| Popis: |
Assimilatory nitrate reductase (NR) (E.C. 1.6.6.1) from eukaryotic sources appears to be similar in major structural features, containing one mole of FAD, haem (cytochrome 6557) and molybdenum per subunit. All higher plant NRs so far examined consist of a homodimer with subunit molecular mass varying from 100 to 145 kD (e.g. Kuo et al. 1982; Fido and Notton 1984; Redinbaugh and Campbell 1985). The reason for this variation in size is not known. Chlorella vulgaris NR is a homotetramer except at low protein concentrations when it behaves as an active homodimer (Howard and Solomonson 1982). Physiologically, NR catalyses the rate limiting step in inorganic nitrogen assimilation, i.e. the reduction of NO3 − to NO2 −. NADH initially donates electrons to the FAD, a process which involves sulphydryl groups. This is followed by electron transfer to the haem and egress to NO3 − via the Mo site. Additionally the enzyme catalyses a number of partial reactions which involve one or more of the prosthetic groups. These reactions utilize various artificial electron donors (reduced flavin, FADH2; methyl viologen, MV; and bromophenol blue, BPB) and acceptors (ferricyanide, F; cytochrome c, C; and dichlorophenol indophenol, DCPIP), and are classified as NO3 −-reducing and NADH-dehydrogenase reactions respectively (Wray and Filner 1970; Solomonson et al. 1975; Hewitt and Notton 1980; Guerrero et al. 1981; Dunn-Coleman et al. 1984; Campbell 1985). |
