Flavin-dependent alcohol oxidase from yeast. Studies on the catalytic mechanism and inactivation during turnover

Alcohol Oxidase from the Methylotrophic Yeast Ogataea polymorpha: Isolation, Purification, and Bioanalytical Application

Alcohol oxidase (EC 1.1.3.13; AOX) is a flavoprotein that catalyzes the oxidation of primary short-chain alcohols to corresponding carbonyl compounds with a concomitant release of hydrogen peroxide. It is a key enzyme of methanol metabolism in methylotrophic yeasts, catalyzing the first step of methanol oxidation to formaldehyde.Here we describe the isolation and purification of AOX from the thermotolerant methylotrophic yeast Ogataea (Hansenula) polymorpha, and using this enzyme in enzymatic assay of ethanol, simultaneous analysis of methanol and formaldehyde, and in construction of amperometric biosensors selective to primary alcohols and formaldehyde.

Light-Driven Catalytic Upgrading of Butanol in a Biohybrid Photoelectrochemical System

This paper reports the design and preparation of a biohybrid photoelectrochemical cell (PEC) that can drive the tandem enzymatic oxidation and aldol condensation of n-butanol (BuOH) to C8 2-ethylhexenal (2-EH). In this work, BuOH was first oxidized to n-butyraldehyde (BA) by the alcohol oxidase enzyme (AOx), concurrently generating hydrogen peroxide (H2O2). To preserve enzyme activity and increase kinetics nearly 2-fold, the H2O2 was removed by oxidation at a bismuth vanadate (BiVO4) photoanode. Organocatalyzed aldol condensation of C4 BA to C8 2-EH improved the overall BuOH conversion to 6.2 ± 0.1% in a biased PEC after 16 h. A purely light-driven, unbiased PEC showed 3.1 ± 0.1% BuOH conversion, or ~50% of that obtained from the biased system. Replacing AOx with the enzyme alcohol dehydrogenase (ADH), which requires the diffusional nicotinamide adenine dinucleotide cofactor (NAD+/NADH), resulted in only 0.2% BuOH conversion due to NAD+ dimerization at the photoanode. Lastly, the application of more positive biases with the biohybrid AOx PEC led to measurable production of H2 at the cathode, but at the cost of lower BA and 2-EH yields due to both product overoxidation and decreased enzyme activity.

Crystal Structure of Alcohol Oxidase from Pichia pastoris

FAD-dependent alcohol oxidases (AOX) are key enzymes of methylotrophic organisms that can utilize lower primary alcohols as sole source of carbon and energy. Here we report the crystal structure analysis of the methanol oxidase AOX1 from Pichia pastoris. The crystallographic phase problem was solved by means of Molecular Replacement in combination with initial structure rebuilding using Rosetta model completion and relaxation against an averaged electron density map. The subunit arrangement of the homo-octameric AOX1 differs from that of octameric vanillyl alcohol oxidase and other dimeric or tetrameric alcohol oxidases, due to the insertion of two large protruding loop regions and an additional C-terminal extension in AOX1. In comparison to other alcohol oxidases, the active site cavity of AOX1 is significantly reduced in size, which could explain the observed preference for methanol as substrate. All AOX1 subunits of the structure reported here harbor a modified flavin adenine dinucleotide, which contains an arabityl chain instead of a ribityl chain attached to the isoalloxazine ring.

Ergot cluster-encoded catalase is required for synthesis of chanoclavine-I in Aspergillus fumigatus

