Aspects of the oxygen-tolerance of nitrogen fixation in Azotobacter vinelandii
1983
Scherings, G.H.
Following the pioneering physiological studies of Haaker et al. on the generation of reducing equivalents for nitrogenase in Azotobactervinelandii, it seemed desirable to further characterize interactions between the likely ultimate electron donors (flavodoxin and/or ferredoxin) and nitrogenase. Isolation of these proteins was therefore necessary. Flavo- and ferredoxin from A.vinelandii have been prepared to purity; as far as nitro genase was concerned, however, it was deemed that the O 2 -tolerant nitrogenase complex as isolated originally by Bulen and LeComte should render information more physiologically relevant than could be the case using highly puri fied nitrogenase components (Av 1 + Av 2 ). In chapter II, results are described suggesting that the Bulen-LeComte nitrogenase complex, with fully reduced fla vodoxin as a source of reducing equivalents, has regulatory properties not exhibited by a more highly purified (Av 1 + Av 2 ) complex, due to the presence of a 'contaminating' third protein.This third protein appeared to be the same as that shown by Haaker etal. to be responsible for the oxygen-tolerance of the nitrogenase complex. The spectral properties and molecular weight are shown in chapter II to be identical to that of the [2Fe-2S] ferredoxin isolated earlier by Shethna etal. who, however, did not ascribe any physiological function to this pro tein. In chapters III and IV, the interactions between Av 1 , Av 2 and the third protein are further characterized, as well as the conditions necessary to induce the reconstitution of an oxygen-tolerant complex from the purified proteins. The relation between the oxygen-tolerant complex and the so-called 'switched-off state' of nitrogenase activity in vivo is discussed.
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