Uncoupling of the cytochrome P-450cam monooxygenase reaction by a single mutation, threonine-252 to alanine or valine: possible role of the hydroxy amino acid in oxygen activation

Abstract

Site-directed mutants of cytochrome P-450cam (the cytochrome P-450 that acts as the terminal monooxygenase in the d-camphor monooxygenase system), in which threonine-252 had been changed to alanine, valine, or serine, were employed to study the role of the hydroxy amino acid in the monooxygenase reaction. The mutant enzymes were expressed in Escherichia coli and were purified by a conventional method. All the mutant enzymes in the presence of d-camphor exhibited optical absorption spectra almost indistinguishable from those of the wild-type enzyme in their ferric, ferrous, oxygenated, and carbon monoxide ferrous forms. In a reconstituted system with putidaredoxin and its reductase, the alanine enzyme consumed O2 at a rate (1100 per min per heme) comparable to that of the wild-type enzyme (1330 per min per heme), whereas the amount of exo-5-hydroxycamphor formed was less than 10% of that formed by the wild-type enzyme. About 85% of the O2 consumed was recovered as H2O2. The valine enzyme also exhibited an oxidase activity to yield H2O2 accompanied by a relative decrease in the monooxygenase activity. On the other hand, the serine enzyme exhibited essentially the same monooxygenase activity as that of the wild-type enzyme. Thus, uncoupling of O2 consumption from the monooxygenase function was produced by the substitution of an amino acid without a hydroxyl group. When binding of O2 to the ferrous forms was examined, the alanine and valine enzymes formed instantaneously an oxygenated form, which slowly decomposed to the ferric form with rates of 5.5 and 3.2 x 10(-3) sec-1 for the former and latter enzymes, respectively. Since these rates were too slow to account for the overall rates of O2 consumption, the formation of H2O2 was considered to proceed not by way of this route but through the decomposition of a peroxide complex formed by reduction of the oxygenated form by reduced putidaredoxin. Based on these findings, a possible mechanism for oxygen activation in this monooxygenase reaction has been discussed.