Sulfhydryl linkages

Sulfhydryl Groups. Sulfhydryl linkages are considered to be important to the structural integrity of membrane proteins. 

The keratinization of epithelial structures has long been recognized as a feature of vitamin A deficiency, but the biochemical and ultrastructural details of this lesion are poorly understood. Keratinization implies accumulation of keratin, an insoluble protein rich in cystine sulfhydryl linkages. The incorporation of methionine- S into the skin of vitamin A deficient rats is markedly increased (173), and the incorporation of inorganic S04, presumably into mucopolysaccharides, is decreased. There is also evidence for an increased conversion of cystine into inorganic sulfate (174), and the total cystine content of skin is depressed (175). 

In the presence of sulfide or sulfhydryl anions, the quinonemethide is attacked and a benzyl thiol formed. The P-aryl ether linkage to the next phenylpropane unit is broken down as a result of neighboring-group attack by the sulfur, eliminating the aryloxy group which becomes reactive phenolate ion (eq. 2). If sulfide is not present, a principal reaction is the formation of the stable aryl enol ether, ArCH=CHOAr. A smaller amount of this product also forms in the presence of sulfhydryl anion. 

Figure 1.10 Sulfhydryl groups may undergo a number of additional reactions, including acylation and alkylation. Thiols also may participate in redox reactions, which generate reversible disulfide linkages.

Figure 1.69 SAMSA is an anhydride compound containing a protected thiol. Reaction with protein amine groups yields amide bond linkages. Deprotection of the acetylated thiol produces free sulfhydryl groups for conjugation.

Figure 1.121 Sodium tetrathionate reacts with thiols to form reactive sulfenylthiosulfate intermediates. Another sulfhydryl-containing molecule may couple to this active group to create a disulfide linkage.

Williams and Ibrahim, 1981). Other nucleophiles also are reactive. Sulfhydryl groups may attack the active species and form thioester linkages, although these are not as stable as the bond formed with an amine. 

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