Aryl-alcohol Oxidase Involved in Lignin Degradation: A mechanistic study based onsteady and pre-steady state kinetics and primary and solvent isotope effects with two alcohol substrates

8 páginas, 6 figuras, 3 tablas, 1 esquema -- PAGS nros. 24840-24847Aryl-alcohol oxidase (AAO) is a FAD-containing enzyme in the GMC (glucose-methanol-choline oxidase) family of oxidoreductases. AAO participates in fungal degradation of lignin, a process of high ecological and biotechnological relevance, by providing the hydrogen peroxide required by ligninolytic peroxidases. In the Pleurotus species, this peroxide is generated in the redox cycling of p-anisaldehyde, an extracellular fungal metabolite. In addition to p-anisyl alcohol, the enzyme also oxidizes other polyunsaturated primary alcohols. Its reaction mechanism was investigated here using p-anisyl alcohol and 2,4-hexadien-1-ol as two AAO model substrates. Steady state kinetic parameters and enzyme-monitored turnover were consistent with a sequential mechanism in which O2 reacts with reduced AAO before release of the aldehyde product. Pre-steady state analysis revealed that the AAO reductive half-reaction is essentially irreversible and rate limiting during catalysis. Substrate and solvent kinetic isotope effects under steady and pre-steady state conditions (the latter showing ∼9-fold slower enzyme reduction when α-bideuterated substrates were used, and ∼13-fold slower reduction when both substrate and solvent effects were simultaneously evaluated) revealed a synchronous mechanism in which hydride transfer from substrate α-carbon to FAD and proton abstraction from hydroxyl occur simultaneously. This significantly differs from the general mechanism proposed for other members of the GMC oxidoreductase family that implies hydride transfer from a previously stabilized substrate alkoxide.Wood and other lignocellulosic materials are the main source of renewable materials in earth. White-rot basidiomycetes are essential contributors to carbon cycling in forest and other land ecosystems because of their ability to degrade lignocellulose to carbon dioxide and water. This ability confers to these fungi and their ligninolytic enzymes high interest in industrial processes, such as bioethanol production and paper pulp manufacturing, where the removal of lignin is a previous and essential step to use the cellulose present in plant biomass as a source for renewable fuels, chemicals, and materials (1). Aryl-alcohol oxidase (AAO)5 is an extracellular FAD-containing enzyme (2) that, in collaboration with myceliar aryl-alcohol dehydrogenases, participates in lignin degradation by some white-rot fungi, such as Pleurotus (and Bjerkandera) species, by generating hydrogen peroxide in the redox cycling of aromatic fungal metabolites, such as p-anisaldehyde (3, 4). Fungal high redox-potential peroxidases catalyze the oxidative degradation of lignin by this extracellular peroxide (5).AAO was cloned for the first time in Pleurotus eryngii (6), a fungus of biotechnological interest because of its ability to degrade lignin selectively (7). The AAO amino acid sequence revealed moderate homology with glucose oxidase from Aspergillus niger (8), a flavoenzyme in the glucose-methanol-choline oxidases (GMC) oxidoreductase family. The reported molecular model of AAO (9), based on the glucose oxidase crystal structure (10), showed common features with the overall structural topology of bacterial choline oxidase and almond hydroxynitrile lyase (a lyase with oxidoreductase structure), as well as with other members of the GMC family; such as the extracellular flavoenzymes pyranose-2-oxidase and cellobiose dehydrogenase from white-rot basidiomycetes, and bacterial cholesterol oxidase (11–15). In particular, P. eryngii AAO conserves two histidine residues, His-502 and His-546 (supplemental Fig. S1), involved in catalysis in different members of this family (the second residue is an asparagine in some of them) (9). Non-glycosylated P. eryngii AAO expressed in Escherichia coli (16) is used for further characterization studies. The enzyme catalyzes the oxidative dehydrogenation of unsaturated alcohols with a primary hydroxyl at Cα, exhibiting broad substrate specificity. In addition to benzyl alcohols, its active site also binds and oxidizes aliphatic polyunsaturated primary alcohols (such as 2,4-hexadien-1-ol), naphthyl, and cinnamyl alcohols, and shows low activity on some aromatic aldehydes (17). Methanol and other saturated alcohols are not AAO substrates, and the monounsaturated allyl alcohol is very slowly oxidized (2).It is suggested that the AAO catalytic mechanism proceeds via electrophilic attack and direct transfer of a hydride to the flavin (17). A recent mutational study confirmed the strict requirement for catalysis of His-502 and His-546 located near the isoalloxazine ring of FAD (supplemental Fig. S1), as well as the involvement of two aromatic residues (18). Here we present the first study on the reaction mechanism of AAO in which substrate and solvent kinetic isotope effect (KIE), in combination with bisubstrate steady state and pre-steady state kinetic approaches, have been used to investigate the mechanism of polyunsaturated primary alcohol oxidation by AAO. Its natural substrate, p-anisyl alcohol, as well as a structurally different (non-aromatic) AAO substrate, 2,4-hexadien-1-ol, were chosen as two models for the different AAO alcohol substratesThis work was supported by the Spanish Projects BIO2005-03569, BIO2007-65890-C02-01, and BIO2008-01533, and the BIORENEW Project of the European Union (Contract NMP2-CT-2006-026456)Peer reviewe