Structure and function of eukaryotic NAD(P)H:nitrate reductase.

Pyridine nucleotide-dependent nitrate reductases (NRs; EC 1.6.6.1-3) are molybdenum-containing enzymes found in eukaryotic organisms which assimilate nitrate. NR is a homodimer with an approximately 100 kDa polypeptide which folds into stable domains housing each of the enzyme's redox cofactors--FAD, heme-Fe molybdopterin (Mo-MPT) and the electron donor NAD(P)H--and there is also a domain for the dimer interface. NR has two active sites: the nitrate-reducing Mo-containing active site and the pyridine nucleotide active site formed between the FAD and NAD(P)H domains. The major barriers to defining the mechanism of catalysis for NR are obtaining the detailed three-dimensional structures for oxidized and reduced enzyme and more in-depth analysis of electron transfer rates in holo-NR. Recombinant expression of holo-NR and its fragments, including site-directed mutagenesis of key acative site and domain interface residues, are expected to make large contributions to this effort to understand the catalytic mechanism of NR.