3-Amylases sulfhydryl groups

Quinone reacts with the sulfhydryl groups of enzymes snch as amylase and carboxylase. This reaction interferes with their activity and, indirectly, with other cellnlar processes such as oxidative phosphorylation. Chloranil and dichlone have been discontinued and are no longer available in the U.S. 

Modification studies of alpha-amylase have indicated that imidazol-ium - - and amino " groups are probably important in the active center, whereas sulfhydryl groups are not. Similar investigations have indicated that, for hefa-amylase, sulfhydryl groups are necessary for maximum activity, - - whereas for phosphorylase, imidazole and amino groups are essential. ... 

The nature of the active site in beta-amylase is not unambiguously known for enzymes from different sources. Early experiments on purified barley and on malted barley first indicated, from studies of the modification of the enzyme with nitrous acid and ketene, that free tyrosine and sulfhydryl groups are essential for activity, whereas free a-amino groups are not. The importance of the sulfhydryl groups was emphasized by the partial recovery of activity of the modified or oxidized enzyme (that is, treated with nitrous acid, iodine, phenyl mercuribenzoate, ferricyanide, and cupric ions) when it was treated with hydrogen sulfide or cysteine. Barley feeto-amylase (not highly purified) has been reported to contain 12—15 sulfhydryl groups per molecule by titration with p-chloromercuribenzoate, and the loss of free sulfhydryl content by treatment with L-ascorbic acid in the presence of cupric ions was found to be directly related to the loss of activity. 

Sulfhydryl groups have also been reported to be essential for the action of crystalline, wheat beta-amylase, and titration of the purified enzyme with iodoacetic acid suggested that there were 3.6—3.8 SH groups per molecule. 

As already oudined, inhibition is essentially complete when caused by reagents that react with sulfhydryl groups (for example, p-chloro-mercuribenzoate, p-mercuribenzoate, o-iodosobenzoate, L-ascorbic acid silver, cupric, and mercuric ions iodine, and ferricyanide) this inactivation can be reversed to some extent by hydrogen sulfide and by cysteine. Lineweaver—Burk graphs have shown that the action of L-ascorbic acid is noncompetitive, and L-ascorbic acid acting in the presence of cupric ions probably causes formation of an inactive cuprous-enzyme. The action of p-chloromercuribenzoate on barley beta-amylase has been shown to be a competitive inhibition. In contrast, the soya-bean p-chloromercuribenzoate inhibition is noncompetitive, and the extent of inhibition is inversely related to the concentration of acetate ion. The latter exhibits a protective effect, and there... 

Sulfhydryl group, 117 in /3-amylases, 334 in phosphorylases, 347 Sulfones, of 5-thioaldopyranoses, 212 Sulfonic anhydrides, sulfonylation of carbohydrates by, 238 Sulfonic esters of carbohydrates, 233 — 280 physical properties and chemical stability of, 253,257 Sulfonylation, of carbohydrates selective, 240, 244 by sulfonyl halides, 236... 

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