Enzymatic deimination of glycogen phosphorylase and a peptide of the phosphorylation site: identification of modification and roles in phosphorylation and activity.

The functional role of arginine residues in glycogen phosphorylase b was probed by enzymatic modification by using peptidylarginine deiminase, which converts arginine residues to citrulline. A peptide with sequence LysArgLysGlnIleSerValArgGlyLeu, corresponding to the phosphorylation site of serine-14 in phosphorylase, was a substrate for the deiminase. Although both arginine residues could be converted to citrulline, modification of arginine-16 occurred more rapidly than modification of arginine-10. Previous studies have implicated a role for arginine, notably arginine-16, in determining phosphorylase kinase activity with the peptide. Deimination altered the phosphorylation of the peptide. Monodeimination of the peptide at arginine-16 slowed down the phosphorylation reaction, but did not diminish the total amount of phosphorylation that could be obtained. Deimination of both arginines produced a peptide that could not be phosphorylated. Modification of phosphorylase b resulted in activation or inactivation of enzyme activity depending on the extent of reaction with peptidylarginine deiminase. A low level of deiminase causes inactivation initially, but after prolonged incubation activation occurs. With high level of deiminase only activation can be observed. Because changes in activity are seen only at subsaturating AMP concentrations (50-100 microM), inactivation and activation are likely due to changes in affinity of the enzyme for AMP. The protein modified with the high level of deiminase has multiple sites of deimination. Arginine 16 was established as a major site of modification. Only protein modified with the high level of deiminase showed modification of arginine-16 and effects on the phosphorylation of phosphorylation b. As with the modified peptide substrate, the reaction was slower with modified phosphorylase in comparison with native phosphorylase b. The results show the importance of the guanidino group of arginine-16 of the protein substrate in modulating the phosphorylase kinase reaction.