We have investigated the use of optically transparent, nanoporous TiO(2) films as substrates for protein immobilization. Immobilization on such films may be readily achieved from aqueous solutions at 4 °C. The nanoporous structure of the film greatly enhances the active surface area available for protein binding (by a factor of 150 for a 4-μm-thick film). We demonstrate that the redox state of immobilized cytochrome c may be modulated by the application of an electrical bias potential to the TiO(2) film and that the fluorescence yield of immobilized fluorophore-labeled maltose-binding protein may be used to monitor specifically maltose concentration. We conclude that nanoporous TiO(2) films may be useful both for basic studies of protein/electrode interactions and for the development of array-based bioanalytical devices employing both optical and electrochemical signal transduction methodologies.

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Human liver CYP3A4 is an endoplasmic reticulum (ER)-anchored hemoprotein responsible for the metabolism of >50% of clinically prescribed drugs. After heterologous expression in Saccharomyces cerevisiae,it is degraded via the ubiquitin (Ub)-dependent 26S proteasomal pathway that utilizes Ubc7p/Cue1p, but none of the canonical Ub-ligases (E3s) Hrd1p/Hrd3p, Doa10p, and Rsp5p involved in ER-associated degradation (ERAD). To identify an Ub-ligase capable of ubiquitinating CYP3A4, we examined various in vitro reconstituted mammalian E3 systems, using purified and functionally characterized recombinant components. Of these, the cytosolic domain of the ER-protein gp78, also known as the tumor autocrine motility factor receptor (AMFR), an UBC7-dependent polytopic RING-finger E3, effectively ubiquitinated CYP3A4 in vitro, as did the UbcH5a-dependent cytosolic E3 CHIP. CYP3A4 immunoprecipitation coupled with anti-Ub immunoblotting analyses confirmed its ubiquitination in these reconstituted systems. Thus, both UBC7/gp78 and UbcH5a/CHIP may be involved in CYP3A4 ERAD, although their relative physiological contribution remains to be established.

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Human flavin-containing monooxygenase 3 (hFMO3) catalyses the oxygenation of a wide variety of compounds including drugs as well as dietary compounds. It is the major hepatic enzyme involved in the production of the N-oxide of trimethylamine (TMAO) and clinical studies have uncovered a striking correlation between plasma TMAO concentration and cardiovascular disease. Certain mutations within the hFMO3 gene cause defective trimethylamine (TMA) N-oxygenation leading to trimethylaminuria (TMAU) also known as fish-odour syndrome. In this paper, the inactivation mechanism of a TMAU-causing polymorphic variant, N61S, is investigated. Transient kinetic experiments show that this variant has a > 170-fold lower NADPH binding affinity than the wild type. Thermodynamic and spectroscopic experiments reveal that the poor NADP+ binding affinity accelerates the C4a-hydroperoxyFAD intermediate decay, responsible for an unfavourable oxygen transfer to the substrate. Steady-state kinetic experiments show significantly decreased N61S catalytic activity towards other substrates; methimazole, benzydamine and tamoxifen. The in vitro data are corroborated by in silico data where compared to the wild type enzyme, a hydrogen bond required for the stabilisation of the flavin intermediate is lacking. Taken together, the data presented reveal the molecular basis for the loss of function observed in N61S mutant.

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This review covers the current state of knowledge regarding artificial fusion constructs of cytochrome P450 enzymes in which the activity of the catalytic heme is driven by reductases of different origins. Cytochromes P450 form a vast family of heme-thiolate proteins, which act as monooxygenases by activating molecular oxygen, resulting in the insertion of one atom into an organic substrate with the concomitant reduction of the other to water. The reducing equivalents are usually supplied by nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate and are transferred in two consecutive steps via the redox partner(s). These include reductases containing flavin mononucleotide and/or flavin adenine dinucleotide and/or Fe-S clusters in different combinations depending on the P450 system. These enzymes catalyze extremely diverse reactions, including regio- and stereospecific oxidations of a large range of substrates in addition to many drugs and xenobiotics, as well as biosynthesis of physiologically important compounds such as various steroids, vitamins, and lipids. Because of their ability to catalyze such a vast range of reactions, they have become the focus of biotechnological interest, but their dependence on the reductase partner has remained one of the challenging limitations for full exploration of their synthetic potential. To address the latter limitation, many researchers have reconstituted functional P450 enzymes by fusion with different reductase proteins; this review will cover their findings.