FIG 5.

Electron transfer in resolved M. tuberculosis cyt-bcc:aa₃. (A) Electron transfer pathway. The left side of the dimer describes the electron transfer pathway, while the right side depicts the distances between the reaction centers. The binding of menaquinol to the Q_o site in QcrB initiates the electron transfer process. The first electron from menaquinol is transferred to FeS, which is ~20 Å from the Q_o site. The second electron from menaquinol is transferred to heme b_L, which is ~20 Å apart from the Q_o site and subsequently transferred to heme b_H. The electron from heme b_H is used to reduce a menaquinone bound at the Q_o site. FeS transfers its electron to the cytochrome c domain present in QcrC, which transfers the electron further to the cyt-aa₃ domain. The electrons are carried in the following order: from FeS to cyt-cI, cyt-cII, and Cu_A. The final electron transfer pathway occurs from Cu_A to heme a followed by Cu_B and heme a₃. The electrons at heme a₃ are used to reduce O₂ to H₂O. (B) Electron transfer in the Q_o site. The Q_o binding site with side chains interacting with menaquinol (M). The electron transfer pathway is illustrated with orange arrows. The binding of menaquinol results in the transfer of one electron to heme b_L and the other to FeS. (C) Amino acid residues that interact with menaquinol at the Q_o site revealed in the M. tuberculosis cyt-bcc:aa₃ and the M. smegmatis cyt-bcc:aa₃ structure. (D) Electron transfer in the Q_N site. The Q_i binding site with side chains is predicted to interact with menaquinone. The electron transfer pathway is illustrated by pink arrows. The electron, which originated from heme b_L, is transferred to heme b_H before being transferred to menaquinone in the Q_i site. The [2Fe-2S] and heme groups are shown as balls and sticks and are labeled in the figures. Mtb, M. tuberculosis.