Multiple steroid-binding orientations: alteration of regiospecificity of dehydroepiandrosterone 2- and 7-hydroxylase activities of cytochrome P-450 2a-5 by mutation of residue 209

Molecular modelling of CYP2A enzymes: application to metabolism of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

1. Tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung carcinogen in a variety of animal models and a putative human lung carcinogen. Its tumorigenic potential is unmasked via cytochrome P450 (CYP)-mediated hydroxylation of the carbon atoms adjacent to the nitroso moiety (i.e. alpha-hydroxylation). Therefore, elucidation of enzyme-substrate interactions that facilitate alpha-hydroxylation is important to gain insight into the tumorigenic mechanism of NNK and to develop potent inhibitors of this detrimental reaction. 2. Molecular models of CYP2A enzymes from mice, rats and humans that are catalysts of NNK bioactivation were constructed and used, in conjunction with docking experiments, to identify active-site residues that make important substrate contacts. 3. Docking studies revealed that hydrophobic residues at positions 117, 209, 365 and 481, among others, play critical roles in orienting NNK in the active site to effect alpha-hydroxylation. These molecular models were then used to rationalize the stereo- and regioselectivity, as well as the efficiency, of CYP2A-mediated NNK metabolism.

Involvement of steroids and cytochromes P(450) species in the triggering of immune defenses

In vertebrates the wide variety of cytochromes P(450) (P(450)) is a key for elimination of low molecular weight xenobiotics and for the production and metabolism of steroid hormones. In contrast, xenobiotics of large molecular weight are processed and eliminated after the immune response. The suppression of immune response by native P(450)-produced glucocorticoid (GC) hormones constitutes a first link between P(450) and immunity. In the last decade, mechanisms and molecules responsible for the triggering of immune response were investigated and results showed that many tissues and organs transform native 3beta-hydroxysteroids into 7-hydroxylated metabolites that trigger immunity. Present data suggest that 7-hydroxysteroids are native anti-GCs that block the GC-induced immunosuppression. Because specific P(450) are responsible for the production of 7-hydroxylated steroids resulting into increased immunity, a second link exists between P(450) and immunity. Taken together, these findings support the proposal that P(450) are keys to all of the known defense mechanisms of vertebrates against all xenobiotic forms.

Intracrinology and the local enzymatic control of hormone distribution and metabolism: dehydroepiandrosterone does not just act as a prohormone for androgens and estrogens

The study of subcellular environments for the interaction of biomolecules and observations of certain features of hormone actions have nourished the concept of 'intracrinology', which describes hormone actions within a singular cell, in contrast to the well-described autocrine, paracrine and endocrine fashion. Synthesis and metabolism of DHEA make it a likely candidate for intracrine actions in target tissues. Recent experimental findings are reviewed in this context.

Molecular modelling of the human cytochrome P450 isoform CYP2A6 and investigations of CYP2A substrate selectivity

(1) The generation of a homology model of CYP2A6, the major catalyst of human hepatic coumarin 7-hydroxylase activity, involves the use of the recently published substrate-bound CYP102 crystal structure as a template. (2) A substantial number of structurally diverse CYP2A6 substrates are found to dock satisfactorily within the putative active site of the enzyme, leading to the formulation of a structural template (or pharmacophore) for CYP2A6 specificity/selectivity. (3) The CYP2A6 model is consistent with available evidence from site-directed mutagenesis studies carried out on CYP2A subfamily isoforms, and enables some explanation of species differences in CYP2A-mediated metabolism of certain substrates. (4) Quantitative structure-activity relationship (QSAR) analysis of CYP2A5 (the mouse orthologue) mutants yields statistically significant correlations between various properties of amino acid residues and coumarin 7-hydroxylase activity.

Comparisons of catalytic selectivity of cytochrome P450 subfamily enzymes from different species

Historically there has been considerable interest in comparing patterns of biotransformation of xenobiotic chemicals in experimental animal models and humans, e.g. in areas such as drug metabolism and chemical carcinogenesis. With the availability of more basic knowledge it has become possible to attribute the oxidation of selected chemicals to individual cytochrome P450 (P450) enzymes in animals and humans. Further, these P450s can be characterized by their classification into distinct subfamilies, which are defined as having > 59% amino acid sequence identity. Questions arise about how similar these enzymes are with regard to structure and function. More practically, how much can be predicted about reaction specificity and catalysis? In order to address these issues, we need to consider not only the relatedness of P450s from different species but also (i) functional similarity within P450 subfamilies and (ii) the effects of small changes imposed by site-directed mutagenesis. Relationships in the P450 1A, 2A, 2B, 2C, 2D, 2E, 3A, and 17A subfamilies are briefly reviewed. Overall functional similarity is generally seen in subfamily enzymes but many examples exist of important changes in catalysis due to very small differences, even a single conservative amino acid substitution. Some general conclusions are presented about predictability within various P450 subfamilies.

