Department of Chemistry & Biochemistry
R. Bruce Banks, Associate Professor
Organic/Biochemistry
415 Science Building, (336) 334-4606, e-mail:rbbanks@uncg.edu
B.A. (1970) Vanderbilt University; M.S. (1973), Ph.D. (1976), Florida State University (organic chemistry). Postdoctoral fellowships: Center in Molecular Toxicology, Department of Biochemistry, Vanderbilt University Medical Center (1976-78); Department of Pharmacology, University of Minnesota (1978-80). Visiting Assistant Professor, Macalester College, St. Paul, Minnesota (1980-82).
Enzymatic catalysis is responsible for all the transformations which occur during the metabolism of endogenous and environmental compounds by an organism. The efficiency and selectivity of enzymes may also be exploited in the laboratory, where they are useful as catalysts for synthesis. Research in my laboratory is currently focused on two general areas of investigation: (1) enzyme-catalyzed oxidation/reduction of compounds by molybdenum hydroxylases (aldehyde oxidase and xanthine oxidase), as well as inhibition of these pathways and (2) applications of these enzymes in organic synthesis.
Molybdenum hydroxylases occur in the cytosol of human and other mammalian cells where they are active in both oxidative and reductive reactions of xenobiotic metabolism. Aldehyde oxidase and xanthine oxidase are genetically related enzymes which are known to contain three redox-active centers--Mo(VI), iron-sulfur clusters, and flavin-adenine dinucleotide (FAD. These enzymes, often in combination with cytochrome P450's, contribute to the metabolic inactivation of certain drugs (e.g., AZT, nicotine), but bioactivate others (e.g., metho- trexate) to more toxic forms. Since aldehyde oxidase and xanthine oxidase both produce oxygen free radicals during their reactions in vivo, there is considerable clinical interest in the role of these enzymes in oxidative stress damage in humans.
Current Projects Include:
1. Drug design based on inhibition of molybdenum hydroxylases
Preliminary experiments in our lab have established that several nitroaromatic compounds, including 2-, 3-, and 5-nitrobenzaldehyde, are potent inhibitors of rabbit liver aldehyde oxidase. We hope to identify similar compounds which have even greater inhibitory activity toward the enzyme. Such inhibitors would be useful experimental tools for study of the enzymes, as well as potential lead compounds for drug development. Data from these experiments will be used to determine whether hydrophobic or more polar interactions are necessary for optimal interaction between inhibitor and enzyme
An extremely potent xanthine oxidase inhibitor, effective at nanomolar concentrations, has recently been reported and an x-ray crystallographic structure has been obtained for the enzyme complexed with this compound (Okamoto et al, J Biol Chem. 2003, 17, 1848). The inhibitor, 2-[3-cyano-4-isobutoxyphenyl]-4-methyl-5-thiazolecarboxylic acid, contains an anionic carboxylate group. The x-ray structure reveals coordination of the inhibitor through this carboxylate group with glutamate-826 near the active site molybdenum, in analogy with the isopropanol-aldehyde oxidase complex previously published by other researchers. This common feature of the two enzymes - - a critical glutamate and its interaction with substrates/inhibitors- - guides our thinking in the design of new inhibitors for aldehyde oxidase and xanthine oxidase. We believe that the similar electronic character of the carboxylate and nitro groups may partially explain why nitraromatic compounds are potent inhibitors of aldehyde oxidase. In order to test this hypothesis, we will compare the enzyme inhibitory activity of structurally similar compounds containing either the carboxy or nitro functional group.
2. Enzymatic catalysis by enzymes in organic solvent media. In the past, enzymes were viewed as being incompatible with the organic solvents used in conventional synthetic procedures. However, in recent years a remarkable number of synthetic transformations have been carried out with enzymes suspended in organic solvents containing very low water content (<5%). Enzymatic reactions in nonaqueous media provide several advantages when compared with their aqueous counterparts: (1) the enzyme, which is insoluble in organic solvents, may be separated by filtration from solvent-soluble reaction products, (2) compounds which are insoluble in water may be used as substrates for reaction, (3) reactions with unfavorable equilibria in water, such as condensations reactions, may be accomplished, and (4) enzymes often display altered selectivities and enhanced thermal stabilities in organic solvents. Lipases and other hydrolytic enzymes are particularly robust enzymes in organic solvents and have been extensively investigated. Other families of enzymatic catalysts have received less attention. We have shown that aldehyde oxidase can catalyze several synthetically useful oxidations when conducted in water-immiscible organic solvents such as dichloromethane and hexane containing 3% water or less. For example, the enzyme suspended in dichloromethane converts phenazine methosulfate to a ring-oxidized product (Eq. 1), 2-methylbutanal to 2-methylbutanoic acid (Eq. 2), and phenanthridine to phenanthridone (Eq. 3).
(1)
(2)
(3)
These reactions, carried out on a micromolar scale, demonstrate the potential utility of oxidations by aldehyde oxidase in organic solvents. We are now ready to scale up these nonaqueous enzyme-catalyzed reactions to synthetically useful proportions, i.e., milligram to gram quantities of reactants.