CRYSTAL-STRUCTURE OF THE FAD-CONTAINING FRAGMENT OF CORN NITRATE REDUCTASE AT 2.5 ANGSTROM RESOLUTION - RELATIONSHIP TO OTHER FLAVOPROTEIN REDUCTASES

Background: In the biological assimilation of nitrate in plants and mi croorganisms, nitrate is reduced to ammonium by transfer of eight elec trons in a two-step process. The first step of the pathway, the reduct ion of nitrate to nitrite, is catalyzed by nitrate reductase, a multi- redox cofactor enzyme which belongs to the class of flavoprotein pyrid ine nucleotide cytochrome reductases. The enzyme can be divided into t hree functional fragments that bind the cofactors molybdopterin, heme- iron and flavin adenine dinucleotide (FAD)/nicotinamide adenine dinucl eotide (NADH). Results: Here we describe the crystal structure of the recombinant cytochrome b reductase fragment of corn nitrate reductase, in complex with the cofactor FAD, determined to 2.5 Angstrom resoluti on. This catalytically competent fragment of nitrate reductase consist s of two domains, the amino-terminal lobe, which binds FAD, and the ca rboxy-terminal lobe, which presumably binds NADH, connected by a linke r region. Conclusions: Nitrate reductase belongs to the class of flavo protein pyridine nucleotide cytochrome reductases, a subgroup in the f amily of ferredoxin reductase-like flavoproteins. Comparison with othe r members of this family reveals that large structural differences are found in the relative orientation of the cofactor binding lobes. This indicates that conformational changes might be important for biologic al function.