Interaction of cAMP derivatives with the 'stable' cAMP-binding site in the cAMP-dependent protein kinase type I

cAMP binding to the 'stable' cAMP-binding sites in the regulatory subunit of the cAMP-dependent protein kinase type I was investigated using a set of 18 selected derivatives. All the tested analogues were competitive with [3H]cAMP and inhibitor constants from 12 nM to 20 microM with the free regulatory subunit were determined. The cAMP molecule seemed to be bound by these specific hydrogen bonds to the 5' and 3' oxygen, the 2' hydroxyl, and an ion pair interaction between the negative charge in equatorial position and a positively charged amino acid side chain. The adenine base is rather unspecifically bound with no hydrogen bonds involved. This binding specificity of the 'stable' site is similar to the requirement for dissociation as determined by the activation of the kinase by a respective analogue. This indicates that occupation of the 'stable' sites leads to activation of the protein kinase. The presence of the catalytic subunit reduced the affinity of most analogues. The binding of one derivative with the negative charge fixed in the axial position is not influenced by the addition of the catalytic subunit and ATP. A plausible model for a conformational change during the activation process in the 'stable' site is discussed.