Nicotinamide-mononucleotide adenylyltransferases from yeast and other microorganisms

The enzyme nicotinamide–mononucleotide adenylyltransferase plays a key role in pyridine nucleotide turnover, being involved in both de novo and salvage synthesis of nicotinamide adenine dinucleotide (NAD). Sustained degradation and resynthesis of NAD is a well-recognized phenomenon both in eukaryotic and in prokaryotic microorganisms. Sulfolobus solfataricus is a thermophilic archaeon living under extreme conditions of temperature and pH. Comparison of the molecular and catalytic properties of nicotinamide–mononucleotide adenylyltransferase from mesophilic and thermophilic microorganisms can provide significant information for the elucidation of the significance of NAD turnover in an evolutionary perspective. In addition, the identification of the molecular determinants of the enzyme thermostability and thermophilicity appears to have possible biotechnological interest. Owing to the high reaction temperature, a continuous spectrophotometric assay cannot be used. Therefore, the activity of the enzyme from Sulfolobus solfataricus is measured with two different discontinuous assays. The effect of temperature on the enzyme activity shows Arrhenius activation energy of 95 kJ/mol, indicating a remarkable thermophilicity, even when compared with that of other enzymes from the same microorganism.