Nitrate Reductase Biochemistry Comes of Age

NR (EC 1.6.6.1-3) was first isolated and characterized more than 40 years ago, and each decade of study of this key enzyme of nitrate assimilation has been associated with a new understanding of its structure and function. Briefly, NR is a homodimeric enzyme (native form = A,) with each subunit containing a 100-kD polypeptide and one each of molybdate, Mo-pterin, Fe, heme, and FAD (Redinbaugh and Campbell, 1985). NR has two active sites, one where NADH donates electrons to FAD to begin the transport of electrons via the heme-Fe to the Mo / Mo-pterin in the second active site, where nitrate is reduced to nitrite. NR has two-site, ping-pong, steady-state kinetics, where the enzyme "pings and pongs" between oxidized and reduced forms, as NADH/NADC bind at the electron donor active site and nitrate/nitrite bind at the electron acceptor site. The most recent advances have resulted from the cloning of the NR gene The deduced amino acid sequence of higher-plant NR showed that it contained about 900 residues with a predicted molecular size of approximately 100 kD. Although this size is a bit smaller than the NR polypeptide appears on SDS-PAGE gels (110-115 kD;