Structure and orientation of two voltage-dependent anion-selective channelisoforms - An attenuated total reflection Fourier-transform infrared spectroscopy study

Two VDAC (voltage-dependent anion-selective channel) isoforms were purified from seed cotyledons of Phaseolus vulgaris by chromatofocusing chromatogra phy. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spec troscopy was used to study the structural properties of the two isoforms re constituted in a mixture of asolectin and 5% stigmasterol. The IR spectra o f the two VDAC isoforms were highly similar indicating 50 to 53% anti-paral lel beta -sheet. The orientation of the beta -strands relative to the barre l axis was calculated from the experimentally obtained dichroic ratios of t he amide I beta -sheet component and the amide II band. Comparing the IR sp ectra of the reconstituted VDAC isoforms with the IR spectra of the bacteri al porin OmpF, for which a high resolution structure is available, provided evidence for a general structural organization of the VDAC isoforms simila r to that of bacterial porins. Hydrogen-deuterium exchange measurements ind icated that the exchange of the amide protons occurs to a higher extent in the two VDAC isoforms than in the OmpF porin.