Mitochondrial proton and electron leaks

Abstract

Mitochondrial proton and electron leak have a major impact on mitochondrial coupling efficiency and production of reactive oxygen species. In the first part of this chapter, we address the molecular nature of the basal and inducible proton leak pathways, and their physiological importance. The basal leak is unregulated, and a major proportion can be attributed to mitochondrial anion carriers, whereas the proton leak through the lipid bilayer appears to be minor. The basal proton leak is cell-type specific and correlates with metabolic rate. The inducible leak through the ANT (adenine nucleotide translocase) and UCPs (uncoupling proteins) can be activated by fatty acids, superoxide or lipid peroxidation products. The physiological role of inducible leak through UCP1 in mammalian brown adipose tissue is heat production, whereas the roles of non-mammalian UCP1 and its paralogous proteins, in particular UCP2 and UCP3, are not yet resolved. The second part of the chapter focuses on the electron leak that occurs in the mitochondrial electron transport chain. Exit of electrons prior to the reduction of oxygen to water at cytochrome c oxidase causes superoxide production. As the mechanisms of electron leak are crucial to understanding their physiological relevance, we summarize the mechanisms and topology of electron leak from complexes I and III in studies using isolated mitochondria. We also highlight recent progress and challenges of assessing electron leak in the living cell. Finally, we emphasize the importance of proton and electron leak as therapeutic targets in body mass regulation and insulin secretion.

Figure 1. Proton pumping and leak across the mitochondrial inner membrane

Protons are pumped out of the matrix into the intermembrane space (IMS) by complexes I, III, and IV of the electron transport chain. This establishes a proton motive force ( Δp) across the inner membrane. Proton re-entry through the ATP synthase (complex V) couples the release of Δp to ATP synthesis. All other means of proton re-entry constitute proton leak, as Δp derived from substrate oxidation is depleted without catalysing ATP synthesis. Mechanisms of proton leak include direct movement of protons across the phospholipid membrane (the “water wires” model), diffusion through or around integral membrane proteins, or inducible transport through the adenine nucleotide translocase (ANT) or uncoupling proteins (UCP1, UCPx).

Figure 2. The kinetics of the proton leak

The rate of oxygen consumption as a function of membrane potential increases approximately exponentionally. This can be mistaken for pseudo-linearly when Δp is low, as indicated by the dashed line. As the membrane potential increases, however, a disproportionately large rate of oxygen consumption becomes apparent to defend the membrane potential ( ΔΨ). Therefore, proton leak across the mitochondrial inner membrane is non-ohmic (full line).

Figure 3. Sites of electron leak (loss) from electron transport chain complexes during electron transport

Electrons carried by NADH are transferred to the flavin mononucleotide(I_F) site in complex I, where they normally pass down a chain of Fe-S centres to the ubiquinone binding site (I_Q). At both the I_F and I_Q sites, these electrons react with O₂, forming superoxide (O₂^•−) within the matrix. In complex III, QH₂ binds to the Q_O site, where its electrons can bypass their normal transfer in the Q-cycle (see text) and react directly with oxygen to form superoxide that is released to both sides of the mitochondrial inner membrane.