Sulfhydryl amino acids

Note that while sulfhydryl amino acids oceui in the intracellular protein myosin, tliey arc probably involved in enzymic activity rather than di.sulfide bonds (Dreizen and Gershman, 1970). 

Assimilatory sulfate reducers reduce sulfate to the sulfhydryl level where it is incorporated into the sulfur amino acids of protein. Many plants and bacteria can do this. 

The amino acid residues in neurotoxins which are important for neurotoxic action are still not entirely clarified. Some neurotoxins contain one free SH group, while others do not. From this fact, it would be logical to assume the sulfhydryl group is not essential. This was actually proven to be the case. 

Humans may be exposed to hydrogen sulfide both from its endogenous production or from exogenous sources. Most endogenous production apparently results from the metabolism of sulfhydryl-containing amino acids, e.g., cysteine, by bacteria present in both the intestinal tract and the mouth (Beauchamp et al. 1994 Tonzetich and Carpenter 1971) however, it is also produced in the brain and several smooth muscles, e.g., thoraic aorta, by enzymes found in these tissues (Abe and Kimura 1996 Hosoki et al. 1997). 

Aryl halide compounds such as fluorobenzene derivatives can be used to form covalent bonds with amine-containing molecules like proteins. The reactivity of aryl halides, however, is not totally specific for amines. Other nucleophiles such as thiol, imidazolyl, and phenolate groups of amino acid side chains also can react (Zahn and Meinhoffer, 1958). Conjugates formed with sulfhydryl groups are reversible by cleaving with an excess of thiol (Shaltiel, 1967). 

The iodoacetyl group of both isomers reacts with sulfhydryls under slightly alkaline conditions to yield stable thioether linkages (Figure 9.7). They do not react with unreduced disulfides in cystine residues or with oxidized glutathione (Gorman et al., 1987). The thioether bonds will be hydrolyzed under conditions necessary for complete protein hydrolysis prior to amino acid analysis. 

FIGURE 3.22 Protection (A) of carboxyl groups of amino acids as ferf-butyl esters,58 (B) of carboxyl groups of /V-substituted amino acids as fert-butyl esters,61 and (C) of phenolic and sulfhydryl groups as ethers. The amino acid esters are isolated as the hydrochlorides. 

FIGURE 5.22 (A) Reaction of an Fmoc-amino acid with 2-chlorotrityl chloride resin.56 The ester bond formed is cleavable by the mild acid, which does not affect tert-butyl-based protectors. (B) Generation of a protected peptide containing cystine by detachment of a chain, deprotection of cysteine residues, and oxidation of the sulfhydryls by the reagent containing iodine. The cations produced are trapped by CF3CH2OH. 

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