📒 Jastroch 2010
Mitochondrial proton and electron leaks1
The chemiosmotic theory
- Electrochemical proton gradient (protonmotive force, Δp) driving mitochondrial ATP production
- Protons can return to the matrix independently of ATP synthase.
- basal leak vs inducible leak
- Non-ohmic manner observed (exponential relation), even in intact cells
- Another explanation is the electron slip: e- transferred without pumping protons (esp. C4). This has not been convincingly demonstrated experimentally to occur under physiological conditions.
Molecular nature of the basal proton leak
- The magnitude of proton conductance also correlates with the abundance of mitochondrial anion carrier proteins (e.g. ANT, up to 2/3). H+ leaking via the interface btween the proteins and the lipid bilayer.
- Not due to the protein activity (i.e. inhibitor-insensitive)
Physiological significance of basal proton leak
- 20–30% of the resting metabolic rate of hepatocytes and up to ~50% of the respiration of skeletal muscle of a rat, contributes significantly to basal metabolic rate (BMR), as the heat generator for mammals.
- protection by mild uncoupling against excessive ROS production
Methodological considerations assaying leak in vivo
- futile proton cycling contributes to ~50% of the respiratory rate
- calculating coupling efficiency by inhibition of ATP synthase by oligomycin
The molecular nature of the inducible proton leak
- ANT: activated by activated by fatty acids and reactive alkenals, such as hydroxynonenal (HNE)
- UCP: activated by thyroxin, fatty acids
Uncoupling protein 1
- In the brown adipose tissue
- Genarates heat, dissipates Δp. For thermal regulations in small mammals.
- Inhibited by purine nucleoside di- and tri-phosphates and activated by free fatty acids
- FA cycling by UCP1
- simple competitive kinetics for activation of UCP1
- regulated by cellular signal transduction
- noradrenergic (sympathetic)
The function of novel uncoupling proteins
- paralogues of UCP1 => UCP2, UCP3
- Superoxide, and derived lipid peroxidation products such as HNE, activate UCP-mediated proton conductance
- Exchange ATP for ADP across the inner mitochondrial membrane (IMM), electrogenic (loses 1 charge in forward mode)
- FAs. superoxide and oxidative compounds (HNE) can also induce uncoupling via the ANT
- From semiquinone radical (QH•) or reduced flavin
- Both superoxide anion (O2•−) and membrane-soluble hydroperoxyl radical (HO2•) exist
- Mainly from C1 & C3 (but there are reports that C2, KGDH, and others also produce some ROS)
- FMN (IF site) vs Q reductase (IQ site)
- During FET (G/M oxidation): FMN (IF site), ROS production enhanced by rotenone. Δp-dependent.
- During RET (succinate oxidation): Sensitive to Δp and IQ site inhibitors
- Bifurcation of electron flow (Q-cycle)
- Electrons taken fron QH2 are distributed rapidly in the cytochrome b, avoiding significant accumulation of QH• in the Qo site and thus superoxide generation
- Qo site inhibitors (stigmatellin and myxothiazol) blocks ROS generation from C3
Topology of ROS formation
- Phospholipid bilayer membranes are highly impermeable to the anionic O2•−, and HO2• concentration is very small at physiological pH.
- ROS formation from complex I is directed solely to the matrix
- Superoxide production by complex III in the presence of antimycin A results in release of ROS to both the matrix and intermembrane
- α-glycerophosphate dehydrogenase results in ROS production to both sides of the mitochondrial inner membrane
Importance of proton and electron leak in disease
Regulation of body weight by proton leak
- chemical uncoupler 2,4-dinitrophenol (DNP) for weight control: narrow margin between therapeutic and toxic doses, and nonspecific to tissues, withdrawn due to deaths.
- brown adipose tissue as a target for obesity treatment: upregulating UCP
Modulating the insulin response through mild uncoupling
- UCP2 was implicated as a negative regulator of insulin secretion, presumably by lowering the ATP/ADP ratio
- Knocking out UCP2 results in chronic oxidative stress