Solution structure of the first and second transmembrane segments of the mitochondrial oxoglutarate carrier

The structures of the first and the second transmembrane segment of the bovine mitochondrial oxoglutarate carrier (OGC) were studied by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. Peptides 21-46 and 78-108 of its primary sequence were synthesized and structurally characterized in membrane-mimetic environments. CD data showed that at high concentrations of TFE (>50%) and SDS (>2%) both peptides assume alpha-helical structures, whereas in more hydrophilic environments only peptide 78-108 has a helical structure. (1)H-NMR spectra of the two peptides in TFE/water and SDS were fully assigned, and the secondary structures of the peptides were obtained from nuclear Overhauser effects, (3)J(alphaH-NH) coupling constants and alphaH chemical shifts. The three-dimensional solution structures of the peptides in TFE/water were generated by distance geometry calculations. A well-defined alpha-helix was found in the region K24-V39 of peptide 21-46 and in the region A86-F106 of peptide 78-108. We cannot exclude that in intact OGC the extension of these helices is longer. The helix of peptide 21-46 is essentially hydrophobic, whereas that of peptide 78-108 is predominantly hydrophilic.