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Oxidative Phosphorylation

Transport into mitochondria (TIMs)

Transported via TIMs (as listed):

  • malate
  • \(\alpha\)-ketoglutarate
  • succinate
  • pyruvate
  • NAD\(^+\)
  • ATP/ADP
  • Ca\(^{2+}\), P\(_i\)

Can move through without TIMs:

  • O\(_2\), CO\(_2\), H\(_2\)O

Cytosolic NADH shuttles

Glycerol-3-phosphate shuttle

  • mainly in skeletal muscle

Steps:

  1. 2 NADH interact with glycerol-3-phosphate dehydrogenase (cytoplasm)

  2. DHAP → glycerol-3-phosphate

  3. glycerol-3-phosphate interacts with glycerol-3-phosphate dehydrogenase (mitochondria)

  4. located on the inner membrane outer side

  5. converts FAD → FADH\(_2\) (inside in matrix)
  6. glycerol-3-phosphate → DHAP
  7. FADH\(_2\) enters ETC at coenzyme Q (CoQ)

Malate–aspartate shuttle

  • mainly in brain and heart muscle

Steps:

  1. 2 NADH reduce oxaloacetate → malate via malate dehydrogenase (cytosol)
  2. malate enters mitochondria via TIMs

  3. malate → oxaloacetate via malate dehydrogenase (mitochondria, TCA)

  4. converts NAD\(^+\) → NADH

  5. oxaloacetate → aspartate via aspartate transaminase

  6. also converts glutamate → \(\alpha\)-ketoglutarate

  7. transport notes:

  8. aspartate, \(\alpha\)-ketoglutarate, and glutamate move through TIMs

  9. aspartate and glutamate are TIM anti-cotransporter substrates
  10. aspartate and \(\alpha\)-ketoglutarate form oxaloacetate and glutamate

Electron transport chain (ETC)

Core idea

  • power source: electrochemical gradient
  • ETC pumps protons from matrix → intermembrane space

Electron entry points:

  • NADH donates to complex I (NADH–CoQ oxidoreductase; NADH dehydrogenase)
  • FADH\(_2\) donates to complex II (succinate dehydrogenase; does not pump protons; touches outside of membrane)

Carrier flow:

  • complex I/II → CoQ → complex III (cytochrome bc\(_1\)) → cytochrome c → complex IV (cytochrome c oxidase)
  • complex IV reduces O\(_2\) → H\(_2\)O

Principle:

  • each redox center has higher affinity for electrons than the previous carrier

Complex I

  • pumps 4 H\(^+\) out

Steps:

  • NADH transfers 2e\(^-\) to FMN (flavin mononucleotide)
  • FMNH\(_2\) transfers e\(^-\) to Fe–S clusters
  • Fe–S clusters pass e\(^-\) to CoQ

Complex II

  • succinate → fumarate, with FAD → FADH\(_2\)
  • FADH\(_2\) transfers e\(^-\) to Fe–S clusters
  • Fe–S clusters transfer e\(^-\) to CoQ
  • no proton pumping

Coenzyme Q (CoQ)

  • lipid-soluble; ubiquinone
  • accepts electrons from multiple sources
  • floats with two electrons received

ROS note:

  • if CoQ holds only one electron, it can generate ROS
  • CoQ with an extra electron is a superoxide (as noted)

Complex III (cytochrome bc\(_1\) complex)

  • cytochromes: proteins containing heme groups
  • heme: heterocyclic ring with Fe in the center

Components (as listed):

  • cytochrome b\(_L\) (low affinity)
  • cytochrome b\(_H\) (high affinity)
  • cytochrome c\(_1\)

Electron movement (Q-cycle description, as listed):

  1. CoQ arrives (pumps out 2 H\(^+\)) and delivers electrons to b\(_L\) then b\(_H\)

  2. then passed to another CoQ (only 1 of the two electrons are passed)

  3. CoQ arrives again (pumps out 2 H\(^+\)) and delivers 1 electron to Fe–S then to c\(_1\) then to cytochrome c

Cytochrome c:

  • holds 1 electron at a time
  • membrane-bound on the outer-facing side of the inner membrane

Complex IV (cytochrome c oxidase)

Components (as listed):

  • cytochrome a
  • cytochrome a\(_3\)
  • Cu A
  • Cu B

Electron flow:

  • receives electrons from 2 cytochrome c
  • electrons go: Cu A → cytochrome a → cytochrome a\(_3\) → Cu B
  • oxidizes half an O\(_2\) molecule per cycle
  • pumps 2 H\(^+\) out each time (as noted)

Proton motive force

  • ETC creates:

  • concentration gradient (ΔpH)

  • electric potential (ΔΨ)

ATP synthase (F\(_0\)/F\(_1\) ATP synthase)

  • multi-unit enzyme

F\(_0\) component

  • lipid-rich, membrane-embedded
  • motor driven by protons

Subunits (as listed):

  • C subunits: motor (rotor)
  • A subunits: proton channel

Mechanism:

  • proton flow through A subunit drives C subunit rotation

F\(_1\) component

  • water-soluble
  • converts ADP → ATP
  • without F\(_0\), F\(_1\) degrades ATP (as noted)

Sites (as listed):

  • \(\alpha\) and \(\beta\) sites
  • \(\beta\) subunits perform ATP synthesis

Coupling:

  • F\(_0\) rotation drives F\(_1\) rotation

Rotary catalysis

  • every 12 protons through the channel:

  • 1 full rotation

  • 3 ATP synthesized

Energy note:

  • much energy is lost as heat
  • metabolism is a significant source of heat

Uncouplers

Uncouplers dissociate proton gradients from ATP production.

Types:

  • chemical (proton ionophores)
  • protein

Chemical uncouplers:

  • pick up H\(^+\) and carry it across the membrane into the matrix
  • work because pH is lower than the pK\(_a\) of the chemical (as noted)

Example:

  • 2,4-dinitrophenol (DNP)