THE ROLE OF ADENYLIC ACID IN THE ACTIVATION OF PHOSPHORYLASE

Abstract

The Km of muscle phosphorylase b for inorganic P and for glycogen decreased progressively as the concentration of 5[image]-AMP was increased. Conversely, the addition of either substrate in increasing concentration caused a decrease in the K for 5[image]-AMP. These mutual interactions are presumably the result of (conformational) alterations of the enzyme protein. No interaction between the two substrate binding sites could be demonstrated when the concentration of 5[image]-AMP was held constant and that of the substrates was varied. Muscle phosphorylase a differs from phosphorylase b in being active in the absence of 5[image] -AMP and in being stimulated by much smaller concentrations of 5[image]-AMP. The degree of stimulation of phosphorylase a by 5[image]-AMP depends on the substrate concentration. This is a consequence of a decrease in Km for substrate when 5[image]-AMP is added while there is no significant change in Vmax- The dependence of muscle phosphorylase on 5[image]-AMP for activity, absolute in the case of phosphorylase b and relative in the case of phosphorylase a, is explained in either case by increased affinity of the enzyme for substrate.