Biochemistry Chapter on Oxidative Phosphorylation

Overview of Oxidative Phosphorylation

• NADH and FADH2 are produced in multiple metabolic processes, including glycolysis, beta-oxidation of fatty acids, the TCA cycle, and other oxidative reactions.
• Electrons transfer from NADH to O2 occurs in stages through large protein complexes embedded in the inner mitochondrial membrane.
• Complexes I, III, and IV use the energy from electron transfer to pump protons to the intermembrane space.
• The pumping of protons creates an electrochemical gradient or proton-motive force.
• Protons re-enter the mitochondrial matrix through ATP synthase, causing ATP to be generated.
• The P-O ratio for NADH is 2.5. This means 2.5 ATP molecules are produced per NADH molecule.
• The P-O ratio for FADH2 is 1.5. This means 1.5 ATP molecules are produced per FADH2 molecule.

Chemiosmotic Coupling Theory

• This theory explains how oxidative phosphorylation works.
• Energy from oxidation of components in the electron transport chain is coupled to proton translocation across the inner mitochondrial membrane.

ATP Synthase (Complex V)

• Functions as a rotary motor to form ATP.
• Embedded in the inner mitochondrial membrane.
• Composed of two subcomplexes:
• Fo subcomplex: Contains a disk of C protein subunits, the gamma subunit attached to Fo and F1, and the oligomycin sensitivity conferring enzyme (OSCP).
• F1 subcomplex: Contains three alpha subunits and three beta subunits.
• Fo spans the membrane and serves as a proton channel.
• F1's beta subunit is the site of ATP synthesis.

Electron Shuttle Systems

• Two known shuttle systems:
• Glycerol-3-phosphate shuttle system: Simpler system using glycerol-3-PO4+ dehydrogenase. Major shuttle system in the brain and myocytes. Uses FAD as a mitochondrial electron carrier with a P-O ratio of 1.5.
• Malate-aspartate shuttle system: More complex system. Uses NAD as a mitochondrial electron carrier with a P-O ratio of 2.5.

ΔG: Change in Free Energy

• Free energy (G) is a measure of useful energy in a system.
• ΔG is the change in free energy and reflects the chemical potential of a system.
• ΔG is negative for exergonic reactions (energy released) and positive for endergonic reactions (energy absorbed).
• ΔG of zero indicates that the system is at equilibrium.
• ΔGo' is the standard free energy change at pH 7.0.

Standard Free Energy Change

• Endergonic reactions require energy (ΔGo' is positive).
• Exergonic reactions release energy (ΔGo' is negative).
• Isoergonic reactions are at equilibrium (ΔGo' is zero).

Substrate Level Phosphorylation

• Produces ATP through direct transfer of a phosphate group from a substrate to ADP.
• Only produces 1 ATP molecule at a time.
• Does not involve electron transport chain.

Importance of ATP

• Intermediate position of ATP allows it to play a critical role in energy transfer.
• ATP allows the coupling of thermodynamically unfavorable reactions to favorable ones.

Coupling Exergonic and Endergonic Reactions

• Energy released from an exergonic reaction can be used to drive an endergonic reaction, by transferring energy via a high-energy compound like ATP.

Creatine Phosphate

• Found in skeletal muscles.
• Acts as an energy source.
• Creatine kinase (CK) facilitates the reversible transfer of a phosphate group from creatine phosphate to ADP, producing ATP.

Importance of Pyrophosphate (PPi)

• Pyrophosphate can be further hydrolyzed to release additional energy (-4.6 kJ/mol).
• When AMP + PPi are completely hydrolyzed, the overall energy release is -12.3 kJ/mol.
• The presence of PPi in reactions can make them irreversible.