Study Notes on Oxidative Phosphorylation

What happens to NADH and FADH2 in the cell?
- Originates from glycolysis, fatty acid oxidation, amino acid catabolism, and the Citric Acid Cycle (CAC).
What's the connection to ATP?
- ATP is synthesized through oxidative phosphorylation, where the energy from NADH and FADH2 leads to ATP formation.
Where did the numbers 2.5 and 1.5 come from?
- These numbers represent the amount of ATP produced per mole of NADH (2.5 ATP) and FADH2 (1.5 ATP) during oxidative phosphorylation.

Most ATP synthesized in organisms primarily occurs through oxidative phosphorylation.
The Final Electron Acceptor in the Electron Transport Chain (ETC) is oxygen.

It is the final stage of cellular respiration.
Takes place in the mitochondria.
Reaction: Glucose ( $C_6H_{12}O_6$ ) reacts with oxygen ( $6O_2$ ) to produce carbon dioxide ( $6CO_2$ ) and water ( $6H_2O$ ).
Couples the oxidation of NADH or FADH2 to the reduction of O2 (forming H2O) and the formation of ATP (from ADP and inorganic phosphate).

Stage 1: Acetyl-CoA Production
- Glycolysis leads to the formation of pyruvate, which is converted to Acetyl-CoA by the pyruvate dehydrogenase complex, releasing CO2.
Stage 2: Acetyl-CoA Oxidation
- Acetyl-CoA enters the Citric Acid Cycle, leading to production of reduced electron carriers NADH and FADH2.
Stage 3: Electron Transfer and Oxidative Phosphorylation
- Electrons flow through a series of membrane-bound carriers, resulting in proton movement and contributing to ATP synthesis.

Outer Membrane:
- Freely permeable to small molecules and ions due to porin channels.
Inner Membrane:
- Impermeable to most small molecules and ions, including H+.
- Contains respiratory electron carriers (Complexes I-IV) and ATP synthase (FoF1).
Matrix:
- Contains various enzymes including those for the citric acid cycle, fatty acid β-oxidation, and amino acid oxidation, as well as mitochondrial DNA and ribosomes.

Electron Flow:
- Electrons are transferred through a chain of membrane-bound carriers leading to the movement of protons (H+) from the matrix to the intermembrane space.
Proton Gradient:
- The proton motive force drives ATP synthesis as protons flow back into the mitochondrial matrix through ATP synthase.
Role of NADH and FADH2:
- NADH drops two electrons directly to FMN in Complex I, while FADH2 from succinate enters via Complex II, contributing to electron transport.

Classes: a, b, c (each can accept one electron per molecule).
All contain an iron-containing heme prosthetic group.
- Classes a and b are typically bound non-covalently, while class c is covalently bound.

Participate in electron transfer by undergoing redox reactions where Fe²+ is oxidized to Fe³+.

Consists of 4 complexes: Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc₁), and Complex IV (cytochrome c oxidase).
Stepwise Process:
- Electrons move from NADH/FADH2 through several complexes, culminating with the reduction of oxygen to water.
- Protons move across the membrane, creating a gradient that powers ATP synthesis.

NADH: Moves electrons into Complex I, contributing to proton pumping.
FADH2: Enters the chain at Complex II, resulting in less proton pumping compared to NADH, thus producing fewer ATPs (1.5 ATP).

Malate-Aspartate Shuttle:
- Predominantly in liver, kidney, and heart; transports reducing equivalents from cytosolic NADH into mitochondria and promotes NADH regeneration through malate.
Glycerol-3-Phosphate Shuttle:
- Found in skeletal muscle and brain; transfers reducing equivalents efficiently but reduces ATP yield.

The shuttle transfers intermediate molecules directly into the mitochondrial matrix for utilization in oxidative phosphorylation.
Transporters and enzymes involved include:
- Malate dehydrogenase (transfers electrons from NADH),
- Aspartate-Glutamate Transporter (moves aspartate in exchange for glutamate).

Cytochrome c carries electrons from Complex III to Complex IV, where they are ultimately transferred to oxygen, completing the electron transport process.
This reaction leads to the production of water and additionally pumps protons into the intermembrane space.

P/O ratio: Describes ATP formation relative to oxygen consumption.
- 2.5 ATP per NADH and 1.5 ATP per FADH2, established based on translocation of protons.
ATP Synthase: Uses energy from the proton motive force to convert ADP and inorganic phosphate into ATP, driven by the flow of protons back into the matrix from the intermembrane space.

Oxygen and ADP levels control the rate of ATP production through acceptor control.
- Higher ADP concentrations accelerate respiration and ATP synthesis.
- Low ATP usage leads to a decrease in [ADP] and rate of electron transfer.

Examples include Rotenone, Antimycin A, CO, CN-: These block electron flow, affecting the function of ATP synthase.
DNP (2,4-Dinitrophenol) acts as an uncoupler, allowing protons to flow back into the matrix without generating ATP, leading to heat production instead.

Mitochondrial diseases result from issues in the respiratory chain or oxidative phosphorylation.
Understanding oxidative phosphorylation is crucial for insights into metabolic diseases and energy regulation.

All aspects discussed emphasize the integration of substrate oxidation, oxygen consumption, and ATP production within the mitochondria, highlighting the importance of each component in cellular energetics.