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In order to obtain a better understanding of the allosteric site of rabbit muscle phosphorylase b, nine AMP analogs having a bulky hydrophobic benzene ring were synthesized and tested for activity as activators or inhibitors. N6-Benzyl-AMP derivatives activated phosphorylase b to the same extent as AMP but bound to the enzyme more tightly than AMP. N6-p-nitrobenzyl-AMP had the highest affinity (Ka = 7.7 X 10(-7) M) for the AMP site. In an attempt to irreversibly modify the allosteric site of phosphorylase b, N6-p-bromoacetaminobenzyl-AMP was synthesized. Phosphorylase b was maximally activated upon incorporation of 1.0 mol of N6-p-bromoacetaminobenzyl-AMP per enzyme subunit, and its activity was approximately 90% of that of native phosphorylase b measured in the presence of AMP. The modified enzyme showed characteristics (e.g., kinetic parameters, stability, solubility, inhibition by glucose-6-phosphate, and state of aggregation) quite similar to those observed for the native enzyme in the presence of AMP. These results indicate that the AMP site of phosphorylase was specifically labeled by N6-p-bromoacetaminobenzyl-AMP. The nature of the allosteric site of phosphorylase b is discussed based on the results obtained.
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