ETC

• The first protein complex that receives electrons from NADH is the complex I
• Products of Glycolysis and Krebs cycle

  • ATP
  • NADH
  • FADH2
• Electrons carried by NADH and FADH2 are transported to the inner mitochondrial membrane.
• Complex I

  • These proteins receive and transfer electrons from the oxidation of NADH
  • Electrons are then transferred to ubiquinone (labeled as Q), thus producing QH2
• Complex II and Ubiquinone

  • Succinate dehydrogenase (covalently attached to FAD) and Fe-S receive electrons from the oxidation of FADH2 to FAD. 
  • The succinate dehydrogenase enzyme also oxidizes succinate to fumarate during the Krebs cycle.
• Complex III

  • contains three molecules, namely, cytochrome b (Cyt b), Rieske center (2Fe-2S center), and cytochrome c1 (Cyt c1). 
  • The first electron released by QH2 is transferred to Rieske center to Cyt c1. Then, the Cyt c1 transfers the electron to the cytochrome c (Cyt c).
  • Cyt c transfers electrons from complex III to complex IV.
• Complex III (2nd electron)

  • Since QH2 carries two electrons, one of its electrons passes through another pathway. The second electron is transferred to Cyt b, then to another molecule of Q located in complex III
  • Q is converted into its reduced form semiquinone radical ion (Q–).
• Complex III (another)

  • When another QH2 arrives at the complex III, it is oxidized again to Q
  • One electron of QH2 is transferred to the Rieske center then to Cyt c1 and Cyt c.
  • The second electron is transferred to Cyt b and then to Q–. Since Q– receives another electron, it becomes QH2. This QH2 molecule is eventually released to the inner membrane of the mitochondrion
• Complex IV (Step 1-2)

  • The electron travels to CuA/CuA to Cyt a, and then to Cyt a3
  • if another Cyt c arrives, it goes the same path but continues to CuB.
• Complex IV (Step 3)

  • Since both Cyt a and CuB receive electrons, they are now both reduced. Once they are reduced, oxygen (O2) molecule binds to them, forming a peroxide bridge.
• Complex IV (Step 4-5)

  • When two more Cyt c arrives, two more electrons are transferred
  • Two more electrons transferred causes the binding of two hydrogen atoms to break the peroxide bridge forming Cyt a3-OH and CuB-OH.
• Complex IV (Step 6-7)

  • Two more hydrogen atoms enter the complex IV to oxidize Cyt a3-OH and CuB-OH into their original forms
  • As a result of the previous step, there are two water molecules formed
• NADH and FADH2 are oxidized to release electrons into the protein complexes
• these electrons pass through protein complexes and cause the pumping of hydrogen ions from the matrix to the intermembrane space
• ATP Synthase

  • The hydrogen ions return to the matrix through ATP synthase.
• in ETC and chemiosmosis, a total of 34 ATP molecules are produced
• 10 NADH produces 30 ATP molecules
• 2 FADH2 produces 4 ATP
• C1, C3, and C4 only gain hydrogen balls (blue) when they receive yellow balls (electrons)
• How do heme groups in cytochromes help in the transport of electrons?

  Heme groups contain iron that attracts and donates electrons.
• There are four proteins embedded in the inner

  membrane of a mitochondrion that forms this transport complex.
• Both the electron transport chain and chemiosmosis constitute the oxidative phosphorylation.
• In Complex I, the first molecule that accepts the electrons from NADH is a flavoprotein.
• Flavoprotein

  • This molecule contains a prosthetic group called flavin mononucleotide (FMN)
• FMN is oxidized, while Fe-S is reduced in complex I
• Ubiquinone (Q) is a mobile hydrophobic molecule that receives electrons from Fe-S found in both complexes I and II.
• Complex II accepts electrons from the oxidation of FADH2 to FAD
• Complex II contains succinate dehydrogenase and Fe-S
• unlike complex I, complex II does not pump protons into the intermembrane space
• Cytochromes are proteins that contain a heme group consisting of an iron atom.
• Cyt b contains two heme groups
• Cyt c1 has one heme group
• When NADH is oxidized to NAD+, electrons are transferred to complex I

  • This causes complex I to pump one hydrogen ion from the matrix to the intermembrane space