Cysteine sulfonic acid

Cystine and cysteine also can be oxidized under the same conditions but the analytical methods to detect these oxidation products in proteins have not been developed yet. The different oxidation products of cystine, which have been synthesized (55-58), are cystine disulfoxide (NH2-CH-(COOH)-CH2-SO-SO-CH2-(COOH)-CH-NH2), cystine disul-fone (NH2-CH- (COOH) -CH2-C02-S02-CH2-( COOH )-CH-NH2), cysteine sulfenic acid (NH2-CH-(COOH)-CH2-SOH), cysteine sul-finic acid (NH2-CH-(C00H)-CH2-S02H), and cysteine sulfonic acid or cysteic acid (NH2-CH-(C00H)-CH2-S03H). It is doubtful that all of these derivatives described in 1935 by Bennett are present in proteins. 

The central nervous system has at least three enzymes capable of decar-hoxylating cysteine sulfonic acid, one of which is glutamate decarboxylase. Glutamate and cysteine sulfinic acid are mumaUy competitive. In some brain regions, more than half the total cysteine sulfinic acid decarboxylase activity may be from glutamate decarboxylase. 

The situation may be further complicated by reactions involving C—S fission in the cystine side chains. Thus cysteine sulfonic acid (CyS—SO3H)... 

Cysteinesulfinic Acid. Cysteine is oxidized by enzyme systems present in bacteria and in liver to the corresponding sulfinic acid. It has been suggested that the unstable sulfenic acid is an intermediate in this oxidation. The nature of the reaction that produces cysteinesulfinic acid is not known. The subsequent metabolism of the sulfinic acid may proceed by any of three pathways. One involves further oxidation to cysteine-sulfonic acid, cysteic acid. The enzyme responsible has not been separated from the system responsible for the formation of cysteinesulfinic acid. Cysteinesulfonic acid is decarboxylated to taurine (I) by the decarboxylase mentioned previously (p. 284). 

Cystine can then be further oxidized as described in the section below [19,47,48,52]. It has also been suggested that cysteine is oxidized to RS [48-50], which is subsequently oxidized to RS03 [49,50]. In addition, RSH adsorbed on platinum is thought to be oxidized directly to cysteic acid (cysteine sulfonic acid), RSO3H, [51] ... 

According to the mechanism of translation outlined in the introduction, it is clear that analogues are activated before being incorporated into protein, and compete with the protein amino acid for the aminoacyl-transfer-RNA synthetases. They are afterwards transferred to specific transfer RNAs. When the natural amino-acid analogue has been transferred to the transfer-RNA molecule, it takes no part in determining the specificity of polypeptide synthesis. The incorporation of Ala in place of Cys, after the chemical reduction of Cys-tRNA has been outlined before (see Para. 8.1.2). Similarly Cys-tRNA can be mildly oxidized to CysSOsH-tRNA thereafter cysteine sulfonic acid becomes incorporated into protein. This appears to be a suitable procedure to incorporate an analogue which per se is not specific enough to be incorporated (CysSOsH itself is not incorporated). Such a procedure is, however, essentially restricted to in vitro studies. 

A quantitatively important pathway of cysteine catabolism in animals is oxidation to cysteine sulfinate (Fig. 24-25, reaction z),450 a two-step hydroxyl-ation requiring 02, NADPH or NADH, and Fe2+. Cysteine sulfinic acid can be further oxidized to cyste-ic acid (cysteine sulfonate),454 which can be decarbox-ylated to taurine. The latter is a component of bile salts (Fig. 22-16) and is one of the most abundant free amino acids in human tissues 455-457 Its concentration is high in excitable tissues, and it may be a neurotransmitter (Chapter 30). Taurine may have a special function in retinal photoreceptor cells. It is an essential dietary amino acid for cats, who may die of heart failure in its absence,458 and under some conditions for humans.459 In many marine invertebrates, teleosts, and amphibians taurine serves as a regulator of osmotic pressure, its concentration decreasing in fresh water and increasing in salt water. A similar role has been suggested for taurine in mammalian hearts. A chronically low concentration of Na+ leads to increased taurine.460 Taurine can be reduced to isethionic acid... 

The analysis of methionine and cysteine is problematic. The sulfur containing side chains of these amino acids are prone to oxidation. The standard hydrochloric acid hydrolysis will cause the partial conversion of these amino acids into cystine, cysteine, cysteine sulfinic acid, cysteic acid, methionine, methionine sulfoxide, and methionine sulfone. The classic strategy (79) for dealing with this problem is simply to drive the oxidative process to completion (i.e., convert all the cyst(e)ine to cysteic acid) and then to analyze chromatographically for cysteic acid and/or methionine sulfone. This is traditionally accomplished by a prehydrolysis treatment of the sample with performic acid. While this method has sufficed over the years, the typical recovery (85 -90%) and precision (4% intra- and 15% interlaboratory) have been poor (80). 

Biological oxidation of cysteine can yield, in addition to the disulfide cystine, cysteine sulfinic acid and the sulfonic acid cysteic acid. 

Mixed anhydride 6b is a sulfonylating rather than a phos-phorylating agent. Thus, hydrolysis with 0 gives acid 6c and propylsulfonic acid in which the - -°0 isotope is incorporated only in the sulfonic acid (Eq. 2). Compound 6b reacts with either alcohols (methanol, ethanol, sec-butanol) or L-cysteine to yield acid 6c and with triethylamine to give the anion of 6c. probably via propylsulfene (4) (Eq. 2). 

Scheme 10. Auxiliaries for Native Chemical Ligation without cysteine (TFMSA, trifluoromethane sulfonic acid).

Abz was combined with a broad variety of non-fluorescent acceptors such as p-nitrobenzyl for leucine aminopeptidase (Carmel et al., 1977), pNA for trypsin (Bratanova and Petkov, 1987), 4-ni-trophenylalanine [Phe(NC>2)] for HIV protease (Toth and Marshall, 1990), and V-(2,4-di n itrophenyl) ethylenediamine (EDDnp) for thermolysin and trypsin (Nishino et al., 1992). Lecaille et al. (2003) described a FRET quench assay based on a specific substrate for cathepsin K labeled with Abz and EDDnp. This substrate is not cleaved by the other Cl cysteine cathepsins and serine proteases in contrast to methoxycoumarin (Mca)-based substrates described earlier (Aibe et al., 1996 Xia et al., 1999) and merely covered the non-primed site of the scissile bond. The 5-[(2-aminoethyl)amino] naphthalene-l-sulfonic acid (EDANS) compound is a second example of a fluorescence donor historically used for many FRET quench-based protease assays, e.g., in combination with tryptophan as a quencher in an ECE activity assay (Von Geldren et al., 1991). The FRET-1 example in Table 2.2 shows the typical dynamic range that can be achieved with an EDANS/DABCYL-based assay. 

Cysteic acid is obtained in nearly quantitative yield from cysteine with aqueous hydrogen peroxide in the presence of iron(II) ions.397 Molybdates and tungstates have also been used as effective catalysts for similar transformations.398 An excellent route for the oxidation of 2-thioethanol to isothionic acid has been developed.399 Heteropolyoxometallates supported on alumina400 can also be used to oxidize a range of organo-sulfur compounds. For example, alkyl monosulfides to sulfoxides and sulfones, and thiols to sulfonic acids are a few possibilities (Figure 3.98). 

As shown in Figure 14.7, taurine is a /3-amino sulfonic acid (2-aminoethane sulfonic acid) and can be synthesized from cysteine by three pathways ... 

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