Fig. 3

Heme arrangement in CydA. Electron density maps of b₅₉₅ and d contoured at 1.5σ show that, as compared to G. thermodenitrificans bd oxidase, b₅₉₅ and d exchange positions in CydA (a, top). Heme b₅₉₅ as found in G. thermodenitrificans bd oxidase (blue carbons) does not fit well into the electron density map, neither does heme d (b top). Exchanging positions of both heme groups (yellow carbons) produces substantially better fits of hemes d (a, bottom) and b₅₉₅ (b, bottom), in particular for the hydroxyl group at the spiro substituted pyrrole of heme d. c Arrangement of heme groups in E. coli bd oxidase, periplasm: top of picture, cytoplasm: bottom of picture. The proposed arrangement is in line with the spectroscopic characterization of E. coli bd variants, in which Glu99^A and Glu445^A, both adjacent to the central Fe ion of heme b₅₉₅ and d, respectively, have been mutated^{8,23,24,25,26}. Mutation of Glu99^A, adjacent to heme d according to our model, did not influence the heme b₅₉₅ content, while mutations of Glu445^A, adjacent to heme b₅₉₅ according to our model, either influenced solely the content of b₅₉₅ or of both hemes in question^{8,23,24,25,26}. Mutation of Glu74^A, close to heme d according to the proposed model, eliminated the signal of heme d (Supplementary Fig. 7)