A model for the chemical interactions of adenosine 3':5'-monophosphate with the R subunit of protein kinase type I. Refinement of the cyclic phosphate binding moiety of protein kinase type I.

The cAMP receptor site in the regulatory subunit of adenosine 3':5'-monophosphate (cAMP)-dependent protein kinase type I was mapped using analogues of cAMP in which the ribose phosphate moiety was systematically modified. Electronical alteration of the cyclophosphate ring at the 3' and 5' positions by sulfur and nitrogen decreased the affinity of these analogues towards the kinase. Substituents at these positions are not tolerated. Testing the separated diastereomers of derivatives in which one of the exocyclic oxygens at the phosphorus has been substituted by sulfur, it was found that one diastereoisomer is preferentially recognized. Based on these results it is proposed that the hydrophylic cyclic phosphate-ribose moiety of cAMP is bound to the kinase via its 3' and 5'-oxygens, the 2'-hydroxy group and the negative charge in a fixed position. Based on our and other published results it is further proposed, that the adenine moiety is bound in a hydrophobic cleft without any hydrogen bond interactions. The chemical interactions between cAMP and the R subunit of protein kinase type I differ from those found for the binding of cAMP to the chemoreceptor of Dictyostelium discoideum [18].