Structure and Substrate Specificity of Protein Kinase

The activity of protein kinase A is controlled by cAMP. In the absence of cyclic AMP, protein kinase A exists as a tetramer composed of two regulatory R subunits and two catalytic C subunits (see Fig. 6.2). The catalytic activity is masked in the holoenzyme C2R2, since an inhibitory structural element of the R subunit blocks the entrance to the 

In mammals, four isoforms of the R subunit (RIa, Rip, Rlla and RIip) and three subtypes of the C subrmit, namely C, CP and Cy, are known. 

The composition of the subrmits is shown schematically in Fig. 7.5. The R subunit has two cAMP binding sites of differing affinity. In addition, the R subunit has a domain containing an autophosphorylation site which is involved in the autoinhibition of protein kinase A. 

The C subrmit has a myristinic acid residue at the amino terminus, the function of which is unknown. In addition, the C subunit has specific Ser/Thr phosphorylation sites, namely Thrl97 and Ser338. Thrl97 is located in the activation loop and is phos-phorylated by an autophosphorylation mechanism. This is linked to an increase in the affinity for ATP and to the catalytic efficiency. 

The consensus sequence for phosphorylation of proteins by protein kinase A is RRXSX. The RII subrmit contains such a sequence in the autoinhibitory domain and is therefore subject to phosphorylation by the C subrmit in the holoenzyme, but without release of inhibition. Inhibition of the C subrmit by the R subrmit is based on binding of the autoinhibitory sequence of R at the substrate binding site and at parts of the active center of the C subrmit. 

---