Selective chemical modification of Cys264 with diiodofluorescein iodacetamide as a tool to study the membrane topology of cytochrome P450scc (CYP11A1)

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

Structure-guided activity restoration of the silkworm glutathione transferase Omega GSTO3-3

Glutathione transferases (GSTs) are ubiquitous detoxification enzymes that conjugate hydrophobic xenobiotics with reduced glutathione. The silkworm Bombyx mori encodes four isoforms of GST Omega (GSTO), featured with a catalytic cysteine, except that bmGSTO3-3 has an asparagine substitution of this catalytic residue. Here, we determined the 2.20-Å crystal structure of bmGSTO3-3, which shares a typical GST overall structure. However, the extended C-terminal segment that exists in all the four bmGSTOs occupies the G-site of bmGSTO3-3 and makes it unworkable, as shown by the activity assays. Upon mutation of Asn29 to Cys and truncation of the C-terminal segment, the in vitro GST activity of bmGSTO3-3 could be restored. These findings provided structural insights into the activity regulation of GSTOs.

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

Complex formation in vesicle-reconstituted mitochondrial cytochrome P450 systems (CYP11A1 and CYP11B1) as evidenced by rotational diffusion experiments using EPR and ST-EPR

Rotational diffusion measurements using EPR and saturation transfer EPR were applied to analyze complex formation between the electron-transfer components of the mitochondrial steroid-hydroxylating cytochrome P450 systems (CYP11A1 and CYP11B1) in phosphatidylcholine/phosphatidylethanolamine/cardiolipin vesicles prepared by octyl glucoside dialysis/adsorption. Octyl glucoside reconstitution of P450SCC results in large vesicles, which have an advantage over small vesicles in that vesicle tumbling does not contribute to measured rotational diffusion rates. Immobilization of spin-labeled adrenodoxin by both P450SCC and adrenodoxin reductase indicates equimolar complexation between P450SCC and adrenodoxin as well as between adrenodoxin reductase and adrenodoxin. Combination of rotational diffusion and antibody cross-linking confirmed the complexation of adrenodoxin with P450SCC and for the first time provided direct evidence of a complex between P450SCC and P45011beta in the membrane. In contrast, no evidence was found for the existence of adrenodoxin reductase-P450SCC complexes or a ternary complex of all three proteins. Thus, these experiments confirm the shuttle mechanism of electron transfer to vesicle-reconstituted P450SCC and P45011beta.

Molecular modelling of steroidogenic cytochromes P450 from families CYP11, CYP17, CYP19 and CYP21 based on the CYP102 crystal structure

The results of homology modelling of mammalian steroidogenic cytochromes P450 (CYP) from families CYP11, CYP17, CYP19 and CYP21 are reported, based on a novel protein sequence alignment with CYP102, a bacterial P450 of known crystal structure. The molecular models generated from the CYP102 crystal structure template are consistent with experimental information from site-directed mutagenesis studies, steroidal substrate specificity and active site inhibitor studies. Interactive docking studies with both substrates and inhibitors of these enzymes indicate key residue interactions with the putative active site regions of each isoform investigated, which point to potential determinants of substrate specificity within these related enzymes.