Mechanism of Activation of the Protein Kinase I from Rabbit Skeletal Muscle

Abstract
The high-affinity binding site for ATP of the holoenzyme of c[cyclic]AMP-dependent protein kinase (type I) from rabbit skeletal muscle was investigated. Binding affinity of a series of ATP derivatives substituted at different sites in the molecule was determined by competition with 3H-ATP. The results were compared with data available from cAMP derivatives with the same substituents, to analyze the electronic and steric features of these 2 sites on the protein kinase. The comparison revealed significant differences of the effect of substituents towards the 2 sites. In particular the N6-derivatives of ATP and substituents affecting the .gamma.-phosphate indicate that the high-affinity ATP site of the protein kinase has similar properties as those found for phosphotransferase sites. The present results are consistent with the supposition that the high-affinity site for ATP on the holoenzyme is congruent with the phosphotransferase site of the catalytic subunit. On combination of catalytic and regulatory subunits this site would be transformed into a high-affinity site for ATP with simultaneous blocking of the phosphotransferase activity.

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