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Cytochrome P450 has been called the Rosetta Stone of iron proteins. Perhaps nowhere else in the biological sciences has the rich interplay between structural, spectroscopic, mechanistic, computational, and chemical modeling techniques led to such a detailed level of understanding of such an important system. The central paradigm of biological oxygen activation is now recognized to involve the formation a ferryl, or oxoiron intermediate. Oxoiron(IV) porphyrin cation radicals have been observed in peroxidase, cytochrome oxidase, CPO, cytochrome P450, and in a variety of model systems. Model system studies, especially those of iron, manganese, and ruthenium porphyrins and related ligands, have led to important advances in catalysis and in catalytic asymmetric oxygenation. Advances in computational studies of such complex, open-shell systems have begun to provide a rigorous physical underpinning for the body of complex and sometimes confusing experimental results. In this chapter, I have tried to weave together all of these aspects to provide for the reader a unified picture of the current understanding in the field of cytochrome P450 research. More detailed presentations are to be found in the chapters that follow.

We have proposed an efficient iterative domain decomposition method that solves general convection-diffusion singular perturbation problems. Our specific application involves a piecewise constant diffusion coefficient. We have established sufficient conditions for the convergence of the method, identified suitable values of the relaxation parameter , and started investigations into the rate of convergence. The method was implemented to (2) to solve test problems.

Full details of the proofs of these assertions will appear in a future publication.

We have proposed an efficient iterative domain decomposition method that solves general convection-diffusion singular perturbation problems. Our specific application involves a piecewise constant diffusion coefficient. We have established sufficient conditions for the convergence of the method, identified suitable values of the relaxation parameter , and started investigations into the rate of convergence. The method was implemented to (2) to solve test problems.

Full details of the proofs of these assertions will appear in a future publication.

We have proposed an efficient iterative domain decomposition method that solves general convection-diffusion singular perturbation problems. Our specific application involves a piecewise constant diffusion coefficient. We have established sufficient conditions for the convergence of the method, identified suitable values of the relaxation parameter , and started investigations into the rate of convergence. The method was implemented to (2) to solve test problems.

Full details of the proofs of these assertions will appear in a future publication.

The rich mechanistic enzymology of the cytochrome P450s has occupied chemists, biochemists, pharmacologists, and toxicologists for over three decades. Are we near to a detailed molecular understanding? We have attempted to convey in this chapter of the recent discoveries that fill many of the lacunas in our understanding of P450-catalyzed substrate oxidations. We now have a precise three-dimensional structure of the ferrous-oxygenated state, and ample spectroscopic characterization of the ferric-peroxo anion and ferric-hydroperoxo intermediates. In the exogenous oxidant driven pathway, an archaeal P450 allowed facile observation of the formation and breakdown of the “Compound I” ferryl-oxo state. Yet much remains. Stabilization and characterization of the “Compound I” state in the dioxygen reaction has not yet been achieved. With the ability to separate, through time and temperature, the population of multiple “active oxygen” intermediates in P4 50 catalysis, it remains to precisely define the reactivity profiles of each state and thereby realize a mapping of observed metabolic profiles to specific states in the reaction cycle. An overriding revelation has been the subtle way in which Nature controls the reactivity of atmospheric dioxygen, electrons, and transition metal through delicate hydrogen-bonding interactions. Thus, in a Periclesian control of mechanism, the cytochromes P450 utilize a variety of proton pathways to finely tune this versatile catalyst for critical physiological processes.

Cytochrome P450 mechanisms continue to surprise and delight, although the field is growing to maturity and the completely unexpected is less frequently encountered. Experimentally, the past few years have seen major progress in characterizing the intermediates that are formed as molecular oxygen is activated to the final oxidizing species. All the intermediates, with the exception of the critical ferryl species, have now been directly observed by various spectroscopic and crystallographic methods. The ferric peroxo anion has been found to act as the oxidizing agent with a growing range of highly electrophilic substrates. In contrast, the proposed role for the ferric hydroperoxo complex as an electrophilic oxidizing agent remains a matter of debate, as the evidence advanced in support of the proposal is circumstantial and contradictory. Although the ferryl species remains elusive, it is increasingly clear that it plays the predominant role as the oxidizing agent in the P450 catalytic cycle. A second area that has recently received considerable attention is the mechanism of hydrocarbon hydroxylation, the key question being whether the radical rebound mechanism that has held sway for three decades is in fact valid. The contradictory results obtained with radical and cation probes, which have provided most of the new evidence, must be resolved by further experimentation in order for this quest ionto be settled. The development of a two-state model for the catalytic action of P450 enzymes may be one of the most important recent advances in the field, as it provides a ready explanation for a variety of otherwise contradictory data, some of which argues for concerted and some for nonconcerted oxidation mechanisms. No doubt, the next few years will uncover novel aspects of P450 function and will lead to deeper and more sophisticated understanding of the catalytic mechanisms of the amazing family of P450 enzymes.

We have proposed an efficient iterative domain decomposition method that solves general convection-diffusion singular perturbation problems. Our specific application involves a piecewise constant diffusion coefficient. We have established sufficient conditions for the convergence of the method, identified suitable values of the relaxation parameter , and started investigations into the rate of convergence. The method was implemented to (2) to solve test problems.

Full details of the proofs of these assertions will appear in a future publication.

Over the last decade, great strides have been made in understanding the roles that the nuclear receptors PXR, CAR, PPARα, and AHR play in the induction of CYP genes. The ability of xenobiotics to bind and activate NRs to induce the expression of the CYP enzymes involved in their metabolism provides a mechanism by which an organism can mount an adaptive response to its changing chemical environment. The identification of endogenous ligands for some NRs indicates that these receptors play important roles in regulating CYP levels during physiological processes as well. It has become clear that the expression of many CYP genes is dependent on more than one NR. Recent studies have demonstrated that NRs often share xenobiotic ligands, response elements, and target CYP genes. The existence of multiple xenobiotic receptors with broad and sometimes overlapping functions likely increases the ability of an organism to detect and respond to a wide range of chemicals. The challenge for the future will be to understand how the NRs participate in a complex network to regulate CYP gene expression and to mediate the physiological response to xenobiotics.

The studies of the P450 AA monooxygenase have uncovered new and important roles for P450 in the metabolism of endogenous substrates, and added P450 to the list of enzymes that participate in the metabolism of AA, a fatty acid that serves as the precursor for the biosynthesis of several physiologically important lipid mediators. The functional relevance of this metabolic pathway is suggested by the many important biological activities attributed to its products. These studies, as well as the documented endogenous roles of P450s in cholesterol, steroid, and vitamin metabolism are contributing to establish this enzyme system as a major participant in the regulation of cell, organ, and body physiology. Among these, the phenotypic analysis of mice carrying disrupted copies of the CYP4a14 gene unveiled new and important roles for the P450 enzymes in cardiovascular physiology and the control of systemic blood pressures, and suggested the human homologs of the rodent CYP 2C and 4A AA epoxygenases and ω-hydroxylases as candidate genes for the study of their role in the pathophysiology of hypertension, and cardiovascular disease.

We have proposed an efficient iterative domain decomposition method that solves general convection-diffusion singular perturbation problems. Our specific application involves a piecewise constant diffusion coefficient. We have established sufficient conditions for the convergence of the method, identified suitable values of the relaxation parameter , and started investigations into the rate of convergence. The method was implemented to (2) to solve test problems.

Full details of the proofs of these assertions will appear in a future publication.