Reciprocal size-effect relationship of the key residues in determining regio- and stereospecificities of DHEA hydroxylase activity in P450 2a5

Collectively, the P450 2a4/2a5 system hyrdoxylates DHEA in at least three positions (7alpha, 7beta, and 2alpha). An individual P450, however, exhibits high specificity to one of these products. Using site-directed mutagenesis of mP450 2a5 from the wild mouse Mus minutoides and bacterial expression, we have associated the function of residues 117, 209, and 481 with the respective specificity observed in each P450. Ala at position 117 determines the 7beta-hydroxylase activity, whereas Val at this position defines the 2alpha-hydroxylase activity. Leu at position 209 is essential for high DHEA 7alpha-hydroxylase activity. The substitutions of residue 481 with various hydrophobic amino acids elicited a profound alteration of the specific hydroxylation rates, but did not influence the regio- and stereospecificities at either of the three positions of DHEA. The alterations caused by residue 481 also depended on the residue identity at position 117 or 209. The results indicate that the sizes of several key residues obey a concerted reciprocal relationship whereby the substrate pocket of the P450s adjusts to accommodate DHEA. A limited molecular modeling study successfully correlates DHEA binding to experimental DHEA hydroxylase activities for a series of mutants at key positions.

Cloning of wound-induced cytochrome P450 monooxygenases expressed in pea

Cytochrome P450 monooxygenases (P450s) mediate a wide range of oxidative reactions involved in the biosynthesis of plant secondary metabolites including phenylpropanoids and phytoalexins. To investigate the regulation of these P450s in the phenylpropanoid biosynthetic pathway of pea (Pisum sativum), partial cDNAs representing four distinct P450s expressed in pea seedlings were cloned using a reverse transcription-polymerase chain reaction strategy. One of the corresponding full-length cDNA clones, designated CYP73A9, encodes pea trans-cinnamic acid 4-hydroxylase, which catalyzes the second core reaction in the phenylpropanoid pathway. As expected from its central role in the production of lignin precursors and defense compounds, northern analysis of poly(A)+ mRNA demonstrates that transcripts encoding CYP73A9 are induced appreciably within 3 h after wounding. A second cDNA clone, designated CYP82, encodes a novel P450, whose transcripts are also induced in response to wounding at approximately the same time as CYP73A9 transcripts. Despite the multitude of environmental stimuli known to induce expression of phenylpropanoid pathway enzymes, genomic DNA Southern analysis indicates that each of these P450s is encoded by a low copy number (possibly a single copy) gene family.

Structural flexibility and functional versatility of cytochrome P450 and rapid evolution

P450 represents a large group of heme-thiolate enzymes that exhibit remarkably diverse activities for the metabolism of numerous endogenous and exogenous chemicals. Recent site-directed mutagenesis studies indicate that a single mutation at any of the key residues can be enough to alter the substrate and/or product specificities in the P450 activities. Molecular modeling predicts that these key residues are located within the substrate heme pocket. Structural elements involved in diversifying P450 activity appear to correspond to the B' helix, the F helix and the F/G interhelical loop in the bacterial P450s. Structures represented by these regions are extremely variable despite the fact that the core of the P450 substrate pocket is well conserved. A mutation within these regions may result in a significant geometrical alteration of the pocket and lead to diversify the P450 activity. Phylogenetical analysis shows a relatively high rate of nonsynonymous substitution within these substrate binding regions. The functional versatility of P450 can thus be largely accounted for in terms of pocket change brought about by rapid mutations.

The roles of individual amino acids in altering substrate specificity of the P450 2a4/2a5 enzymes

A single amino acid substitution is sufficient to alter substrate specificity of P450 enzymes. Mouse P450 2a5, for example, has its substrate specificity converted from coumarin 7- to testosterone 15 alpha-hydroxylase activity by the substitution of Phe at position 209 to Leu. Furthermore, placing Asn at this position confers a novel corticosterone 15 alpha-hydroxylase activity to this P450. Recent site-directed mutational studies show the presence of the topologically common residues, each of which can determine the specificities of various mammalian P450s. For instance, residue 209 (in 2a5) corresponds to a residue at position 206 in rat P4502B1 that regulates its steroid hydroxylase activity. High substrate specificity often observed in an individual P450, therefore, can be determined and altered by the identities of a few critical residues. The structural flexibility of the substrate-heme pocket may also provide P450 enzymes with the ability to display a broad range of substrate specificities. Understanding the underlying principles whereby the flexible pocket determines P450 activities may lead us to the prediction of P450 activities based on the identities of key amino acid residues.