Cysteine reduction

Screening of over 66,000 compounds from the MLSMR by scientists at the PCMD for inhibitors of Cathepsin B resulted in the identification and characterization of an alternate substrate, SID 16952359 [29]. This study also describes issues relating to the nucleophilicity of dithiothreitol (DTT) and cysteine, reductants frequently used in HTS protocols, and the potential for reactivity with electrophilic sites of probe molecules. 

Whatever the mechanism of cysteine reduction, it is evident that, if the above reaction takes place, cystine metabolism approaches cysteine metabolism. This is admitted by Pirie (96) in his scheme to illustrate the oxidation of cystine to sulfate by rat liver slices ... 

HSCH -CHNHj-COjH. Cysteine is a reduction product of cystine. It is the first step in the breakdown of cystine in the body, one molecule of cystine splitting to give two molecules of cysteine. Cysteine is soluble in water but the solution is unstable, and is reoxidized to cystine. 

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

Cysteine is used as a reductant for cold wave treatment in place of thioglycoHc acid. A/-Lauroylarginine ethyl ester [48076-74-0] is appHed as the hydrochloride as a preservative. Urocanic acid [104-98-3] hich. is derived from histidine is used in skin cream as a uv absorber (235). 

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

During the synthesis of peptides that contain 4-methoxybenzyl-protected cysteine residues, sulfoxide formation may occur. These sulfoxides, when treated with HF/ anisole, form thiophenyl ethers that cannot be deprotected therefore, the peptides should be subjected to a reduction step prior to deprotection. ... 

A second kind of covalent bonding in peptides occurs when a disulfide linkage, RS-SR, is formed between two cysteine residues. As we saiv in Section 18.8, a disulfide is formed by mild oxidation of a thiol, RSH, and is cleaved by mild reduction. 

The action of a peptidase can be neutralized by an inhibitor. Some inhibitors are very broad in their action and are capable of inhibiting many different peptidases, including peptidases of different catalytic types. Some inhibitors are assumed to be specific for a particular catalytic type, but can inhibit peptidases of different types. Leupeptin, for example, is widely used as an inhibitor of serine peptidases from family SI, but it is also known to inhibit cysteine peptidases from family Cl. Cysteine pqrtidase inhibitors such as iodoacetic acid interact with the thiol of the catalytic cysteine. However, this reduction can occur on any thiol group and can affect other, predominantly intracellular, peptidases with a thiol dependency. One example is thimet oligopepti-dase. Metal chelators such as EDTA can inhibit meta-llopeptidases, but can also affect peptidases that have a requirement for metal ions that is indq>endent of their catalytic activity, such as the calcium-dependent cysteine endopqrtidase calpain 1. 

Homocitrate is bound to the molybdenum atom by its 2-carboxy and 2-hydroxy groups and projects down from the molybdenum atom of the cofactor toward the P clusters. This end of FeMoco is surrounded by several water molecules (5, 7), which has led to the suggestion that homocitrate might be involved in proton donation to the active site for substrate reduction. In contrast, the cysteine-ligated end of FeMoco is virtually anhydrous. 

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

Clusters Fa and Fb of photosystem I from cyanobacteria and chloro-plasts are distinguished by their EPR signatures (26, 27) and their reduction potentials (-520 mV for Fa and -580 mV for Fb Ref. (28). The assignment of cysteines in the primary sequence as ligands to individual clusters has been achieved by site-specific mutagenesis (29, Fig. 3), and structural information with regard to the environment of both clusters has been obtained by NMR (24). 

Using a combination of techniques such as EPR, resonance Raman, and MCD spectroscopy, the conversion of [2Fe-2S] into [4Fe—4S] centers has been found to take place under reducing conditions in E. coli biotin synthase 281). The as-prepared form of this enzyme has been thought to contain one [2Fe-2S] center per monomer, coordinated by the three cysteine residues of the motif Cys-X3-Cys-X2-Cys and by a fourth, noncysteinyl ligand. Upon reduction, a [4Fe-4S] cluster bridging two monomers may be formed in the active enzyme. In the reduced state, the [4Fe-4S] center is characterized by a mixture of S = I and S = k spin states giving EPR features at g 5.6 and... 

Chiral tricyclic fused pyrrolidines 29a-c and piperidines 29d-g have been synthesized starting from L-serine, L-threonine, and L-cysteine taking advantage of the INOC strategy (Scheme 4) [19]. L-Serine (23 a) and L-threonine (23 b) were protected as stable oxazolidin-2-ones 24a and 24b, respectively. Analogously, L-cysteine 23 c was converted to thiazolidin-2-one 24 c. Subsequent N-allylation or homoallylation, DIBALH reduction, and oximation afforded the ene-oximes, 27a-g. Conversion of ene-oximes 27a-g to the desired key intermediates, nitrile oxides 28 a-g, provided the isoxazolines 29 a-g. While fused pyrrolidines 29a-c were formed in poor yield (due to dimerization of nitrile oxides) and with moderate stereoselectivity (as a mixture of cis (major) and trans (minor) isomers), corresponding piperidines 29d-g were formed in good yield and excellent stereoselectivity (as exclusively trans isomers, see Table 3). 

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