3-DIMENSIONAL STRUCTURE OF HUMAN ELECTRON-TRANSFER FLAVOPROTEIN TO 2.1-ANGSTROM RESOLUTION

Mammalian electron transfer flavoproteins (ETF) are heterodimers conta ining a single equivalent of flavin adenine dinucleotide (FAD). They f unction as electron shuttles between primary flavoprotein dehydrogenas es involved in mitochondrial fatty acid and amino acid catabolism and the membrane-bound electron transfer flavoprotein ubiquinone oxidoredu ctase. The structure of human ETF solved to 2.1-Angstrom resolution re veals that the ETF molecule is comprised of three distinct domains: tw o domains are contributed by the alpha subunit and the third domain is made up entirely by the beta subunit, The N-terminal portion of the a lpha subunit end the majority of the beta subunit have identical polyp eptide folds, in the absence of any sequence homology, FAD lies in a c left between the two subunits, with most of the PAD molecule residing in the C-terminal portion of the alpha subunit. Alignment of all the k nown sequences for the ETF alpha subunits together with the putative F ixB gene product shows that the residues directly involved in FAD bind ing are conserved, A hydrogen bond is formed between the N5 of the PAD isoalloxazine ring and the hydroxyl side chain of alpha T266, suggest ing why the pathogenic mutation, alpha T266M, affects ETF activity in patients with glutaric acidemia type II. Hydrogen bonds between the 4' -hydroxyl of the ribityl chain of FAD and N1 of the isoalloxazine ring , and between alpha H286 and the C2-carbonyl oxygen of the isoalloxazi ne ring, may play a role in the stabilization of the anionic semiquino ne. With the known structure of medium chain acyl-CoA dehydrogenase, w e hypothesize a possible structure for docking the two proteins.