Oxidative Phosphorylation

Electron Transport Chain and ATP Synthesis

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane that facilitate the transfer of electrons from NADH and FADH2 to oxygen, generating a proton gradient used for ATP synthesis.

• Complexes I-IV: Transfer electrons and pump protons from the matrix to the intermembrane space, creating an electrochemical gradient.

• ATP Synthesis: The proton-motive force drives ATP production via ATP synthase.

• Key Equation:

• Proton Gradient: Chemical and electrical potential differences across the membrane power ATP synthesis.

F1-Fo ATP Synthase Structure and Function

ATP synthase is a multi-subunit enzyme complex responsible for synthesizing ATP from ADP and inorganic phosphate (Pi), utilizing the proton gradient established by the ETC.

• F1 Component: Peripheral to the membrane, contains 9 subunits (α3β3γδε). Each β subunit has a catalytic site for ATP synthesis.

• Fo Component: Integral to the membrane, composed of three subunits (ab2c8-12). The c subunit forms concentric circles and provides a proton channel.

• γ Subunit: Forms a central stalk, interacts with αβ pairs, and induces conformational changes necessary for catalysis.

• Rotational Catalysis: Protonation of Asp-61 in the c subunit causes conformational changes, resulting in rotation of the γ subunit and cycling of αβ pairs through empty, ADP-bound, and ATP-bound states.

• ATP Yield: 3 ATP per 360° rotation, requiring 12 H+ translocated (for a c12 ring).

Equation:

Proton Requirement for ATP Synthesis

The number of protons required to synthesize one ATP molecule is determined by the structure of ATP synthase.

• 10 protons are pumped out per NADH oxidized; 6 protons per FADH2 (succinate) oxidized.

• 4 protons are needed to synthesize 1 ATP (for a c12 ring).

• ATP Yield: 2.5 ATP per NADH, 1.5 ATP per FADH2.

Equation:

Shuttling of ADP, Pi, and ATP

Transport of adenine nucleotides and phosphate across the mitochondrial membrane is essential for ATP synthesis.

• Adenine Nucleotide Translocase (Antiporter): Exchanges ATP4- from the matrix with ADP3- from the intermembrane space.

• Phosphate Translocase (Symporter): Co-transports H2PO4- and H+ into the matrix.

• ATP Synthase: Utilizes the proton gradient to synthesize ATP from ADP and Pi.

Shuttling of NADH

NADH produced in the cytosol during glycolysis cannot directly cross the inner mitochondrial membrane. Specialized shuttles transfer its reducing equivalents into the matrix.

• Malate-Aspartate Shuttle: Transfers electrons from cytosolic NADH to mitochondrial NAD+ via malate and aspartate intermediates. Yields 2.5 ATP per electron pair.

• Glycerol-3-Phosphate Shuttle: Transfers electrons from cytosolic NADH to mitochondrial FAD via glycerol-3-phosphate. Yields 1.5 ATP per electron pair.

ATP Production from Glucose Oxidation

The total ATP yield from complete oxidation of glucose depends on the shuttle system used for cytosolic NADH.

• 30 ATP: If only the glycerol-3-phosphate shuttle is used.

• 32 ATP: If only the malate-aspartate shuttle is used.

• 31 ATP: If both shuttles are used.

Regulation of Oxidative Phosphorylation

Oxidative phosphorylation is tightly regulated to meet cellular energy demands.

• ATP, ADP, AMP, and NADH: Allosterically regulate glycolysis, the citric acid cycle, and oxidative phosphorylation.

• High [ATP]: Inhibits glycolysis, TCA cycle, and Ox-Phos; low [ADP] and [AMP] have similar effects.

• High [ADP] and [AMP]: Accelerate all pathways.

• ADP and Pi: Primary regulators of Ox-Phos.

Principles of Metabolic Regulation

Chemistry and Regulation of Glycogen Metabolism

Glycogen metabolism involves the synthesis and breakdown of glycogen, a major storage form of glucose in animals, and is subject to complex regulation.

Glycogen Structure and Function

• Glycogen: Highly branched polymer of glucose, stored in granules in liver and skeletal muscle.

• Storage: Up to 10% of liver weight and 1–2% of muscle weight.

• Granules: Contain glycogen, enzymes for synthesis and degradation, and regulatory machinery.

• Reducing/Nonreducing Ends: Glycogen has ~55,000 glucose residues and ~2,000 nonreducing ends, facilitating rapid mobilization.

Glycogen Synthesis

• Activation of Glucose: Glucose is converted to glucose 6-phosphate (G6P), then to glucose 1-phosphate (G1P) by phosphoglucomutase.

• UDP-Glucose Formation: UDP-glucose pyrophosphorylase catalyzes the reaction:

• Glycogen Synthase: Adds glucose from UDP-glucose to nonreducing ends of glycogen chains.

• Branching Enzyme: Transfers 6–7 glucose residues to form α(1→6) branches, increasing solubility and accessibility.

• Primer Requirement: Glycogen synthase requires a primer of at least 8 glucose residues, provided by glycogenin.

Glycogen Breakdown

• Glycogen Phosphorylase: Removes glucose residues from nonreducing ends, producing glucose 1-phosphate.

• Debranching Enzyme: Transfers three residues to a nearby chain and releases the final residue as free glucose.

• Phosphoglucomutase: Converts glucose 1-phosphate to glucose 6-phosphate.

• Glucose-6-Phosphatase: In liver and kidney, converts G6P to glucose for export to blood.

Regulation of Glycogen Metabolism

• Glycogen Phosphorylase: Regulated allosterically (inhibited by glucose) and hormonally (phosphorylation activates).

• Phosphorylase b Kinase: Activated by glucagon/epinephrine via cAMP/PKA signaling; phosphorylates and activates glycogen phosphorylase.

• Phosphorylase a Phosphatase: Dephosphorylates and inactivates glycogen phosphorylase.

• Glycogen Synthase: Active when dephosphorylated; regulated by the same phosphatase (PP1).

• Reciprocal Regulation: Glycogen synthase and phosphorylase are never fully active simultaneously, ensuring efficient control of glycogen metabolism.

Example: During fasting, glucagon stimulates glycogen breakdown in the liver to maintain blood glucose; during exercise, epinephrine stimulates glycogen breakdown in muscle for energy.

Additional info: The notes cover key aspects of oxidative phosphorylation and glycogen metabolism, including enzyme structure, mechanism, and regulation, suitable for biochemistry exam preparation.