Structure and thermal denaturation of crystalline and noncrystalline cytochrome oxidase as studied by infrared spectroscopy.

Fourier-transform infrared spectroscopy has been applied to the study of lipid vesicle-supported two-dimensional crystals and noncrystalline preparations of beef heart cytochrome oxidase. At room temperature, no conformational differences are seen between the noncrystalline and crystalline proteins, whose conformation is shown to consist of ca. 40% alpha-helix, 20% extended structures (including beta-sheet), 17% beta-turns, and 22% open loops plus nonstructured conformations. A novel infrared approach that combines quantitative spectral band decomposition with the study of the thermal behavior of each component has been applied. The procedure allows the independent examination of temperature-induced changes in individual structural elements (alpha-helix, beta-sheet, beta-turns, and unordered). All these reflect, upon heating the protein from 20 to 80 degrees C, a major irreversible thermal event centred at 55-60 degrees C, leading to a molecular state devoid of enzyme activity but with a defined secondary structure; in addition, when the band position, percent area (integrated intensity), and bandwidth of the various amide I components are separately plotted versus temperature, each component is seen to behave in a characteristic way. Thermal denaturation in D2O buffer shows a decrease in nonstructured conformations and an increase in beta-turns without major changes in the proportion of alpha-helix. Temperature-induced changes are not the same in amorphous and crystalline structures, the latter being in general more stable toward the thermal challenge. The above data extend and confirm previous structural studies on cytochrome oxidase using cryo-electron microscopy.