A possible involvement of sulfhydryl groups in the conversion of lysine monooxygenase to an oxidase

Dual activities of oxidation and oxidative decarboxylation by flavoenzymes

Specific flavoenzyme oxidases catalyze oxidative decarboxylation in addition to their classical oxidation reactions in the same active sites. The mechanisms underlying oxidative decarboxylation by these enzymes and how they control their two activities are not clearly known. This article reviews the current state of knowledge of four enzymes from the l-amino acid oxidase and l-hydroxy acid oxidase families, including l-tryptophan 2-monooxygenase, l-phenylalanine 2-oxidase and l-lysine oxidase/monooxygenase and lactate monooxygenase which catalyze substrate oxidation and oxidative decarboxylation. Apart from specific interactions to allow substrate oxidation by the flavin cofactor, specific binding of oxidized product in the active sites appears to be important for enabling subsequent decarboxylation by these enzymes. Based on recent findings of l-lysine oxidase/monooxygenase, we propose that nucleophilic attack of H2O2 on the imino acid product is the mechanism enabling oxidative decarboxylation.

Mechanistic insights into the dual activities of the single active site of l-lysine oxidase/monooxygenase from Pseudomonas sp. AIU 813

l-Lysine oxidase/monooxygenase (l-LOX/MOG) from Pseudomonas sp. AIU 813 catalyzes the mixed bioconversion of l-amino acids, particularly l-lysine, yielding an amide and carbon dioxide by an oxidative decarboxylation (i.e. apparent monooxygenation), as well as oxidative deamination (hydrolysis of oxidized product), resulting in α-keto acid, hydrogen peroxide (H₂O₂), and ammonia. Here, using high-resolution MS and monitoring transient reaction kinetics with stopped-flow spectrophotometry, we identified the products from the reactions of l-lysine and l-ornithine, indicating that besides decarboxylating imino acids (i.e. 5-aminopentanamide from l-lysine), l-LOX/MOG also decarboxylates keto acids (5-aminopentanoic acid from l-lysine and 4-aminobutanoic acid from l-ornithine). The reaction of reduced enzyme and oxygen generated an imino acid and H₂O₂, with no detectable C4a-hydroperoxyflavin. Single-turnover reactions in which l-LOX/MOG was first reduced by l-lysine to form imino acid before mixing with various compounds revealed that under anaerobic conditions, only hydrolysis products are present. Similar results were obtained upon H₂O₂ addition after enzyme denaturation. H₂O₂ addition to active l-LOX/MOG resulted in formation of more 5-aminopentanoic acid, but not 5-aminopentamide, suggesting that H₂O₂ generated from l-LOX/MOG in situ can result in decarboxylation of the imino acid, yielding an amide product, and extra H₂O₂ resulted in decarboxylation only of keto acids. Molecular dynamics simulations and detection of charge transfer species suggested that interactions between the substrate and its binding site on l-LOX/MOG are important for imino acid decarboxylation. Structural analysis indicated that the flavoenzyme oxidases catalyzing decarboxylation of an imino acid all share a common plug loop configuration that may facilitate this decarboxylation.

A novel enzyme, L-tryptophan oxidase, from a basidiomycete, Coprinus sp. SF-1: purification and characterization

A basidiomycete, Coprinus sp. SF-1, was found to produce an L-Trp-oxidizing enzyme by screening from the culture collection of our laboratory. After solubilization by 1 M NaSCN from the particulate fraction of disrupted cells of the strain, the enzyme was purified about 76-fold to essential homogeneity. The enzyme had a molecular mass of about 420 kDa and the subunit molecular mass was 68 kDa. The enzyme contained 1 mol of non-covalently bound FAD per mol of the subunit. It catalyzed the simultaneous reactions of oxidative deamination and oxygenative decarboxylation of L-Trp to form indolepyruvic acid and indole-3-acetamide, the former of which was further oxidized to indole-3-acetic acid. The molar ratio of the respective reaction products was about 9:1. The enzyme specifically oxidized L-Trp, and slightly acted on L-Phe and L-Tyr. The Km for L-Trp was about 0.5 mM in both oxidase and oxygenase reactions. Thus, the enzyme is a novel one and was tentatively designated "L-Trp oxidase (deaminating and decarboxylating)". The optimum pHs of oxidase and oxygenase activities were 7.0 and 9.0, respectively. The optimum temperatures of both activities were 50 degrees C. The enzyme was stable at pH 6.0-10.5 and below 50 degrees C, and at 4 degrees C for 1 year.