Complex Interplay of Metabolic Substrates, Points of Entry into the Mitochondrial Electron Chain, and ROS Generation: A critical analysis of "Active control of mitochondrial network morphology by metabolism-driven redox state" by Singh et al. and studies replacing ETC components with yeast counterparts

Abstract

Recently, a fascinating, well-executed, molecular study regarding the direct influence of mitochondrial reactive oxygen species (ROS) formation by the electron transport chain (ETC) on mitochondrial morphology in baker's yeast appeared in PNAS. The findings highlight some very interesting connections between the choice of metabolic substrates, points of entry into the ETC, ROS formation, efficiency of ATP generation, and mitochondrial structures. These reflect both ancient eukaryotic constraints and later specific adaptations of Saccharomyces cerevisiae. However, by not addressing these adaptations, the important wider implications of the article's findings run the risk of being overlooked. There are illuminating connections to the FADH₂/NADH ratio concept and new studies replacing ETC components with yeast counterparts in diverse metazoan cells, which will also be discussed.

FIGURE 1

A depiction of the extensive mitochondrial oxidative phosphorylation system, highlighting specific adaptations in S. cerevisiae impacting ATP and ROS generation. The respiratory chain starts with Complex I (NADH dehydrogenase), oxidizing NADH (forming NAD⁺) and reducing Q (forming QH₂). QH₂ is used by Complex III (cytochrome c reductase; dimer and Q‐cycle depicted), reducing cytochrome c (CC; indicated in red). Cytochrome c is re‐oxidized by Complex IV, using O₂ as the final electron acceptor. These complexes (light blue) translocate protons (H⁺), coupling electron transport to the membrane potential (Δp), also influencing ROS formation. The Δp is used (light orange channels) by Complex V, ATP synthase (A) to make ATP, by “uncoupling” proteins (U), or by NNT (the transhydrogenase exchanging NADH for NADPH, involved in ROS scavenging). Complex V can function as an ATPase, sustaining Δp, in the absence of respiration chain activity. In turn, unwanted ATPase activity is inhibited by the IF‐1 inhibitor protein (*). Apart from being a substrate of Complex I, Q can also be reduced by electrons coming from the FAD(H₂) prosthetic co‐factor of Complex II (succinate dehydrogenase, a TCA cycle enzyme), complex “F” (the ETF complex involved in fatty acid oxidation) and “G” (glycerol‐3‐phosphate dehydrogenase using cytoplasmic NADH). Cytoplasmic NADH can also donate electrons to Q more directly, using external NADH dehydrogenases facing the intermembrane space (encoded by the yeast Nde1/Nde2 genes); not shown. Two other eukaryotic enzymes reducing Q are also not shown (and absent from S. cerevisiae): malate:quinone oxidoreductase (MQO) and dihydroorotate dehydrogenase (DHOD). All such complexes (turquoise) do not contribute to Δp. In plants and some other organisms, QH₂ can be oxidized by an alternative oxidase (AOX; again, not shown), bypassing both III and IV, and thus not contributing to Δp either. Specific S. cerevisiae adaptations: A single subunit, non‐proton pumping, FAD containing, dehydrogenase (encoded by the yeast ndi1 gene; Iy). It oxidizes matrix NADH to reduce Q instead of Complex I. A flavonoid (FMN) containing L‐lactate cytochrome c oxidoreductase (encoded by the yeast cyb2 gene; Ly) shuttling electrons from lactate directly to CC. Several proteins and complexes are absent (indicated by red crosses): Transhydrogenase, uncoupling protein, Complexes I and F (beta‐oxidation is peroxisomal in this yeast). Normally, ROS is mostly formed by Complexes I and III. Proton translocation: thin black arrows; electron transport, thick brown arrows. Adapted and extended from [61].