Structure of electron transfer flavoprotein-ubiquinone oxidoreductase and electron transfer to the mitochondrial ubiquinone pool

doi:10.1073/pnas.0604567103

Structure of electron transfer flavoprotein-ubiquinone oxidoreductase and electron transfer to the mitochondrial ubiquinone pool

*Department of Biochemistry, Medical College of Wisconsin, Milwaukee, WI 53226; and
^†Department of Pediatrics, University of Colorado Health Sciences Center, Denver, CO 80262

Edited by Douglas C. Rees, California Institute of Technology, Pasadena, CA, and approved September 11, 2006 (received for review June 2, 2006)

Abstract

Electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) is a 4Fe4S flavoprotein located in the inner mitochondrial membrane. It catalyzes ubiquinone (UQ) reduction by ETF, linking oxidation of fatty acids and some amino acids to the mitochondrial respiratory chain. Deficiencies in ETF or ETF-QO result in multiple acyl-CoA dehydrogenase deficiency, a human metabolic disease. Crystal structures of ETF-QO with and without bound UQ were determined, and they are essentially identical. The molecule forms a single structural domain. Three functional regions bind FAD, the 4Fe4S cluster, and UQ and are closely packed and share structural elements, resulting in no discrete structural domains. The UQ-binding pocket consists mainly of hydrophobic residues, and UQ binding differs from that of other UQ-binding proteins. ETF-QO is a monotopic integral membrane protein. The putative membrane-binding surface contains an α-helix and a β-hairpin, forming a hydrophobic plateau. The UQ—flavin distance (8.5 Å) is shorter than the UQ—cluster distance (18.8 Å), and the very similar redox potentials of FAD and the cluster strongly suggest that the flavin, not the cluster, transfers electrons to UQ. Two possible electron transfer paths can be envisioned. First, electrons from the ETF flavin semiquinone may enter the ETF-QO flavin one by one, followed by rapid equilibration with the cluster. Alternatively, electrons may enter via the cluster, followed by equilibration between centers. In both cases, when ETF-QO is reduced to a two-electron reduced state (one electron at each redox center), the enzyme is primed to reduce UQ to ubiquinol via FAD.

Footnotes

^‡To whom correspondence should be addressed. E-mail: jjkim{at}mcw.edu
Author contributions: J.Z. and J.-J.P.K. designed research; J.Z. and J.-J.P.K. performed research; J.Z. and J.-J.P.K. analyzed data; F.E.F. contributed new reagents/analytic tools; and J.Z., F.E.F., and J.-J.P.K. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Data deposition: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org [PDB ID codes 2GMH (UQ-bound structure) and 2GMJ (UQ-free structure)].
Abbreviations:

ETF,

electron transfer flavoprotein;

ETF-QO,

ETF-ubiquinone oxidoreductase;

UQ,

ubiquinone;

ETF1e−,

ETF semiquinone.