-based hydrogenases. Additionally, the mononuclear complex [Ni(psnet)]+ (15) of known structure and a mildly negative redox potential can stoichiometrically evolve H2 from protic sources. On the basis of kinetics analysis, the reaction paths considered most probable involve steps of protic oxidative addition to Ni(I) to generate Ni -H , and electron transfer to Ni(III) followed... 

Aconitase is an iron-sulfur protein, or nonheme iron protein. It contains four iron atoms that are not incorporated as part of a heme group. The four iron atoms are complexed to four inorganic sulfides and three cysteine sulfur atoms, leaving one iron atom available to bind citrate and then isocitrate through their carboxylate and hydroxyl groups (Figure 17,12). This iron center, in conjunction with other groups on the enzyme, facilitates the dehydration and rehydration reactions. We will consider the role of these iron-sulfur clusters in the electron-transfer reactions of oxidative phosphorylation subsequently (Section 18.3.1). 

Oakes RS, Clilford AA, Bartle KD, Petti MT, Rayner CM. Sulfur oxidation in supercritical carbon dioxide dramatic pressure dependant enhancement of diastereoselectivity for sulfoxidation of cysteine derivatives. Chem Commun 1999 247-248. 

The known occurrences of thioaldehydes in biochemistry are few. One well-studied example is the involvement of a thioaldehyde in the decarboxylation of cysteine in phosphopantothenoyl-cysteine during coenzyme A biosynthesis. In the proposed mechanism for this decarboxylation, a thioaldehyde is generated at the cysteine sulfur by a flavin-dependent oxidation of the thiol. The resulting /3-thioketo acid, acting like a /3-keto acid, facilitates the decarboxylation of the amide-bound cysteine in phosphopantothenoyl-cysteine substrate. Finally, the flavinH2 produced in the thiol oxidation is used to reduce the thioaldehyde back to the thiol. 

Rubredoxin. This sort of substance was first isolated from C.pasteurianum where it appears to participate in a number of biological reactions in which ferredoxin is also active. It contains only one iron atom, four cysteine units, no inorganic sulfur, and has a molecular weight of about 6,000. Its structure in the oxidized form has been investigated in some detail by X-ray crystallography.33 The iron atom is tetrahedrally coordinated by the four cysteine-sulfur atoms, as shown in Fig. 25-E-4. The tetrahedron is markedly distorted. The Fe—S distances, which should be reliable to within less than 0.05 A, are 2.39, 2.33, 2.31 and 1.97 A the angles range between 101° and 118°. 

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