Purification and characterization of intracellular and extracellular, thermostable and alkali-tolerant alcohol oxidases produced by a thermophilic fungus, Thermoascus aurantiacus NBRC 31693

An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols

Structure-guided directed evolution of choline oxidase has been carried out by using the oxidation of hexan-1-ol to hexanal as the target reaction. A six-amino-acid variant was identified with a 20-fold increased k_cat compared to that of the wild-type enzyme. This variant enabled the oxidation of 10 mm hexanol to hexanal in less than 24 h with 100 % conversion. Furthermore, this variant showed a marked increase in thermostability with a corresponding increase in T_m of 20 °C. Improved solvent tolerance was demonstrated with organic solvents including ethyl acetate, heptane and cyclohexane, thereby enabling improved conversions to the aldehyde by up to 30 % above conversion for the solvent-free system. Despite the evolution of choline oxidase towards hexan-1-ol, this new variant also showed increased specific activities (by up to 100-fold) for around 50 primary aliphatic, unsaturated, branched, cyclic, benzylic and halogenated alcohols.

The substrate tolerance of alcohol oxidases

Alcohols are a rich source of compounds from renewable sources, but they have to be activated in order to allow the modification of their carbon backbone. The latter can be achieved via oxidation to the corresponding aldehydes or ketones. As an alternative to (thermodynamically disfavoured) nicotinamide-dependent alcohol dehydrogenases, alcohol oxidases make use of molecular oxygen but their application is under-represented in synthetic biotransformations. In this review, the mechanism of copper-containing and flavoprotein alcohol oxidases is discussed in view of their ability to accept electronically activated or non-activated alcohols and their propensity towards over-oxidation of aldehydes yielding carboxylic acids. In order to facilitate the selection of the optimal enzyme for a given biocatalytic application, the substrate tolerance of alcohol oxidases is compiled and discussed: Substrates are classified into groups (non-activated prim- and sec-alcohols; activated allylic, cinnamic and benzylic alcohols; hydroxy acids; sugar alcohols; nucleotide alcohols; sterols) together with suitable alcohol oxidases, their microbial source, relative activities and (stereo)selectivities.

Quinone-dependent alcohol dehydrogenases and FAD-dependent alcohol oxidases

This review considers quinone-dependent alcohol dehydrogenases and FAD-dependent alcohol oxidases, enzymes that are present in numerous methylotrophic eu- and prokaryotes and significantly differ in their primary and quaternary structure. The cofactors of the enzymes are bound to the protein polypeptide chain through ionic and hydrophobic interactions. Microorganisms containing these enzymes are described. Methods for purification of the enzymes, their physicochemical properties, and spatial structures are considered. The supposed mechanism of action and practical application of these enzymes as well as their producers are discussed.

Purification and properties of alcohol oxidase from Pichia putida

Alcohol oxidase (AO) was extracted from the methylotrophic yeast Pichia putida and purified using various methods. AO purified by crystallization was homogeneous based on analytical centrifugation with subsequent gel filtration and SDS-PAGE. The molecular weight of the enzyme was around 600 kDa. SDS-PAGE revealed a single protein band (74 +/- 4 kDa), and 8-9 bands of native protein with similar specific AO activities and substrate specificities were identified by PAGE without SDS. Electron microscopy of a single molecule revealed eight subunits located on the top of a regular tetragon with dotted symmetry of 422 D4 providing evidence that AO consists of eight subunits. Apparently, each molecule of AO has two types of subunits with very similar molecular weights and differing from each other by the number of acidic and basic amino acid residues. Each subunit includes one molecule of FAD and 2-3 cysteine residues. The pH optimum was within 8.5-9.0. Specific activity of the enzyme varied from 10 to 50 micromol methanol/min per mg protein from batch to batch depending on separation methods and had linear relationship with protein concentration. The AO was quickly inactivated at 20 degrees C and seemed to be stable in phosphate-citrate buffer with 30-50% (w/v) of sucrose. Different forms of 0.1-1 mm crystals of the enzyme were obtained. However the crystals did not yield X-ray reflections, apparently as a result of their molecular microheterogeneity.

New oxidase from Bjerkandera arthroconidial anamorph that oxidizes both phenolic and nonphenolic benzyl alcohols

A new flavooxidase is described from a Bjerkandera arthroconidial anamorph. Its physicochemical characteristics, a monomeric enzyme containing non-covalently bound flavin adenine dinucleotide (FAD), and several catalytic properties, such as oxidation of aromatic and polyunsaturated aliphatic primary alcohols, are similar to those of Pleurotus eryngii aryl-alcohol oxidase (AAO). However, it also efficiently oxidizes phenolic benzyl and cinnamyl alcohols that are typical substrates of vanillyl-alcohol oxidase (VAO), a flavooxidase from a different family, characterized by its multimeric nature and presence of covalently-bound FAD. The enzyme also differs from P. eryngii AAO by having extremely high efficiency oxidizing chlorinated benzyl alcohols (1000-1500 s(-1) mM(-1)), a feature related to the different alcohol metabolites secreted by the Pleurotus and Bjerkandera species including chloroaromatics, and higher activity on aromatic aldehydes. What is even more intriguing is the fact that, the new oxidase is optimally active at pH 6.0 on both p-anisyl and vanillyl alcohols, suggesting a mechanism for phenolic benzyl alcohol oxidation that is different from that described in VAO, which proceeds via the substrate phenolate anion formed at basic pH. Based on the above properties, and its ADP-binding motif, partially detected after N-terminus sequencing, the new enzyme is classified as a member of the GMC (glucose-methanol-choline oxidase) oxidoreductase family oxidizing both AAO and VAO substrates.

