Paper
Insights into Flavin-based Electron Bifurcation via the NADH-dependent Reduced Ferredoxin:NADP Oxidoreductase Structure*
Published Jul 2, 2015 · Julius K Demmer, Haiyan Huang, Shuning Wang
The Journal of Biological Chemistry
72
Citations
1
Influential Citations
Abstract
Background: Flavin-based electron bifurcation is a vital process in microbial energy metabolism. Results: The NfnAB complex structure determines the positions of the prosthetic groups and the substrates. Conclusion: The environment of the central FAD and its distance to the next redox centers of the two electron routes control electron bifurcation. Significance: The first complete structure of a flavin-based electron bifurcating enzyme provides insights into this ancient catalytic process. NADH-dependent reduced ferredoxin:NADP oxidoreductase (NfnAB) is found in the cytoplasm of various anaerobic bacteria and archaea. The enzyme reversibly catalyzes the endergonic reduction of ferredoxin with NADPH driven by the exergonic transhydrogenation from NADPH onto NAD+. Coupling is most probably accomplished via the mechanism of flavin-based electron bifurcation. To understand this process on a structural basis, we heterologously produced the NfnAB complex of Thermotoga maritima in Escherichia coli, provided kinetic evidence for its bifurcating behavior, and determined its x-ray structure in the absence and presence of NADH. The structure of NfnAB reveals an electron transfer route including the FAD (a-FAD), the [2Fe-2S] cluster of NfnA and the FAD (b-FAD), and the two [4Fe-4S] clusters of NfnB. Ferredoxin is presumably docked onto NfnB close to the [4Fe-4S] cluster distal to b-FAD. NAD(H) binds to a-FAD and NADP(H) consequently to b-FAD, which is positioned in the center of the NfnAB complex and the site of electron bifurcation. Arg187 is hydrogen-bonded to N5 and O4 of the bifurcating b-FAD and might play a key role in adjusting a low redox potential of the FADH•/FAD pair required for ferredoxin reduction. A mechanism of FAD-coupled electron bifurcation by NfnAB is proposed.
Highly CitedThe structure of NfnAB reveals a mechanism of flavin-coupled electron bifurcation, which plays a crucial role in microbial energy metabolism.
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