On the model controversy for substrate-induced spin-state transition in cytochrome P450: (a new perspective)

New cytochrome P450 mechanisms: implications for understanding molecular basis for drug toxicity at the level of the cytochrome

IMPORTANCE OF THE FIELD

Cytochrome (CYP) P450 is a collective name for a very large group of heme enzymes, which catalyze largely oxidative reactions, including those of pharmacological and toxicological importance. Their efficient operation requires coupling of specific electron donor and O(2) consumption and substrate hydroxylation. Many drug oxidation reactions are partially uncoupled, leading to the formation of highly toxic reactive oxygen species, which can cause unpredictable toxic effects on the cell. Rational approaches to avoid uncoupling require knowledge of the underlying mechanisms.

AREAS COVERED IN THIS REVIEW

In this communication, attempts have been made to bring together past as well as present information indicating that i) the P450 active site has two differently accessible allosterically interacting subsites geared for entirely different types of functionally relevant interactions; and ii) substrate binding to the specific protein residues (Site I) forming the reducible high-spin complex and product binding at L(6) (Site II) of the heme iron forming inhibited low-spin complex can regulate the functional state of the enzyme during catalysis.

WHAT THE READER WILL GAIN

Since P450 enzymes catalyze a wide variety of reactions, understanding the molecular basis for their efficient operation is of interest to many fields, including rational approaches to design safer drugs, tailoring P450 for a given task (e.g., bioremediation).

TAKE HOME MESSAGE

It is important to take into account that the two sub-sites function as interacting sites rather than parts of a site functioning as single site for rational approaches to P450 mechanisms. This is important especially in regard to interpretation of the observed effects of drugs, products and inhibitors on these enzymes.

Kinetics of electron transfer in the complex of cytochrome P450 3A4 with the flavin domain of cytochrome P450BM-3 as evidence of functional heterogeneity of the heme protein

We used a rapid scanning stop-flow technique to study the kinetics of reduction of cytochrome P450 3A4 (CYP3A4) by the flavin domain of cytochrome P450-BM3 (BMR), which was shown to form a stoichiometric complex (K(D)=0.48 microM) with CYP3A4. In the absence of substrates only about 50% of CYP3A4 was able to accept electrons from BMR. Whereas the high-spin fraction was completely reducible, the reducibility of the low-spin fraction did not exceed 42%. Among four substrates tested (testosterone, 1-pyrenebutanol, bromocriptine, or alpha-naphthoflavone (ANF)) only ANF is capable of increasing the reducibility of the low-spin fraction to 75%. Our results demonstrate that the pool of CYP3A4 is heterogeneous, and not all P450 is competent for electron transfer in the complex with reductase. The increase in the reducibility of the enzyme in the presence of ANF may represent an important element of the mechanism of action of this activator.

Conformational dynamics of the cytochrome P450 BM3/N-palmitoylglycine complex: the proposed "proximal-distal" transition probed by temperature-jump spectroscopy

The ferric spin state equilibrium of the heme iron was analyzed in wild-type cytochrome P450 BM3 and its F87G mutant by using temperature (T)-jump relaxation spectroscopy in combination with static equilibrium experiments. No relaxation process was measurable in the substrate-free enzyme indicating a relaxation process with a rate constant>10,000 s(-1). In contrast, a slow spin state transition process was observed in the N-palmitoylglycine (NPG)-bound enzyme species. This transition occurred with an observed rate constant (298 K) of approximately 800 s(-1) in the wild-type, and approximately 2500 s(-1) in the F87G mutant, suggesting a significant contribution of the phenylalanine side chain to a reaction step rate limiting the actual spin state transition. These findings are discussed in terms of an equilibrium between different binding modes of the substrate, including a position 7.5 A away from the heme iron ("distal") and the catalytically relevant "proximal" binding site, and are in accordance with results from X-ray crystallography, NMR studies, and molecular dynamics simulations.

Differential behavior of the sub-sites of cytochrome 450 active site in binding of substrates, and products (implications for coupling/uncoupling)

The cytochrome P450 catalyzes hydroxylation of many substrates in the presence of O(2) and specific electron transport system. The ternary complex S-Fe(+)O(2) with substrate and O(2) bound to their respective sites on the reduced enzyme is an important intermediate in the formation of the hydroxylating species. Then the active site may be considered as having two sub-sites geared for entirely different types of functionally relevant interactions. The two sites are the substrate binding site, the specific protein residues (Site I), and the L(6) position of the iron (Site II) to which O(2) binds upon reduction. In the ferric enzyme, when substrate binds to Site I, the low spin six-coordinated P450 is converted to the readily reducible high spin five coordinated state. Certain amines and OH compounds, such as products of P450-catalyzed reactions, can bind to Site II resulting in six coordinated inhibited complexes. Then the substrate and product interactions with the two sub-sites can regulate the functional state of the enzyme during catalysis. Product interactions have received very little attention. CYP101 is the only P450 in which X-ray and spectroscopic data on all three structures, the substrate-free, camphor-bound and the 5-exo-OHcamphor-bound are available. The substrate-free CYP101 is low spin and six-coordinated with a water molecule ligated at the L(6) position of the iron. The substrate camphor binds to Site I, and releases the L(6) water despite its inability to bind to this site, indicating that Site I binding can inhibit Site II ligation. The product 5-exo-OHcamphor in addition to binding to Site I, binds to Site II through its -OH group forming Fe-O bond, resulting in the low spin six-coordinated complex. New temperature-jump relaxation kinetic data indicating that Site II ligation inhibits Site I binding are presented. It appears that the Site I and Site II function as interacting sub-sites. The inhibitory allosteric interactions between the two sub-sites are also reflected in the data on binding of the substrate camphor (S) in the presence of the product 5-exo-OH camphor (P) to CYP101 (E). The data are in accordance with the two-site model involving the ternary complex ESP. The affinity of the substrate to the product-bound enzyme as well as the affinity of the product to the substrate-bound enzyme decreased with increase in product concentration, which is consistent with mixed inhibition indicative of inhibitory allosteric interactions between the two sub-sites. Implications of these observations for coupling/uncoupling mechanisms are discussed in the light of the published findings consistent with the two-site behavior of the P450 active site. In addition, kinetic data indicating that the transient high spin intermediate may have to be taken into account for understanding how some P450s have been able to express appreciable hydroxylation activities in the absence of substrate-induced low to high spin transition, observable by the traditional static spectroscopy, are presented.