Characteristics of Gloeophyllum trabeum alcohol oxidase, an extracellular source of H2O2 in brown rot decay of wood

A novel alcohol oxidase (AOX) has been purified from mycelial pellets of the wood-degrading basidiomycete Gloeophyllum trabeum and characterized as a homooctameric nonglycosylated protein with native and subunit molecular masses of 628 and 72.4 kDa, containing noncovalently bonded flavin adenine dinucleotide. The isolated AOX cDNA contained an open reading frame of 1,953 bp translating into a polypeptide of 651 amino acids displaying 51 to 53% identity with other published fungal AOX amino acid sequences. The enzyme catalyzed the oxidation of short-chain primary aliphatic alcohols with a preference for methanol (K(m) = 2.3 mM, k(cat) = 15.6 s(-1)). Using polyclonal antibodies and immunofluorescence staining, AOX was localized on liquid culture hyphae and extracellular slime in sections from degraded wood and on cotton fibers. Transmission electron microscopy immunogold labeling localized the enzyme in the hyphal periplasmic space and wall and on extracellular tripartite membranes and slime, while there was no labeling of hyphal peroxisomes. AOX was further shown to be associated with membranous or slime structures secreted by hyphae in wood fiber lumina and within the secondary cell walls of degraded wood fibers. The differences in AOX targeting compared to the known yeast peroxisomal localization were traced to a unique C-terminal sequence of the G. trabeum oxidase, which is apparently responsible for the protein's different translocation. The extracellular distribution and the enzyme's abundance and preference for methanol, potentially available from the demethylation of lignin, all point to a possible role for AOX as a major source of H(2)O(2), a component of Fenton's reagent implicated in the generally accepted mechanisms for brown rot through the production of highly destructive hydroxyl radicals.

Purification and characterization of a novel alcohol oxidase from Paenibacillus sp. AIU 311

An oxidase catalyzing the conversion of glycolaldehyde to glyoxal was purified to the homogeneous state from Paenibacillus sp. AIU 311, and its properties were revealed. This enzyme was specific to glycolaldehyde and glyceraldehyde, and the reaction rates to other alcohols and aldehydes were less than 6% of that of glycolaldehyde. The Km values for glycolaldehyde and glyceraldehyde were estimated to be 13.2 and 7.5 mM, respectively. The glycolaldehyde oxidation was optimum at pH 6.5 and 50 degrees C. The molecular mass of this enzyme was 49 kDa, and it consisted of two identical subunits of 24 kDa. The NH2-terminal sequence was not homologous to those of alcohol oxidases. This is the first report of an oxidase exhibiting high specificity to a hydroxy group of aldehyde alcohols.

Flavoenzymes

Flavoenzymes are colourful oxidoreductases that catalyze a large variety of different types of reactions. Flavoenzymes have been extensively studied for their structural and mechanistic properties and are gaining momentum in industrial biocatalytic applications. Some of these enzymes catalyze the oxidative modification of protein substrates. New insights in oxidative flavoenzymes and in particular in novel family members point towards their potential application in the pharmaceutical, fine-chemical and food industries.

Characterization of alcohol oxidase from Aspergillus ochraceus AIU 031

An alcohol oxidase (AOD) was found from Aspergillus ochraceus AIU 031, and its characteristics were revealed. This enzyme oxidized short-chain primary alcohols and ethylene glycol, and belonged to the same group as AOD from methylotrophic yeast. However, it differed in the following properties. The K(m) value for ethanol was larger and that for ethylene glycol was smaller than those of AODs derived from methylotrophic yeasts. The ethanol oxidation was optimal at pH 5-7 and 50-55 degrees C. The molecular mass of this enzyme was 262 kDa and consisted of four identical subunits of 68 kDa, which were much smaller than those of methylotrophic yeasts.

Distribution, diversity and regulation of alcohol oxidase isozymes, and phylogenetic relationships of methylotrophic yeasts

In this study, we attempted to classify the methylotrophic yeasts based on diversities of alcohol oxidase (AOD), i.e. zymogram patterns and partial amino acid sequences. According to zymogram patterns for AOD, members of the methylotrophic yeasts separate into two major lineages, one group involving strains having a single AOD and the other group, including Pichia methanolica, Candida pignaliae and C. sonorensis, showing nine AOD isozymes. Based on partial amino acid sequences of AOD, the methylotrophic yeasts could be divided into five groups, and this classification agrees mostly with grouping based on 26S domain D1/D2 rDNA nucleotide sequences, except for some strains. Moreover, the strains having AOD isozymes constitute one group with P. trehalophila, P. glucozyma and Pichia sp. strain BZ159, although these strains are divided into two types, based on amino acid sequences of second AODs. On the other hand, these AOD isozymes consist of two subunits; the first subunits are induced not only by methanol but also by glycerol and pectin, although the second subunits are mainly induced by methanol. These data indicate that AOD isozymes and second AOD genes distribute widely in several methylotrophic yeasts in the natural environment, and second AOD genes may have evolved as methylotrophic genes that can adapt to the environmental conditions of higher methanol concentrations.