The Periplasmic Nitrate Reductase of Thiosphaera pantotropha
Format of Original
Journal of Inorganic Biochemistry
Original Item ID
The aerobically-active periplasmic respiratory nitrate reductase (Nap) from the bacterium Thiosphaera pantotropha is a heterodimer of NapA (90kDa), which binds molybdopterin guanine dinucleotide (MGD) and a [4Fe-4S] cluster (Em,7 = -160mV), and NapB (16kDa) which contains two, probably bis - His co-ordinated, c-type haem groups (Em,7 = -15mV and +80mV) [1-4]. Mov EPR has detected sets of signals typical of molybdenum-dependent hydroxylases as well as signals typical of molybdenum-containing reductases . The two sets of signals are proposed to reflect differences in the cofactor pterin oxidation state. Oxidation by nitrate in the presence of cyanide of enzyme displaying hydroxylase-like signals generates a very unusual Mov signal with gav >2.0. Magnetic interactions between Mo and the [Fe-S] cluster were not observed. The napEDABC locus coding for the periplasmic nitrate reductase system has been cloned and sequenced . Sequence comparisons with other MGD-dependent enzymes allows proposal of a region of NapA involved in formation of the active site. Mov EPR and active site sequence comparisons indicate that the Mo environment in Nap is similar to that in bacterial assimilatory nitrate reductase but distinct from that in bacterial membrane-bound respiratory nitrate reductases and support the presence of a cysteine Mo ligand, napC appears to code for a membrane-anchored tetrahaem cytochrome c of the NirT family. NapC is likely to be the direct physiological electron donor to NapAB and may also be a ubiquinol oxidase.