Fourier transform infrared spectroscopic studies on gastric H +/K +-ATPase

Abstract

Suspensions of membrane-bound H +/K +-ATPase in both H 2O and 2H 2O were investigated using Fourier transform infrared (FT-IR) spectroscopy. Second-derivative techniques were used to reveal the overlapping bands in the 1800-1500 cm −1 region. Analysis of the amide I band shows that the protein component contains substantial amounts of both α-helical and β-sheet structures. Addition of 10 mM KCl to a suspension in 2H 2O does not significantly affect the amide I band, indicating that the E 1–E 2 conformational transition of the enzyme, induced by K +, does not involve a gross change in protein secondary structure. Analysis of the amide II band in the spectra of suspensions in 2H 2O shows that inhibition of the enzyme with omeprazole increases the rate of 1H– 2H exchange, indicating an increase in conformational flexibility. Furthermore, an additional feature at 1628 cm −1 in the spectra of the inhibited samples in 2H 2O could either support a conformational change or arise from a vibrational mode of omeprazole in its enzyme-bound form. The frequency of the band due to the symmetric stretching vibrations of the methylene groups of the lipid acyl chains increases steadily with increasing temperature indicating that there is no co-operative melting process in the lipid component of the membrane over the temperature range 9–50°C. For comparison, FT-IR studies on aqueous suspensions of Na +/K +-ATPase were also carried out. These show that the protein components in the Na +/K +- and H +/K +-ATPases have similar secondary structures.