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Proc Natl Acad Sci U S A. 2017 Nov 28;114(48):12725-12730. doi: 10.1073/pnas.1713698114. Epub 2017 Nov 13.

Unprecedented pathway of reducing equivalents in a diflavin-linked disulfide oxidoreductase.

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Metabolic Engineering Group, Departamento de Microbiología y Genética, Universidad de Salamanca, 37007 Salamanca, Spain.
Instituto de Recursos Naturales y Agrobiología de Salamanca, Consejo Superior de Investigaciones Científicas (CSIC), 37008 Salamanca, Spain.
Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, 41092 Sevilla, Spain.
Institute of Biocomputation and Physics of Complex Systems (BIFI), Universidad de Zaragoza, 50009 Zaragoza, Spain.
Joint Units Instituto de Química-Física Rocasolano (IQFR)-CSIC-BIFI and Grupo de Bioquímica, Biofísica y Biología Computacional (GBsC)-CSIC-BIFI, Universidad de Zaragoza, 50009 Zaragoza, Spain.
Department of Biochemistry and Molecular and Cell Biology, Universidad de Zaragoza, 50009 Zaragoza, Spain.
Aragon Institute for Health Research, 50009 Zaragoza, Spain.
Biomedical Research Networking Centre for Liver and Digestive Diseases, 28029 Madrid, Spain.
Fundación Aragonesa para la Investigación y Desarrollo, 50018 Zaragoza, Spain.
Instituto de Biología Molecular y Celular del Cáncer, CSIC-Universidad de Salamanca, 37007 Salamanca, Spain.
Laboratoire de Biologie Moléculaire et Cellulaire, Université de Neuchâtel, 2000 Neuchâtel, Switzerland.
Department of Plant and Microbial Biology, University of California, 94720 Berkeley
Instituto de Recursos Naturales y Agrobiología de Salamanca, Consejo Superior de Investigaciones Científicas (CSIC), 37008 Salamanca, Spain;


Flavoproteins participate in a wide variety of physiologically relevant processes that typically involve redox reactions. Within this protein superfamily, there exists a group that is able to transfer reducing equivalents from FAD to a redox-active disulfide bridge, which further reduces disulfide bridges in target proteins to regulate their structure and function. We have identified a previously undescribed type of flavin enzyme that is exclusive to oxygenic photosynthetic prokaryotes and that is based on the primary sequence that had been assigned as an NADPH-dependent thioredoxin reductase (NTR). However, our experimental data show that the protein does not transfer reducing equivalents from flavins to disulfides as in NTRs but functions in the opposite direction. High-resolution structures of the protein from Gloeobacter violaceus and Synechocystis sp. PCC6803 obtained by X-ray crystallography showed two juxtaposed FAD molecules per monomer in redox communication with an active disulfide bridge in a variant of the fold adopted by NTRs. We have tentatively named the flavoprotein "DDOR" (diflavin-linked disulfide oxidoreductase) and propose that its activity is linked to a thiol-based transfer of reducing equivalents in bacterial membranes. These findings expand the structural and mechanistic repertoire of flavoenzymes with oxidoreductase activity and pave the way to explore new protein engineering approaches aimed at designing redox-active proteins for diverse biotechnological applications.


Rossmann fold; flavoprotein; redox active disulfide; sulfhydryl; transfer of reducing equivalents

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