https://phys.org/news/2018-11-ready-close-upa-bacterium-electron-pathway.html

Quinones and cytochromes are two types of electron carriers in ETCs used to shuttle electrons to and from large macromolecular structures embedded in the membrane. Four membrane oxidoreductases are involved in the mitochondrial respiratory chain for electron transfer. These include complex I (NADH:ubiquinone oxidoreductase, CI), complex II (succinate:ubiquinone oxidoreductase, CII), complex III (bc1-type ubiquinol:cytochrome c oxidoreductase, bc1-type CIII) and complex IV (aa3-type cytochrome c oxidase, aa3-type CIV). By function, CIII can oxidize ubiquinol to ubiquinone and pass the electrons to soluble cytochrome c. Electrons are then shuttled to CIV, where oxygen is reduced to water. The transmembrane PMF is generated by proton pumping in CI, CIII and CIV.

In the prokaryotic respiratory chain, the situation is more complicated. A complete pathway of electron flow has not yet been determined in the cell type due to its complexity. It is therefore necessary to understand the complete structure of a "supercomplex" involved during bacterial electron transfer to assist the goal. In the study, the researchers extracted and purified the complex from M. smegmatis to visualize the architecture using cryo-electron microscopy (cryo-EM) at a resolution of 3.5 Å. The structure provided crucial insights into the mechanism of direct electron transfer within a respiratory supercomplex. The dimensions of the supercomplex were in the range of 200 x 70 x 120 Å, in a symmetrized linear architecture completely different from previously reported respiratory supercomplexes. By composition, the linear dimeric CIV1-CIII2-CIV1 was arranged such that individual CIVs flanked the central CIII dimer on either side. The information revealed a direct link between enzymes during electron transfer, representing a new mode of respiratory chain catalysis. The detailed structural findings have potential to assist with antimycobacterial drug discovery efforts.