SH groups oxidation

Each functional group of an amino acid exhibits all of its characteristic chemical reactions. For carboxylic acid groups, these reactions include the formation of esters, amides, and acid anhydrides for amino groups, acylation, amidation, and esterification and for —OH and —SH groups, oxidation and esterification. The most important reaction of amino acids is the formation of a peptide bond (shaded blue). 

Mecfianism of Action The mechanism of action is not fully understood. Monobenzone maybe converted to hydroquinone, which inhibits the enzymatic oxidation of tyrosine to DOPA it may have a direct action on tyrosinase, or it may act as an antioxidant to prevent SH-group oxidation so that more SH groups are available to inhibit tyrosinase. Therapeutic Effect Depigmentation in extensive vitiligo. 

As for the stability of the molecules to use, the product of the -SH group oxidation depends on the experimental conditimis different species containing oxygen have been reported, all of them weakly adsorbed on metal surfaces. In addition, thiols are easily oxidatively destroyed by OH radicals [74]. 

Fig. 4 Comparison of the dynamic viscosity ( r) ) of a chitosan-TGA/H202 system at pH 6.0 and 37°C. The molar ratios of SH groups oxidizing agent were 1 0.28, 1 0.63, and 1 1.9 and are represented by white, black, and gray bars, respectively. The means SD of three experiments are shown (Adopted from [42])...

All the individual steps are catalyzed by enzymes NAD" (Section 15 11) is required as an oxidizing agent and coenzyme A (Figure 26 16) is the acetyl group acceptor Coen zyme A is a thiol its chain terminates m a sulfhydryl (—SH) group Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A... 

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. 

CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O Disulfide bonds form between the side chains of two cysfeine residues. Two SH groups from cysteine residues, which may be in different parts of the amino acid sequence but adjacent in the three-dimensional structure, are oxidized to form one S-S (disulfide) group. 

FIGURE 5.18 Methods for cleavage of disulfide bonds in proteins, (a) Oxidative cleavage by reaction with performic acid, (b) Reductive cleavage with snlfliydryl compounds. Disulfide bridges can be broken by reduction of the S—S link with snlfliydryl agents such as 2-mercaptoethanol or dithiothreitol. Because reaction between the newly reduced —SH groups to re-establish disulfide bonds is a likelihood, S—S reduction must be followed by —SH modification (1) alkylation with iodoac-etate (ICH,COOH) or (2) modification with 3-bromopropylamine (Br— (CH,)3—NH,). 

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

Pihl, A., and Lange, R. (1962) The interaction of oxidized glutathione, cystamine mono-sulfoxide, and tetrathionate with the -SH groups of rabbit muscle D-glyceraldehyde 3-phosphate./. Biol. Chem. 237, 1356-1362. 

The determination is done by oxidation of the -SH group with 0.1N iodine solution in NaHCC>3 solution visually with respect to strength or potentiometrically (the equivalence point is difficult to recognize) or in NaOH solution potentiometrically, which is more favorable. The error of the method for a weight of lOmg amounts is approximately 0.5%. 

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