Phosphorylases sulfhydryl groups

GSH is involved in the regulation (glutathionylation) of action of several key-enzymes and other proteins (e.g., phosphorylase, creatinine kinase, ras) and in the restoration of the sulfhydryl groups of proteins (maintenance of enzymes and proteins in active form) ... 

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 role of sulfhydryl groups in phosphorylase activity is not yet known. Modification with p-chloromercuribenzoate certainly causes loss of activity, but it is uncertain whether die sulfhydryl groups form a part of the active center, or are more important for the maintenance of the active conformation of the protein molecule. The second possibility seems the more likely, as p-chloromercuribenzoate causes rabbit-muscle phosphorylase to split up into subunits. Also, the inhibition by this reagent is not parallel to mercaptide formation, and adenosine 5 -phosphate (which causes changes in the conformation of the enzyme) protects against the inhibition. Unmodified sulfhydryl groups are believed to be important for the binding of adenosine 5 -phosphate. ... 

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... 

The chemistry of phosphorylase a is complex. The molecule contains eighteen sulfhydryl groups, four binding sites for adenosine 5-phosphate, four residues of pyridoxal 5-phosphate attached to lysyl residues, and four phosphoserine residues. Treatment with p-(chloromercuri)benzoate splits the molecule into four parts, each of molecular weight 1.35 X 10 this probably represents a monomeric form. This fact, and other evidence, indicates that phosphorylase a is a tetramer. Removal of p-(chloromercuri)-benzoate by cysteine causes reassociation of the monomeric units. ... 

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