Use of arginine compounds to examine the role of an essential arginine in the mechanism of glycogen phosphorylase

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Abstract
The possible role of arginine in the mechanism of [rabbit] muscle glycogen phosphorylase was studied by examining the effect arginine compounds. Guanidino compounds with an aromatic group inhibit native phosphorylase .beta., reduced phosphorylase b, phosphorylase b'' and phosphorylase a. The inhibition was uncompetitive with respect to glucose-1-phosphate and noncompetitive toward glycogen for phosphorylase .beta.. This is consistent with a kinetic mechanism where the inhibitor binds after the substrate, glucose 1-phosphate. In the presence of citrate and L-cysteine, N-.alpha.-tosylarginine methyl ester, a good inhibitor, promotes the removal of tightly bound pyridoxal phosphate. Potato phosphorylase has many similarities to the muscle enzyme, but it lacks the regulatory sites and does not have a polysaccharide storage site. N-.alpha.-Tosylarginine methyl ester [TAME] inhibited the potato enzyme, was uncompetitive with glucose 1-phosphate and was competitive with starch; it seems likely that TAME is binding near the active site of both the potato and muscle enzyme. The different inhibitory patterns with respect to polysaccharide for potato and muscle phosphorylase can be explained by the absence of the polysaccharide storage site on the potato enzyme. Inhibition by arginine compounds is related to the pKa of the guanidino function, i.e., the lower the pKa value, the greater the inhibition. On the basis of these studies and those of Dreyfus et al. who found that Arg-568 was essential for activity, arginyl compounds apparently inhibit when an unprotonated guanidino group competes for the binding site of Arg-568.