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The pathways through which NADPH, NADH and H2 provide electrons to nitrogenase were examined in anaerobically isolated heterocysts. Electron donation in freeze-thawed heterocysts and in heterocyst fractions was studied by measuring O2 uptake, acetylene reduction and reduction of horse heart cytochrome c. In freeze-thawed heterocysts and membrane fractions, NADH and H2 supported cyanide-sensitive, respiratory O2 uptake and light-enhanced, cyanide-insensitive uptake of O2 resulting from electron donation to O2 at the reducing side of Photosystem I. Membrane fractions also catalyzed NADH-dependent reduction of cytochrome c. In freeze-thawed heterocysts and soluble fractions from heterocysts, NADPH donated electrons in dark reactions to O2 or cytochrome c through a pathway involving ferredoxin:NADP reductase; these reactions were only slightly influenced by cyanide or illumination. In freeze-thawed heterocysts provided with an ATP-generating system, NADH or H2 supported slow acetylene reduction in the dark through uncoupler-sensitive reverse electron flow. Upon illumination, enhanced rates of acetylene reduction requiring the participation of Photosystem I were observed with NADH and H2 as electron donors. Rapid NADPH-dependent acetylene reduction occurred in the dark and this activity was not influenced by illumination or uncoupler. A scheme summarizing electron-transfer pathways between soluble and membrane components is presented. 相似文献
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John D. Mills David Crowther Rudolf E. Slovacek Geoffrey Hind Richard E. McCarty 《BBA》1979,547(1):127-137
Addition of NADPH to osmotically lysed spinach chloroplasts results in a reduction of the primary acceptor (Q) of Photosystem II. This reduction of Q reaches a maximum of 50% in chloroplasts maintained under weak illumination and requires added ferredoxin and Mg2+. The reaction is inhibited by (i) an antibody to ferredoxin-NADP+ reductase (EC 1.6.7.1), (ii) treatment of chloroplasts with N-ethylmaleimide in the presence of NADPH, (iii) disulfodisalicylidenepropanediamine, (iv) antimycin, and (v) acceptors of non-cyclic electron transport. Uncouplers of phosphorylation do not affect NADPH-driven reduction of Q.It is proposed that electron flow from NADPH to Q may occur in the dark by a pathway utilising portions of the normal cyclic and non-cyclic electron carrier sequences. The possible in vivo role for such a pathway in redox poising of cyclic electron transport and hence in controlling the ATP/NADPH supply ratio is discussed. 相似文献
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