Structure of nicotinic acid mononucleotide adenylyltransferase from Bacillus anthracis

Nicotinic acid mononucleotide adenylyltransferase (NaMNAT; EC 2.7.7.18) is the penultimate enzyme in the biosynthesis of NAD⁺ and catalyzes the adenylation of nicotinic acid mononucleotide (NaMN) by ATP to form nicotinic acid adenine dinucleotide (NaAD). This enzyme is regarded as a suitable candidate for antibacterial drug development; as such, Bacillus anthracis NaMNAT (BA NaMNAT) was heterologously expressed in Escherichia coli for the purpose of inhibitor discovery and crystallography. The crystal structure of BA NaMNAT was determined by molecular replacement, revealing two dimers per asymmetric unit, and was refined to an R factor and R_free of 0.228 and 0.263, respectively, at 2.3 Å resolution. The structure is very similar to that of B. subtilis NaMNAT (BS NaMNAT), which is also a dimer, and another independently solved structure of BA NaMNAT recently released from the PDB along with two ligated forms. Comparison of these and other less related bacterial NaMNAT structures support the presence of considerable conformational heterogeneity and flexibility in three loops surrounding the substrate-binding area.