Figure 4.

Illustrative scheme of the main (A) and alternative (B) pathways of Cytc reduction with the transport of electrons and proton pumping in mETC. In the main pathway (A), electrons enter the mETC through Complexes I and II and may be used for UQ reduction. This is commonly referred to as the main pathway of UQ reduction by Complexes I and II. Then, UQH₂ can reduce Cytc, passing electrons through Complex III. Cytc is re-oxidized transferring electrons to Complex IV, which can reduce molecular oxygen. In the alternative pathway (B), electrons enter the mETC through alternative NAD(P)H dehydrogenase [NAD(P)H DH] and through l-GalLDH. Electrons entering through alternative NAD(P)H DH are mostly transferred to AOX via UQ/UQH₂, reducing molecular oxygen. This is called the alternative pathway of UQ reduction by alternative NAD(P)H dehydrogenases. Electrons entering through l-GalLDH are not engaged with UQ/UQH₂ and are directly transferred to Cytc, and this is thus called the alternative pathway of Cytc reduction by l-GalLDH. Finally, Cytc would be re-oxidized by Complex IV coupled with reduction of the molecular oxygen. The black dotted arrows indicate the electron transport pathways within the mETC. The brown arrows represent the site of proton transport that occurs from the matrix to the mitochondrial intermembrane space. The transmembrane location of the l-GalLDH enzyme is indicated by the yellow folded shape, and the enzyme activity shows the association of the enzyme with electron flow in the mETC. In green are the four mETC Complexes indicated by the corresponding Roman numerals, I-IV. AOX is represented in red and the internal and external NADP(H) dehydrogenases in purple. The ubiquinone pool and cytochrome c are represented in magenta and pink, respectively.