Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2010
Publication Date: 6/4/2010
Citation: Bowman, M.J., Jordan, D.B., Vermillion, K., Braker, J.D., Moon, J., Liu, Z. 2010. Stereochemistry of Furfural Reduction by a Saccharomyces cerevisiae Aldehyde Reductase That Contributes to In Situ Furfural Detoxification. Applied and Environmental Microbiology. 76(15):4926-4932.
Interpretive Summary: Increased energy demands and need for diversified and renewable energy sources have strengthened interest in conversion of lignocellulosic biomass to simple sugars which can be converted to ethanol for use as biofuel. As an essential step preceding deconstruction of lignocellulosic biomass by enzymatic means, several different pretreatment strategies are employed. Dilute-acid pretreatment provides considerable benefits, including exposure of cellulose fibrils to enzymatic degradation; however, use of acidic pretreatment strategies leads to conditions which generate furans from breakdown of sugars. Presence of these side products is problematic as furans, such as furfural, inhibit ethanol production in yeast. In situ detoxification by reduction of furfural is an important means to address inhibitor challenges involved in achieving efficient biofuel production. This paper determines the spatial requirements of an enzyme, aldehyde reductase, from Saccharomyces cerevisiae that has demonstrated a contribution to in situ detoxification of furfural. This information was used to develop a binding model to aid in the design of mutagenesis studies for optimization of the catalyst for furfural reduction. This research will enhance the development of strategies to reduce inhibitor toxicity and improve the efficiency of biomass to ethanol fermentations.
Technical Abstract: Ari1p from Saccharomyces cerevisiae, recently identified as an intermediate subclass short-chain dehydrogenase/reductase, contributes in situ to the detoxification of furfural. Furfural inhibits efficient ethanol production by the yeast, particularly when the carbon source is acid-treated lignocellulose, which contains furfural at relatively high concentration. Nicotinamide adenine dinucleotide phosphate (NADPH) is Ari1p’s best known hydride donor. Here we report the stereochemistry of the hydride transfer step, determined by using (4R)-[4-**2H]NADPD and (4S)-[4-**2H]NADPD and unlabelled furfural in Ari1p-catalyzed reactions and following the deuterium atom into products 2-furanmethanol or NADP+. Analysis of the products demonstrates unambiguously that Ari1p directs hydride transfer from the si face of NADPH to the re face of furfural. The singular orientation of substrates enables construction of a model of the Michaelis complex in the Ari1p active site. The model reveals hydrophobic residues near the furfural binding site that, upon mutation, may increase specificity for furfural and enhance enzyme performance. Using (4S)-[4-**2H]NADPD and NADPH as substrates, primary deuterium kinetic isotope effects of 2.2 and 2.5 were determined for the steady-state parameters kcat**NADPH and kcat/Km**NADPH, respectively, indicating that hydride transfer is partially rate limiting to catalysis.