SUBUNIT-III OF CYTOCHROME-C-OXIDASE INFLUENCES THE CONFORMATION OF SUBUNIT-I AND SUBUNIT-II - AN INFRARED STUDY

The secondary structure of wild-type Paracoccus denitrificans cytochro me c oxidase obtained by decomposition of the infrared amide I band co ntains 44% alpha-helix, 18% beta-sheet, 14% beta-turns, 18% loops, and 6% nonordered segments. The mutant lacking subunit III presents a sma ll but significant increase (from 18% to 24%) in the percentage of loo ps and slight differences in the other components, Using band/area rat ios and tyrosine side chain absorption as an inner standard, it is sho wn that in the absence of subunit III the structure of subunits I and II is altered although no changes in their alpha-helix or beta-sheet c ontent are observed. In the bacterial oxidase, thermal infrared studie s show a complex denaturation pattern characterized by the presence of a partially denatured intermediate state. Of the seven predicted subu nit III alpha-helices, only four are resistant toward the thermal chal lenge and behave as expected for typical transmembrane helices. The ob servation that the absence of subunit III influences the conformation of loop regions in the two other subunits suggests that part of the in teraction surface between subunit III and the catalytic subunits might be located outside the lipid bilayer.