A propionitrile-induced nitrilase of Rhodococcus sp. NDB 1165 and its application in nicotinic acid synthesis

Rhodococcus sp. NDB 1165, a nitrile-transforming organism was isolated from temperate forest soil of Himalayas. The nitrilase (EC 3.5.5.2) activity of this organism had higher substrate specificity toward aromatic nitriles (benzonitrile, 3-cyanopyridine and 4-cyanopyridine) and unsaturated aliphatic nitrile (acrylonitrile) in comparison to saturated aliphatic nitriles (acetonitrile, propionitrile, butyronitrile and isobutyronitrile) nitrile and arylacetonitrile (phenylacetonitrile and indole-3-acetonitrile). The nitrilase of Rhodococcus sp. NDB 1165 was inducible in nature and propionitrile proved to be an efficient inducer. However, the salts of ferrous and cobalt ions had an inhibitory effect. Under optimized reaction conditions (pH 8.0 and temperature 45°C) the nitrilase activity of this organism was 2.39 ± 0.07 U/mg dry cell mass (dcm). The half-life of this enzyme was 150 min and 40 min at 45°C and 50°C respectively. However, it was quite stable at 40°C and around 58 % activity was retained even after 6 h at this temperature. The V_max and K_m value of this nitrilase were 1.67 μmol/ml min and 0.1 M respectively using 3-cyanopyridine as substrate. However, the decrease in V_max and K_m values (0.56 μmol/ml min and 0.02 M, respectively) were observed at >0.05 M 3-cyanopyridine which revealed that this enzyme experienced uncompetitive inhibition at higher substrate concentrations. Under optimized reaction conditions, 1.6 M 3-cyanopyridine was successfully converted in to nicotinic acid using 2.0 mg resting cells (dcm)/ml reaction mixture in 11 h. This is the highest production of nicotinic acid i.e. 8.95 mg/mg resting cells (dcm)/h as compared to nitrilase systems reported hitherto.