Comparison of cyanide-degrading nitrilases

Recombinant forms of three cyanide-degrading nitrilases, CynD from Bacillus pumilus C1, CynD from Pseudomonas stutzeri, and CHT from Gloeocercospora sorghi, were prepared after their genes were cloned with C-terminal hexahistidine purification tags and expressed in Escherichia coli, and the enzymes purified using nickel-chelate affinity chromatography. The enzymes were compared with respect to their pH stability, thermostability, metal tolerance, and kinetic constants. The two bacterial genes, both cyanide dihydratases, were similar with respect to pH range, retaining greater than 50% activity between pH 5.2 and pH 8 and kinetic properties, having similar K _m (6-7 mM) and V _max (0.1 mmol min^-1 mg^-1). They also exhibited similar metal tolerances. However, the fungal CHT enzyme had notably higher K _m (90 mM) and V _max (4 mmol min^-1 mg^-1) values. Its pH range was slightly more alkaline (retaining nearly full activity above 8.5), but exhibited a lower thermal tolerance. CHT was less sensitive to Hg²⁺ and more sensitive to Pb²⁺ than the CynD enzymes. These data describe, in part, the current limits that exist for using nitrilases as agents in the bioremediation of cyanide-containing waste effluent, and may help serve to determine where and under what conditions these nitrilases may be applied.