Nicotinamidase from the thermophilic archaeon Acidilobus saccharovorans: structural and functional characteristics

Nicotinamidase is involved in the maintenance of NAD+ homeostasis and in the NAD+ salvage pathway of most prokaryotes, and it is considered as a possible drug target. The gene (ASAC_0847) encoding a hypothetical nicotinamidase has been found in the genome of the thermophilic archaeon Acidilobus saccharovorans. The product of this gene, NA_As0847, has been expressed in Escherichia coli, isolated, and characterized as a Fe(2+)-containing nicotinamidase (k(cat)/K(m) = 427 mM(-1)·sec(-1))/pyrazinamidase (k(cat)/K(m) = 331 mM(-1)·sec(-1)). NA_As0847 is a homodimer with molecular mass 46.4 kDa. The enzyme has high thermostability (T(1/2) (60°C) = 180 min, T(1/2) (80°C) = 35 min) and thermophilicity (T(opt) = 90°C, E(a) = 30.2 ± 1.0 kJ/mol) and broad pH interval of activity, with the optimum at pH 7.5. Special features of NA_As0847 are the presence of Fe2+ instead of Zn2+ in the active site of the enzyme and inhibition of the enzyme activity by Zn2+ at micromolar concentrations. Analysis of the amino acid sequence revealed a new motif of the metal-binding site (DXHXXXDXXEXXXWXXH) for homological archaeal nicotinamidases.