Phenylalanine 393 exerts thermodynamic control over the heme of flavocytochrome P450 BM3.

Site-directed mutants of the phylogenetically conserved phenylalanine residue F393 were constructed in flavocytochrome P450 BM3 from Bacillus megaterium. The high degree of conservation of this residue in the P450 superfamily and its proximity to the heme (and its ligand Cys400) infers an essential role in P450 activity. Extensive kinetic and thermodynamic characterization of mutant enzymes F393A, F393H, and F393Y highlighted significant differences from wild-type P450 BM3. All enzymes expressed to high levels and contained their full complement of heme. While the reduction and subsequent treatment of the mutant P450s with carbon monoxide led to the formation of the characteristic P450 spectra in all cases, the absolute position of the Soret absorption varied across the series WT/F393Y (449 nm), F393H (445 nm), and F393A (444 nm). Steady-state turnover rates with both laurate and arachidonate showed the trend WT > F393Y >> F393H > F393A. Conversely, the trend in the pre-steady-state flavin-to-heme electron transfer was the reverse of the steady-state scenario, with rates varying F393A > F393H >> F393Y approximately wild-type. These data are consistent with the more positive substrate-free [-312 mV (F393A), -332 mV (F393H)] and substrate-bound [-151 mV (F393A), -176 mV (F393H)] reduction potentials of F393A and F393H heme domains, favoring the stabilization of the ferrous-form in the mutant P450s relative to wild-type. Elevation of the heme iron reduction potential in the F393A and F393H mutants facilitates faster electron transfer to the heme. This results in a decrease in the driving force for oxygen reduction by the ferrous heme iron, so explaining lower overall turnover of the mutant P450s. We postulate that the nature of the residue at position 393 is important in controlling the delicate equilibrium observed in P450s, whereby a tradeoff is established between the rate of heme reduction and the rate at which the ferrous heme can bind and, subsequently, reduce molecular oxygen.

Structural and spectroscopic analysis of the F393H mutant of flavocytochrome P450 BM3.

In the preceding paper in this issue [Ost, T. W. B., Miles, C. S., Munro, A. W., Murdoch, J., Reid, G. A., and Chapman, S. K. (2001) Biochemistry 40, 13421-13429], we have established that the primary role of the phylogenetically conserved phenylalanine in flavocytochrome P450 BM3 (F393) is to control the thermodynamic properties of the heme iron, so as to optimize electron-transfer both to the iron (from the flavin redox partner) and onto molecular oxygen. In this paper, we report a detailed study of the F393H mutant enzyme, designed to probe the structural, spectroscopic, and metabolic profile of the enzyme in an attempt to identify the factors responsible for causing the changes. The heme domain structure of the F393H mutant has been solved to 2.0 A resolution and demonstrates that the histidine replaces the phenylalanine in almost exactly the same conformation. A solvent water molecule is hydrogen bonded to the histidine, but there appears to be little other gross alteration in the environment of the heme. The F393H mutant displays an identical ferric EPR spectrum to wild-type, implying that the degree of splitting of the iron d orbitals is unaffected by the substitution, however, the overall energy of the d-orbitals have changed relative to each other. Magnetic CD studies show that the near-IR transition, diagnostic of heme ligation state, is red-shifted by 40 nm in F393H relative to wild-type P450 BM3, probably reflecting alteration in the strength of the iron-cysteinate bond. Studies of the catalytic turnover of fatty acid (myristate) confirms NADPH oxidation is tightly coupled to fatty acid oxidation in F393H, with a product profile very similar to wild-type. The results indicate that gross conformational changes do not account for the perturbations in the electronic features of the P450 BM3 heme system and that the structural environment on the proximal side of the P450 heme must be conformationally conserved in order to optimize catalytic function.

Rational re-design of the substrate binding site of flavocytochrome P450 BM3.

Bacillus megaterium P450 BM3 is a fatty acid hydroxylase with selectivity for long chain substrates (C(12)-C(20)). Binding or activity with substrates of chain length 13-fold with butyrate, while the L75T/L181K double mutant has k(cat)/K(M) increased >15-fold with hexanoate and binding (K(d)) improved >28-fold for butyrate. Removing the arginine 47/lysine 51 carboxylate binding motif at the mouth of the active site disfavours binding of all fatty acids, indicating its importance in the initial recognition of substrates.

Protein engineering of cytochromes P-450.

The cytochromes P-450 are an immensely important superfamily of heme-containing enzymes. They catalyze the monooxygenation of an enormous range of substrates. In bacteria, cytochromes P-450 are known to catalyze the hydroxylation of environmentally significant substrates such as camphor, phenolic compounds and many herbicides. In eukaryotes, these enzymes perform key roles in the synthesis and interconversion of steroids, while in mammals hepatic cytochromes P-450 are vital for the detoxification of many drugs. As such, the cytochromes P-450 are of considerable interest in medicine and biotechnology and are obvious targets for protein engineering. The purpose of this article is to illustrate the ways in which protein engineering has been used to investigate and modify the properties of cytochromes P-450. Illustrative examples include: the manipulation of substrate selectivity and regiospecificity, the alteration of membrane binding properties, and probing the route of electron transfer.