title: The role of cysteinyl residues in the phosphorylase kinase activity as revealed by iodacetamide modification.
abstract: In native nonactivated phosphorylase kinase [14C] iodacetamide interacts with 50 cysteinyl residues per enzyme molecule (alpha beta gamma delta)4. According to their reactivity towards iodacetamide these residues can be classified into 3 groups. The most reactive cysteinyl residues are involved in the enzyme activation caused by modification of SH-groups. The enzyme inhibition is biphasic. The fast and slow inactivation reactions follow the pseudo-first order kinetics. The rate of inactivation is increased by Ca2+. Mg-ATP effectively protects the enzyme against the inactivation and chemical modification of three SH-groups per protomer (apha beta gamma delta). The kinetics of inactivation and of the [14C] iodacetamide label incorporation demonstrate that two cysteinyl residues per enzyme protomer (alpha beta gamma delta) are essential for the enzyme activity. These residues are located near the ATP-binding site of the beta and gamma subunits of phosphorylase kinase.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002250:Carbon Radioisotopes; D003545:Cysteine; D007460:Iodoacetamide / Q000494:pharmacology*; D007461:Iodoacetates / Q000494:pharmacology*; D007700:Kinetics; D009132:Muscles / Q000201:enzymology*; D010764:Phosphorylase Kinase / Q000378:metabolism*; D010763:Phosphorylase b / Q000378:metabolism; D011817:Rabbits
chemical_list: D002250:Carbon Radioisotopes; D007461:Iodoacetates; D010763:Phosphorylase b; D010764:Phosphorylase Kinase; D003545:Cysteine; D007460:Iodoacetamide