Genes required for ergot alkaloid biosynthesis are clustered in the genomes of several fungi. Several conserved ergot cluster genes have been hypothesized, and in some cases demonstrated, to encode early steps of the pathway shared among fungi that ultimately make different ergot alkaloid end products. The deduced amino acid sequence of one of these conserved genes (easC) indicates a catalase as the product, but a role for a catalase in the ergot alkaloid pathway has not been established. We disrupted easC of Aspergillus fumigatus by homologous recombination with a truncated copy of that gene. The resulting mutant (ΔeasC) failed to produce the ergot alkaloids typically observed in A. fumigatus, including chanoclavine-I, festuclavine, and fumigaclavines B, A, and C. The ΔeasC mutant instead accumulated N-methyl-4-dimethylallyltryptophan (N-Me-DMAT), an intermediate recently shown to accumulate in Claviceps purpurea strains mutated at ccsA (called easE in A. fumigatus) (Lorenz et al. Appl Environ Microbiol 76:1822-1830, 2010). A ΔeasE disruption mutant of A. fumigatus also failed to accumulate chanoclavine-I and downstream ergot alkaloids and, instead, accumulated N-Me-DMAT. Feeding chanoclavine-I to the ΔeasC mutant restored ergot alkaloid production. Complementation of either ΔeasC or ΔeasE mutants with the respective wild-type allele also restored ergot alkaloid production. The easC gene was expressed in Escherichia coli, and the protein product displayed in vitro catalase activity with H(2)O(2) but did not act, in isolation, on N-Me-DMAT as substrate. The data indicate that the products of both easC (catalase) and easE (FAD-dependent oxidoreductase) are required for conversion of N-Me-DMAT to chanoclavine-I.

Cholesterol oxidase: biochemistry and structural features

Cholesterol oxidases are bifunctional flavoenzymes that catalyze the oxidation of steroid substrates which have a hydroxyl group at the 3beta position of the steroid ring system. The enzyme is found, in a wide range of bacterial species, in two forms: one with the FAD cofactor bound noncovalently to the enzyme; and one with the cofactor linked covalently to the protein. Here we discuss, compare and contrast the salient biochemical properties of the two forms of the enzyme. Specifically, the structural features are discussed that affect the redox potentials of the flavin cofactor, the chemical mechanism of substrate dehydrogenation by active-center amino acid residues, the kinetic parameters of both types of enzymes and the reactivity of reduced enzymes with molecular dioxygen. The presence of a molecular tunnel that is proposed to serve in the access of dioxygen to the active site and mechanisms of its control by a 'gate' formed by amino acid residues are highlighted.

Catalytic mechanism of the oxidative demethylation of 4-(methoxymethyl)phenol by vanillyl-alcohol oxidase. Evidence for formation of a p-quinone methide intermediate

The catalytic mechanism for the oxidative demethylation of 4-(methoxymethyl)phenol by the covalent flavoprotein vanillyl-alcohol oxidase was studied. Using H218O, it was found that the carbonylic oxygen atom from the product 4-hydroxybenzaldehyde originates from a water molecule. Oxidation of vanillyl alcohol did not result in any incorporation of 18O. Enzyme-monitored turnover experiments revealed that for both substrates a process involving flavin reduction is rate determining. During anaerobic reduction of vanillyl-alcohol oxidase by 4-(methoxymethyl)phenol, a relatively stable spectral intermediate is formed. Deconvolution of its spectral characteristics showed a typical pH-independent absorption maximum at 364 nm (epsilon364 nm = 46 mM-1 cm-1). A similar transient species was observed upon anaerobic reduction by vanillyl alcohol. The rate of flavin reduction and synchronous intermediate formation by 4-(methoxymethyl)phenol is 3.3 s-1 and is fast enough to account for turnover (3.1 s-1). The anaerobic decay of the intermediate was too slow (0.01 s-1) to be of catalytical relevance. The reduced binary complex is rapidly reoxidized (1.5 x 10(5) M-1 s-1) and is accompanied with formation and release of product. Oxidation of free-reduced enzyme is an even faster process (3.1 x 10(5) M-1 s-1). The kinetic data for the oxidative demethylation of 4-(methoxymethyl)phenol are in accordance with a ternary complex mechanism in which the reduction rate is rate-limiting. It is proposed that, upon reduction, a binary complex is produced composed of the p-quinone methide of 4-(methoxymethyl)phenol and reduced enzyme.

