Contents

📒 Jastroch 2010

Mitochondrial proton and electron leaks1

Sciwheel

The chemiosmotic theory

  • Electrochemical proton gradient (protonmotive force, Δp) driving mitochondrial ATP production https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3122475/bin/nihms300003f1.jpg

Proton leak

  • 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 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3122475/bin/nihms300003f2.jpg
  • 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
  • Mechanisms
    1. Cofactor
    2. FA cycling by UCP1
    3. simple competitive kinetics for activation of UCP1
  • regulated by cellular signal transduction
    1. noradrenergic (sympathetic)
    2. thyroxines

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

ANT

  • 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

Electron leak

  • 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) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3122475/bin/nihms300003f3.jpg

Complex I

  • 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

Complex III

  • 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

  1. Jastroch M, Divakaruni AS, Mookerjee S, Treberg JR, Brand MD. Mitochondrial proton and electron leaks. Essays Biochem. 2010;47:53-67. PMC3122475 ↩︎