›

Oxidative Phosphorylation

Notes

Oxidative Phosphorylation

Sections

OXIDATIVE PHOSPHORYLATION

Electron transport chain

  • Series of controlled redox reactions; pumps H+ into inter-membrane space

Chemiosmosis

  • Couples e- transport w/ ATP synthesis

ELECTRON TRANSPORT CHAIN

Complex I: NADH dehydrogenase

  • aka NADH-CoQ reductase
  • NADH delivers 2 electrons to the complex I and is oxidized to NAD+

CoQ (aka Q10 and ubiquinone)

  • Lipid-soluble
  • Mobile carrier
  • NOT a protein

Complex II: succinate dehydrogenase

  • aka Succinate-CoQ reductase
  • Also part of citric acid cycle
  • FADH2 delivers two electrons to complex II

Complex III: Cytochrome bc1 complex

  • aka CoQ-cytochrome c reductase

Cytochrome C

  • Water-soluble
  • Mobile carrier

Complex IV: cytochrome c oxidase

  • Produces one H2O from 2 H+ plus ½ O2 + 2e-
  • Complexes I, III & IV pump H+ from matrix to inter-membrane space
    (NOT complex II, cyt. C or CoQ)

CHEMIOSMOSIS

Complex V: ATP synthase

  • Inner mitochondrial membrane IMPERMEABLE to most small molecules
  • H+ that is pumped across membrane cannot diffuse back through the bilayer
  • H+ diffuses down gradient through ATP synthase into the matrix
  • Produces 30-34 ATP per glucose molecule (NADH = 3 ATP, FADH2 = 2 ATP

Full-Length Text

  • Here we will learn about oxidative phosphorylation, the final stage in cellular respiration and glucose breakdown; it generates about 90% of the ATP produced by cellular respiration, which has to keep up with our demands of about 40kg of ATP a day!
  • To begin, start a table.
  • Denote that oxidative phosphorylation comprises two key processes:
    • Electron transport chain, which transports electrons in a series of controlled redox reactions.
    • Chemiosmosis, which couples electron transport with ATP synthesis.

Now, let's illustrate each of these processes.

  • First, draw the mitochondrion, the primary site of cellular respiration.
    • Draw an outer membrane; it is a phospholipid bilayer.
    • Draw an inner membrane with invaginations called cristae; it's also a phospholipid bilayer.
    • Label the space inside the inner membrane "matrix."
    • Label the space between the membranes intermembrane space.
  • Finally, show that the mitochondrion is suspended in the cytosol.
  • Show that pyruvate is converted to acetyl CoA.
  • Draw a circle of arrows in the matrix to represent the citric acid cycle.
  • Indicate that acetyl CoA enters the citric acid cycle, which produces NADH and FADH2.
  • Now, draw the electron transport chain on the inner membrane.
  • Show that NADH and FADH2 shuttle electrons to it.
  • Show that electronegative oxygen pulls electrons down the chain.
    • The electron chain pumps protons into the intermembrane space, which sets up a chemiosmotic gradient or a protein motive force that pumps protons back into the matrix via an ATP Synthase complex that produces ATP.
  • Show that chemiosmosis occurs across the inner membrane and produces ATP.
  • Indicate that oxidative phosphorylation comprises both the electron transport chain and chemiosmosisof these processes.

Now let's illustrate the two key processes in oxidative phosphorylation. To do this, let's take a closer look at the inner mitochondrial membrane.

  • Draw the inner membrane: show the matrix above it and the intermembrane space below it.
  • Begin with the electron transport chain, which comprises four protein complexes and two mobile carriers.
  • Return to our table and denote the four complexes (all oxidoreductase enzymes; they catalyze redox reactions):
    • Complex I: NADH dehydrogenase (aka NADH-CoQ reductase)
    • Complex II: succinate dehydrogenase (aka Succinate-CoQ reductase)
    • Complex III: Cytochrome bc1 complex (aka CoQ-cytochrome c reductase)
    • Complex IV: cytochrome c oxidase.
  • Next, denote the two mobile carriers, which shuttle electrons between the large complexes:
    • CoQ (aka Q10 and ubiquinone), which is lipid-soluble (and is NOT a protein).
    • Cytochrome C, which is water-soluble.

Now, let's add these complexes and carriers to our diagram.

  • Draw the protein complexes I through IV in the membrane.
    • Show that the second embeds in the matrix leaflet of the bilayer, while the remaining three extend across both leaflets.
  • Now, draw CoQ between complex I and III, being lipid-soluble, it lies within the membrane.
  • Next, draw Cyt C in the intermembrane space between complex III and IV, being water-soluble it lies in the intermembrane space.
  • Now, show that NADH delivers 2 electrons to the first complex and is oxidized to NAD+; it's a redox reaction – the first protein complex is reduced (gains electrons) and NADH is oxidized (loses electrons).
  • Use an arrow to show that complex I transfers its electrons to CoQ.
    • The electrons transport from complex to complex in a series of controlled redox reactions.
  • Illustrate that CoQ delivers the electrons from complex I to complex III.
  • Indicate that Cytochrome C delivers electrons from complex III reduces to complex IV.

What about complex II?

  • Show that FADH2 delivers two electrons to complex II (instead of complex I).
    • Complex II is succinate dehydrogenase, which produces FADH2 in the citric acid cycle.
    • Thus FADH2 doesn't just deliver electrons to the transport chain (like NADH), but rather the enzyme that produces FADH2 is actually part of the chain.
  • Now, show that CoQ delivers electrons from complex II to complex III.
    • As with NADH, cytochrome c then delivers electrons from complex III to complex IV.
  • Illustrate that oxygen is the final electron acceptor in this chain.
    • Imagine that oxygen, being electronegative, pulls electrons down the chain.
  • Write that 2 hydrogen ions plus ½ O2 + 2e- produces one molecule of water at the end of the chain.
    • Thus, two molecules of water are produced per molecule of oxygen.

How does this chain produce ATP?

  • This brings us to the second key process in oxidative phosphorylation: chemiosmosis.
  • Denote that chemiosmosis involves complex V: ATP synthase.

Now, return to our diagram.

  • Use arrows to show that complexes I, III and IV pump protons from the matrix into the intermembrane space.
    • These complexes couple redox reactions (electron transport) with the transport of protons from the matrix to the intermembrane space.
  • Indicate that complex II and the two mobile carriers do NOT pump protons.
  • And indicate that the inner mitochondrial membrane is IMPERMEABLE to most small molecules.
    • Thus, protons that are pumped across the membrane cannot diffuse back through the bilayer.
  • Indicate that this creates a gradient, with more protons in the intermembrane space and less in the matrix.
    • Indicate that this gradient is called a chemiosmotic gradient or a protein motive force.
  • Next, draw a fifth protein on the inner membrane and label it complex V (ATP synthase).
    • It is a channel through which hydrogen ions can diffuse down their concentration gradient.
  • Indicate that this facilitates chemiosmosis.
  • Show that hydrogen ions use this channel to diffuse from the intermembrane space into the matrix.
  • Illustrate that ATP synthase harnesses this energy to phosphorylate ADP.
  • Indicate that this produces about 30-34 ATP per glucose molecule.
    • This value is an approximation and varies intertextually.
  • Write that NADH produces approximately 3ATP molecules via this process.
  • Write that FADH2 produces about 2 ATP -- FADH2 joins the chain at complex II, which does not pump protons, thus it results in fewer hydrogen ions being pumped into the intermembrane space, which generates less ATP.