Structure of nicotinamide mononucleotide adenylyltransferase: a key enzymein NAD(+) biosynthesis

Background: Nicotinamide adenine dinucleotide (NAD(+)) is an essential cofa ctor involved in fundamental processes in cell metabolism. The enzyme nicot inamide mononucleotide adenylyltransferase (NMN AT) plays a key role in NAD (+) biosynthesis, catalysing the condensation of nicotinamide mononucleotid e and ATP, and yielding NAD(+) and pyrophosphate. Given its vital role in c ell life, the enzyme represents a possible target for the development of ne w antibacterial agents. Results: The structure of NMN AT from Methanococcus jannaschii in complex w ith ATP has been solved by X-ray crystallography at 2.0 Angstrom resolution , using a combination of single isomorphous replacement and density modific ation techniques. The structure reveals a hexamer with 32 point group symme try composed of alpha/beta topology subunits. The catalytic site is located in a deep cleft on the surface of each subunit, where one ATP molecule and one Mg2+ are observed. A strictly conserved HXGH motif (in single-letter a mino acid code) is involved in ATP binding and recognition. Conclusions: The structure of NMN AT closely resembles that of phosphopante theine adenylyltransferase. Remarkably, in spite of the fact that the two e nzymes share the same fold and hexameric assembly, a striking difference in their quaternary structure is observed. Moreover, on the basis of structur al similarity including the HXGH motif, we identify NMN AT as a novel membe r of the newly proposed superfamily of nucleotidyltransferase alpha/beta ph osphodiesterases. Our structural data suggest that the catalytic mechanism does not rely on the direct involvement of any protein residues and is like ly to be carried out through optimal positioning of substrates and transiti on-state stabilisation, as is proposed for other members of the nucleotidyl transferase alpha/beta phosphodiesterase superfamily.