Conformationally constrained analogues of diacylglycerol. 20. The search for an elusive binding site on protein kinase C through relocation of the carbonyl pharmacophore along the sn-1 side chain of 1,2-diacylglycerol lactones.
Previous studies with 1,2-diacylglycerol (DAG) lactones, which behave as high-affinity ligands for protein kinase C (PK-C), have established the importance of maintaining intact the pharmacophore triad of two carbonyl moieties (sn-1 and sn-2) and the primary alcohol. In addition, docking studies of DAG-lactones into an empty C1b receptor of PK-Cdelta (as it appears in complex with phorbol 13-O-acetate) have revealed that in either of the two possible binding alternatives (sn-1 or sn-2) only one carbonyl group of the DAG-lactone is involved in binding. Therefore, the unknown receptor for the orphaned carbonyl appears to lie outside the boundaries of this binary complex, possibly residing at the membrane or near the membrane-protein interface. A strategy to locate the optimal location of the unengaged carbonyl was conceived by utilizing a small group of DAG-lactones (1-4) with a highly branched chain adjacent to the sn-2 carbonyl such that sn-2 binding is favored. With these compounds, various locations of the sn-1 carbonyl along the side chain were tested for their binding affinity for PK-C. The results indicate that the location of the side chain sn-1 carbonyl in a DAG-lactone must have perfect mimicry to the sn-1 carbonyl of the parent DAG for it to display high binding affinity. A proposed model from this work is that the missing pharmacophore in the ternary complex, which includes the membrane, is close to the membrane-protein interface.