Characterization of the FAD-Containing N-Methyltryptophan Oxidase from Escherichia coli

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Abstract
N-Methyltryptophan oxidase (MTOX) is a flavoenzyme that catalyzes the oxidative demethylation of N-methyl-l-tryptophan and other N-methyl amino acids, including sarcosine, which is a poor substrate. The Escherichia coli gene encoding MTOX (solA) was isolated on the basis of its sequence homology with monomeric sarcosine oxidase, a sarcosine-inducible enzyme found in many bacteria. These studies show that MTOX is expressed as a constitutive enzyme in a wild-type E. coli K-12 strain, providing the first evidence that solA is a functional gene. MTOX expression is enhanced 3-fold by growth on minimal media but not induced by N-methyl-l-tryptophan, l-tryptophan, or 3-indoleacrylate. MTOX forms an anionic flavin semiquinone and a reversible, covalent flavin−sulfite complex (Kd = 1.7 mM), properties characteristic of flavoprotein oxidases. Rates of formation (kon = 5.4 × 10-3 M-1 s-1) and dissociation (koff = 1.3 × 10-5 s-1) of the MTOX−sulfite complex are orders of magnitude slower than observed with most other flavoprotein oxidases. The pKa for ionization of oxidized FAD at N(3)H in MTOX (8.36) is two pH units lower than that observed for free FAD. The MTOX active site was probed by characterization of various substrate analogues that act as competitive inhibitors with respect to N-methyl-l-tryptophan. Qualitatively similar perturbations of the MTOX visible absorption spectrum are observed for complexes formed with various aromatic carboxylates, including benzoate, 3-indole-(CH2)n-CO2- and 2-indole-CO2-. The most stable complex with 3-indole-(CH2)n-CO2- is formed with 3-indolepropionate (Kd = 0.79 mM), a derivative with the same side chain length as N-methyl-l-tryptophan. Benzoate binding is enhanced upon protonation of a group in the enzyme−benzoate complex (pKEL = 6.87) but blocked by ionization of a group in the free enzyme (pKE = 8.41), which is attributed to N(3)H of FAD. Difference spectra observed for the aromatic carboxylate complexes are virtually mirror images of those observed with sarcosine analogues (N,N‘-dimethylglycine, N-benzylglycine). Charge-transfer complexes are formed with 3-indoleacrylate, pyrrole-2-carboxylate, and CH3XCH2CO2- (X = S, Se, Te).