Effects of group-selective reagents on rabbit muscle glycogen synthase

A series of amino acid reagents was tested on the glucose-6-P dependent D, and independent I forms of glycogen synthase (UDPG: glycogen alpha-4-glucosyltransferase, EC 2.4.I. II) from rabbit skeletal muscle, at two levels of purification. Whereas blocking of aliphatic hydroxyl groups did not result in any inhibition of the enzyme(s), blocking of aromatic hydroxyl groups resulted in a gradual and complete inhibition. Under the stated assay conditions both forms of the enzyme were similarly affected in terms of activity, but the tyrosines of the D form were found to react more readily chemically. Tyrosine appears to be "essential" for catalysis. No desensitization to the allosteric modulator glucose-6-P was detected.