Biochemical and Genetic Characterization of a Gentisate 1,2-Dioxygenase from <i>Sphingomonas</i> sp. Strain RW5

ABSTRACT A 4,103-bp long DNA fragment containing the structural gene of a gentisate 1,2-dioxygenase (EC 1.13.11.4 ), gtdA , from Sphingomonas sp. strain RW5 was cloned and sequenced. The gtdA gene encodes a 350-amino-acid polypeptide with a predicted size of 38.85 kDa. Comparison of the gtdA gene product with protein sequences in databases, including those of intradiol or extradiol ring-cleaving dioxygenases, revealed no significant homology except for a low similarity (27%) to the 1-hydroxy-2-naphthoate dioxygenase ( phdI ) of the phenanthrene degradation in Nocardioides sp. strain KP7 (T. Iwabuchi and S. Harayama, J. Bacteriol. 179:6488–6494, 1997). This gentisate 1,2-dioxygenase is thus a member of a new class of ring-cleaving dioxygenases. The gene was subcloned and hyperexpressed in E. coli . The resulting product was purified to homogeneity and partially characterized. Under denaturing conditions, the polypeptide exhibited an approximate size of 38.5 kDa and migrated on gel filtration as a species with a molecular mass of 177 kDa. The enzyme thus appears to be a homotetrameric protein. The purified enzyme stoichiometrically converted gentisate to maleylpyruvate, which was identified by gas chromatography-mass spectrometry analysis as its methyl ester. Values of affinity constants ( K m ) and specificity constants ( K cat / K m ) of the enzyme were determined to be 15 μM and 511 s −1 M −1 × 10 4 for gentisate and 754 μM and 20 s −1 M −1 × 10 4 for 3,6-dichlorogentisate. Three further open reading frames (ORFs) were found downstream of gtdA . The deduced amino acid sequence of ORF 2 showed homology to several isomerases and carboxylases, and those of ORFs 3 and 4 exhibited significant homology to enzymes of the glutathione isomerase superfamily and glutathione reductase superfamily, respectively.