3.D.11. The Periplasmic Nitrate Reductase Complex (NAP) Complex Family
The nap operon of Escherichia coli K-12, encoding a periplasmic nitrate reductase (NapABCDFGH), encodes seven proteins. The catalytic complex in the periplasm, NapA-NapB, receives electrons from the quinol pool via the membrane-bound cytochrome NapC. Like NapA, B and C, a fourth polypeptide, NapD, is also essential for Nap activity (Brondijk et al. 2002). However, none of the remaining three polypeptides, NapF, G and H, which are predicted to encode non-haem, iron-sulphur proteins, are essential for Nap activity. The relative rates of growth and electron transfer from physiological substrates to Nap have been investigated using strains defective in the two membrane-bound nitrate reductases, and also defective in either ubiquinone or menaquinone biosynthesis. Nap is coupled more effectively to menaquinol oxidation than to ubiquinol oxidation. Conversely, parallel experiments revealed that nitrate reductase A couples more effectively with ubiquinol than with menaquinol. NapF, NapG and NapH play no role in electron transfer from menaquinol to the NapAB complex but, in the Ubi+Men- background, deletion of napF, napGH or napFGH all resulted in total loss of nitrate-dependent growth. Electron transfer from ubiquinol to NapAB was totally dependent upon NapGH, but not on NapF. NapC was essential for electron transfer from both ubiquinol and menaquinol to NapAB. Thus, NapG and H, but not NapF, are essential for electron transfer from ubiquinol to NapAB. The decreased yield of biomass resulting from loss of NapF in a Ubi+Men+ strain implicates NapF in an energy-conserving role coupled to the oxidation of ubiquinol. Brondijk et al. 2002 proposed that NapG and H form an energy-conserving quinol dehydrogenase functioning as either components of a proton pump or in a Q cycle, as electrons are transferred from ubiquinol to NapC.