Disulfide bridge-forming enzymes

In an enzymic reaction catalysed by glutathione peroxidase, GSH reacts with peroxides and becomes oxidized to form a dimer (GSSG) linked by a disulfide bridge. 

Since hemoproteins such as lactoperoxidase and catalase are inhibited more rapidly than the sulfhydryl oxidation occurs, it is unlikely that the rapid activation of guanylate cyclase occurs by sulfhydryl oxidation [132]. Prolonged incubation of the papain or dehydrogenase enzymes with substrate and nitroprusside yielded a turbidity which indicated denaturation of the enzyme to an insoluble form, possibly by the formation of disulfide bridges via the dimerization of thiyl radicals [132]. 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

Sulfur (S) Sulfur is a yellow solid. Iron-sulfur clusters are found in cytochrome enzymes. The sulfur-containing amino acid cysteine is common in hair. Cysteine residues can connect to each other via disulfide bridges, giving hair a natural curl. Permanent waves are achieved by artificially removing and then re-forming these disulfide bridges. 

Urease is an -SH group (thiol) containing enzyme. The cysteine residues of the protein molecule must be in the reduced -SH form in order for the enzyme to be active. Oxidation of these groups will form -S-S-, disulfide bridges, and the enzyme loses its activity. Reducing agents such as cysteine or glutathione can reactivate the enzyme. 

Amino acid and nucleotide sequences of CuZn-SOD have been determined for enzymes from many organisms. The amino acid residues of the metal ligands and the cysteine residues which form disulfide bridge are conserved in all CuZn-SODs so far sequenced. Furthermore, the charged arginine and lysine residues which participate in the catalytic function for attracting anionic O2 to the Cu site are also conserved (Fig. 10.3). The amino... 

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