Regioselectivity in the cytochromes P-450: Control by protein constraints and by chemical reactivities

Three alicyclic compounds (d-camphor, adamantanone, adamantane) were found to be hydroxylated by the cytochrome P-450 isoenzymes P-450_cam and P-450_LM2. With P-450_cam as the catalyst only one product was formed from each of the substrates: 5-exo-hydroxycamphor, 5-hydroxyadamantanone, and 1-adamantanol. With P-450_LM2 as the catalyst, two or more isomeric products were formed from each substrate: 3-endo-, 5-exo-, and 5-endo-hydroxycamphor; 4-anti- and 5-hydroxyadamantanone; and 1- and 2-adamantanol. The products from P-450_cam hydroxylations were found to be isosteric with one another, suggesting that each of them was attacked at a topologically congruent position within a rigid enzyme-substrate complex. The distribution of products from P-450_LM2 hydroxylations, on the other hand, were similar to the distributions expected during solution-phase hydroxylations. Thus, it would appear that the complex which P-450_LM2 forms with its substrate allows considerable movement of the substrate molecule, such that most of the hydrogens in the substrate are exposed to the enzymatic hydrogen abstractor. Under these conditions, the distribution of products more nearly reflects the rank order of chemical reactivities of the various hydroxylatable positions, with only a moderate protein-based steric constraint being expressed. These suggestions were also evident in the tightness of binding of the substrates to the two enzymes and in the magnitude of coupling between the substrate binding and the spin-state equilibria. Thus, the product from P-450_cam-catalyzed hydroxylation may be predicted by a consideration of the relation of the topology of the prospective substrate to that of d-camphor. The products from P-450_LM2-catalyzed hydroxylations, on the other hand, may be approximately predicted from the chemical reactivities of the various abstractable hydrogens in the prospective substrate.