Subunit III of Cytochrome c Oxidase Influences the Conformation of Subunits I and II: An Infrared Study

Izaskun Echabe,Tuomas Haltia, Prnesto Freire, Felix M. Goni,1 and Jose Luis R. Arrondo Biochemistry 1995, 84. 13565-13569 Summary: The secondary structure of wild-type Paracoccus denitrificans cytochrome oxidase obtained by decomposition of the infrared amide I band contains 44% a-helix, 18% B-sheet, 14% B-turns, 18% loops, and 6% non ordered segments, The mutant lacking subunit III presents a small but significant increase (from 18% to 24%) in the percentage of loops and slight differences in the other components. Using band/area ratios and tyrosine side chain absorption as an inner standard, it is shown that in the absence of subunit III the structure of subunits I and II is altered although no changes in their a-helix or ,B-sheet content are observed. In the bacterial oxidase, thermal infrared studies show a complex denaturation pattern characterized by the presence of a partially denatured intermediate state. Of the seven predicted subunit III a-helices, only four are resistant toward the thermal challenge and behave as expected for typical transmembrane helices. The observation that the absence of subunit III influences the conformation of loop regions in the two other subunits suggests that part of the interaction surface between subunit III and the catalytic subunits might be located outside the lipid bilayer.