Substrate specificity of flavin-dependent vanillyl-alcohol oxidase from Penicillium simplicissimum. Evidence for the production of 4-hydroxycinnamyl alcohols from 4-allylphenols

The substrate specificity of the flavoprotein vanillyl-alcohol oxidase from Penicillium simplicissimum was investigated. Vanillyl-alcohol oxidase catalyzes besides the oxidation of 4-hydroxybenzyl alcohols, the oxidative deamination of 4-hydroxybenzylamines and the oxidative demethylation of 4-(methoxymethyl)phenols. During the conversion of vanillylamine to vanillin, a transient intermediate, most probably vanillylimine, is observed. Vanillyl-alcohol oxidase weakly interacts with 4-hydroxyphenylglycols and a series of catecholamines. These compounds are converted to the corresponding ketones. Both enantiomers of (nor)epinephrine are substrates for vanillyl-alcohol oxidase, but the R isomer is preferred. Vanillyl-alcohol oxidase is most active with chavicol and eugenol. These 4-allylphenols are converted to coumaryl alcohol and coniferyl alcohol, respectively. Isotopic labeling experiments show that the oxygen atom inserted at the C gamma atom of the side chain is derived from water. The 4-hydroxycinnamyl alcohol products and the substrate analog isoeugenol are competitive inhibitors of vanillyl alcohol oxidation. The binding of isoeugenol to the oxidized enzyme perturbs the optical spectrum of protein-bound FAD. pH-dependent binding studies suggest that vanillyl-alcohol oxidase preferentially binds the phenolate form of isoeugenol (pKa < 6, 25 degrees C). From this and the high pH optimum for turnover, a hydride transfer mechanism involving a p-quinone methide intermediate is proposed for the vanillyl-alcohol-oxidase-catalyzed conversion of 4-allylphenols.

Flavin-dependent alcohol oxidase from the yeast Pichia pinus. Spatial localization of the coenzyme FAD in the protein structure: hot-tritium bombardment and ESR experiments

The spatial localization of the coenzyme FAD in the quaternary structure of the alcohol oxidase from the yeast Pichia pinus was studied by tritium planigraphy and ESR methods. In the present paper we measured the specific radioactivity of FAD labelled as a part of the alcohol oxidase complex. The specific-radioactivity ratio for two FAD portions (FMN and AMP) was calculated. ESR experiments show 4 A (0.4 nm) to be the depth of immersion of paramagnetic isoalloxazines into alcohol oxidase octamer molecules. It is suggested that FAD molecules are bound to the surface of the octamer, rather than to the subunit interfaces. The orientation of the prosthetic group FAD in the alcohol oxidase protein is discussed.

NADH-dependent methemoglobin reductase from the obligate aerobe Vitreoscilla: improved method of purification and reexamination of prosthetic groups

The NADH-dependent methemoglobin reductase from the bacterium Vitreoscilla was purified using hydrophobic chromatography on a phenyl-Sepharose column. The new procedure resulted in a purer protein and increased the overall yield of the enzyme by a factor of approximately three. The active site of the enzyme was investigated by ultraviolet/visible, fluorescence, Mössbauer, and electron paramagnetic resonance spectroscopy (EPR) at 9.4 GHz. Prosthetic group analysis revealed the presence of one FAD per active enzyme molecule but no iron in contrast to earlier reports. The NADH-methemoglobin reductase activity of the pure enzyme was in the range of 1.1-1.25 units; its electronic and fluorescence spectra were typical of metal-free flavoproteins. No EPR signals were detected between 5 and 150 K over a field range 0.05-0.5 T, and there was no Mössbauer signal, consistent with the absence of iron. Methemoglobin reductase from Vitreoscilla was reduced by dithionite, NADH, and deazaflavin/EDTA upon illumination. The main species observed during these anaerobic oxidation-reduction experiments was the blue semiquinone radical with an EPR signal at g = 2.005, linewidth 1.5 mT. The fully reduced state of the enzyme, FlredH3, was also observed in the reaction with NADH. The reduction was fully reversible with ferricyanide. The observations reported here are consistent with a redox enzyme interacting both with a two-electron donating agent such as NADH and a one-electron accepting center such as the Fe(III)/Fe(II) couple of Vitreoscilla hemoglobin.

Mechanisms of flavoprotein-catalyzed reactions

Flavoproteins are a class of enzymes catalyzing a very broad spectrum of redox processes by different chemical mechanisms. This review describes the best studied of these mechanisms and discusses factors possibly governing reactivity and specificity.