The Antarctic Psychrobacter sp. TAD1 has two cold-active glutamate dehydrogenases with different cofactor specificities. Characterisation of the NAD+-dependent enzyme.

Abstract Psychrobacter sp. TAD1 is a psychrotolerant bacterium from Antarctic frozen continental water that grows from 2 to 25 °C with optimal growth rate at 20 °C. The new isolate contains two glutamate dehydrogenases (GDH), differing in their cofactor specificities, subunit sizes and arrangements, and thermal properties. NADP + -dependent GDH is a hexamer of 47 kDa subunits and it is comparable to other hexameric GDHs of family-I from bacteria and lower eukaria. The NAD + -dependent enzyme, described in this communication, has a subunit weight of 160 kDa and belongs to the novel class of GDHs with large size subunits. The enzyme is a dimer; this oligomeric arrangement has not been reported previously for GDH. Both enzymes have an apparent optimum temperature for activity of approximately 20 °C, but their cold activities and thermal labilities are different. The NAD + -dependent enzyme is more cold active: at 10 °C it retains 50% of its maximal activity, compared with 10% for the NADP + -dependent enzyme. The NADP + -dependent enzyme is more heat stable, losing only 10% activity after heating for 30 min, compared with 95% for the NAD + -dependent enzyme. It is concluded that in Psychrobacter sp. TAD1 not only does NAD + -dependent GDH have a novel subunit molecular weight and arrangement, but that its polypeptide chains are folded differently from those of NADP + -dependent GDH, providing different cold-active properties to the two enzymes.