Carbazole is a nitrogen-containing heteroaromatic compound that occurs as a widespread and mutagenic environmental pollutant. The 2′aminobiphenyl-2, 3-diol 1,2-dioxygenase involved in carbazole degradation was purified to near electrophoretic homogeneity from Pseudomonas sp. LD2 by a combination of ion-exchange chromatography, ammonium sulfate precipitation, and hydrophobic interaction chromatography. This purification was challenging due to the great instability of the enzyme under many standard conditions. The enzyme was also purified to electrophoretic homogeneity from recombinant Escherichia coli expressing the 2′aminobiphenyl-2,3-diol 1,2-dioxygenase-encoding gene cloned from Pseudomonas sp. LD2. The molecular mass of the native enzyme was determined by gel filtration to be 70 kDa. The subunit molecular masses were determined to be 25 and 8 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, indicating that the dioxygenase is an [α 2β2] heterotetramer. The optimal temperature and pH for the enzymatic production of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) from 2,3-dihydroxybiphenyl were determined to be 40°C and 8.0, respectively. The maximum observed specific activity on 2,3-dihydroxybiphenyl was 48.1 mmol HOPDA min-1 mg-1. This indicated a maximum observed turnover rate of 360,000 molecules HOPDA enz-1 s -1. The Km′, inhibition constant Ks, and Vmax on 2,3 dihydroxybiphenyl were determined to be 5 μM, 37 μM, and 44 mmol min-1 mg-1, respectively. These results show that 2′aminobiphenyl-2,3-diol 1,2-dioxygenase is a meta-cleavage enzyme related to the 4,5-protocatechuate dioxygenase family, with comparable purification challenges posed by intrinsic enzyme instability.
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