A study in UF-membrane reactor on activity and stability of nitrile hydratase from Microbacterium imperiale CBS 498-74 resting cells for propionamide production

The bioconversion of propionitrile to propionamide was catalysed by nitrile hydratase (NHase) using resting cells of Microbacterium imperiale CBS 498-74 (formerly, Brevibacterium imperiale). This microorganism, cultivated in a shake flask, at 28 ◦C, presented a specific NHase activity of 34.4Umg−1 DCW (dry cell weight). The kinetic parameters, Km and Vmax, tested in 50mM sodium phosphate buffer, pH 7.0, in the propionitrile bioconversion was evaluated in batch reactor at 10 ◦C and resulted 21.6mM and 11.04mmol min−1 mg−1 DCW, respectively. The measured apparent activation energy, 25.54 kJ mol−1, indicated a partial control by mass transport, more likely through the cell wall. UF-membrane reactors were used for kinetic characterisation of the NHase catalysed reaction. The time dependence of enzyme deactivation on reaction temperature (from 5 to 25 ◦C), on substrate concentrations (from 100 to 800 mM), and on resting cell loading (from 1.5 to 200mgDCW ml−1) indicated: lower diffusional control (Ea = 37.73 kJ mol−1); and NHase irreversible damage caused by high substrate concentration. Finally, it is noteworthy that in an integral reactor continuously operating for 30 h, at 10 ◦C, 100% conversion of propionitrile (200 mM) was attained using 200mgDCW ml−1 of resting cells, with a maximum volumetric productivity of 0.5 g l−1 h−1.