title: Oxidation--reduction midpoint potentials of the flavin, haem and Mo-pterin centres in spinach (Spinacia oleracea L.) nitrate reductase.
abstract: Oxidation-reduction midpoint potentials have been determined for the flavin, cytochrome b557 and Mo-pterin prosthetic groups of spinach (Spinacia oleracea L.) assimilatory nitrate reductase using visible, c.d. and room-temperature e.p.r. potentiometric titrations. At pH 7 and 25 degrees C, the midpoint potential for the FAD/FADH2 couple was determined by c.d. potentiometry to be -280 +/- 10 mV (n = 2). The redox potential for reduction of the haem was determined by visible potentiometry to be -123 +/- 10 mV (n = 1), significantly lower than the previously published value of -60 mV [Fido, Hewitt, Notton, Jones & Nasrulhaq-Boyce (1979) FEBS Lett. 99, 180-182]. Potentials for the Mo(VI)/Mo(V) and Mo(V)/Mo(IV) redox couples, determined by room-temperature e.p.r. potentiometry, were found to be +2 +/- 20 and -6 +/- 20 mV respectively. These values are very similar to the values previously determined for the FAD, haem and Mo-pterin centres in assimilatory nitrate reductase isolated from the unicellular green alga Chlorella vulgaris and indicate a close thermodynamic similarity between the two enzymes.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Florida, College of Medicine, Tampa, FL 33612.
authors.forename: C J
authors.initials: CJ
authors.lastname: Kay
authors.forename: M J
authors.initials: MJ
authors.lastname: Barber
authors.forename: B A
authors.initials: BA
authors.lastname: Notton
authors.forename: L P
authors.initials: LP
authors.lastname: Solomonson
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism*; D003573:Cytochrome b Group / Q000378:metabolism*; D005293:Ferritins; D005415:Flavins / Q000378:metabolism*; D009565:Nitrate Reductases / Q000378:metabolism*; D010084:Oxidation-Reduction; D011199:Potentiometry; D014675:Vegetables / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D003573:Cytochrome b Group; D005415:Flavins; C050771:mopI protein, Clostridium pasteurianum; D005293:Ferritins; C018032:bacterioferritin; D009565:Nitrate Reductases