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Requirements for the activation of protein kinase C: comparison of the molecular geometries of phorbol and diacylglycerol.

MM2 calculations have been performed on a number of derivatives of phorbol and diacylglycerol (DAG) to establish the molecular features required for the activation of protein kinase C by a detailed comparison of the molecular geometries in these two classes of compounds. For DAG, a dihedral angle of about -60 degrees appears to be required for the oxygens at C2 and C3 because that angle is fixed at this value in phorbols. There is good agreement between the computed Boltzmann distribution for the O1-C1-C2-O2 dihedral angle and NMR results for the same angle in phospholipids, as obtained by others. A conformer of DAG is identified with dihedral angles corresponding to those of beta-phorbols. This conformer, however, is 3.2 kcal/mol above the global minimum found for DAG. The molecular geometry of this conformer is consistent with that of a number of active and inactive rigid analogues of DAG. The preferred conformation in beta-phorbol diesters is found to be stabilized by an antiparallel stacking of the ester carbonyl groups. The lack of activity of alpha-phorbol esters appears to be due to differences in a portion of the molecule containing the five-membered/seven-membered rings, which are far from the DAG-like end of the phorbol molecule. It is proposed that some of the biological activities of phorbol diesters may be due to this portion of the beta-phorbol molecule, which might represent a second active region, distinct from that resembling DAG.