Cell as a Factory: Oxidative Phosphorylation and Metabolic Regulation

Oxidative Phosphorylation and Electron Transport Chain

• The lecture is divided into two parts: the third lecture in cell as a factory and secretion.
• The focus is on oxidative phosphorylation, including the electron transport chain.
• NADH is an electron carrier (NAD is reduced to NADH).
  • The hydrogen atom is attached to the NAD molecule.
  • Electrons are captured in that bond.
  • Breaking the bond releases electrons.

Electron Transport Chain

• Releasing all free energy at once from a glucose molecule would be too much.
• Instead, energy is released in small packets.
• The electron transport chain (or respiratory chain) is located in:
  • Eukaryotic cells: inner membrane of the mitochondria.
  • Bacteria: inner membrane of the bacterial cell.
• Electrons flow through carrier proteins, doing work within the mitochondria.

Complexes and Mobile Elements

• Complexes: 1, 2, 3, and 4.
• Mobile elements:
  • Ubiquinone (Q) - not a protein, a small molecule.
  • Cytochrome c - a protein.
• Complexes 1-4 are proteins embedded in the membrane.
• Electrons combine with oxygen to make water, releasing free energy.
• Things become more oxidized, giving up free energy.
• Energy is used by some proteins to transport protons across the inner membrane of the mitochondria (active transport).
• Protons build up in the intermembrane space, creating a store of potential energy.

Mitochondria as Rechargeable Batteries

• Mitochondria can be thought of as rechargeable batteries.
• Potential energy from food molecules (e.g., glucose) is stored in a proton gradient across the inner membrane.
• This creates a proton motive force.
• Proton motive force: the difference in the concentration of protons on either side of the membrane.
• Protons flow back down their concentration gradient into the mitochondrial matrix through ATP synthase.
• ATP synthase is like a turbine.
• The movement of protons through ATP synthase is coupled to the generation of ATP.

ATP Synthase Mechanism

• The molecular turbine spins around in the mitochondrial membrane.
• This causes conformational changes that enable the protein to stitch a phosphate onto an ADP molecule, making ATP.
• The buildup of protons in the mitochondria acts like a rechargeable battery, doing work by producing ATP.

Recap of Energy Production

• Glucose is split into two pyruvate molecules, yielding a small amount of ATP (2 ATP per glucose).
• Pyruvate is oxidized into acetyl CoA, capturing electrons.
• Acetyl CoA feeds into the citric acid cycle, producing more electron carriers.
• Electron carriers donate electrons to the electron transport process.
• This enables protons to build up in the mitochondrial membrane.
• The proton gradient drives ATP production.
• Glucose is broken down into carbon dioxide and water.

Detailed Look at the Electron Transport Chain

• Electrons are removed from NADH (by complex 1) or FADH2 (by complex 2).
• Electrons are passed on to ubiquinone (coenzyme Q10).
• Ubiquinone moves in the membrane, passing electrons to complex 3.
• Electrons are passed on to cytochrome c.
• Electrons are passed on to complex 4, where they combine with molecular oxygen to make water.
• Oxygen acts as the final electron acceptor.
• Carbon dioxide comes mainly from the citric acid cycle.
• Electron flow allows protons to be pumped across the membrane, building a gradient.

Chemiosmotic Mechanism and ATP Generation

• The membrane-bound portion of ATP synthase spins around.
• A prop shaft spins in the middle.
• The F1 portion of the protein stays still.
• Movement of the shaft forces subunits of the F1 portion to change conformation.
• These conformational changes allow ADP and phosphate to react, forming ATP.
• ATP leaves the mitochondria, maintaining a low concentration.
• The proton gradient is maintained by breaking down food molecules (sugars, fatty acids).

Uncoupling ATP Synthesis from Electron Transport

• ATP synthesis can be uncoupled from electron transport.
• Brown fat tissue contains channel proteins in the inner membrane.
• These proteins prevent the proton gradient from building up.
• Energy is released as heat instead of ATP.
• Brown fat helps maintain body temperature in infants and hibernating animals.

ATP Synthase Structure and Function

• ATP synthase has two main parts:
  • F0: membrane-bound part.
  • F1: projects into the mitochondrial matrix, where ATP is generated.
• The proton gradient is essential for ATP production.

Anaerobic Conditions and Fermentation

• Without oxygen, glucose can be split into two pyruvate molecules via glycolysis.
• This occurs in the cytoplasm.
• Fermentation is needed to regenerate NAD, which is required for glycolysis.
• Pyruvate is reduced to lactate (lactic acid) in muscle cells to regenerate NAD.
• Microorganisms also use fermentation reactions.
• Pyruvate acts as an electron acceptor.

Redox Balance and NAD Regeneration

• Glycolysis splits glucose into two pyruvates, requiring NAD.
• NAD is converted to NADH.
• Pyruvate is converted to lactic acid, requiring NADH.
• This regenerates NAD to allow glycolysis to continue.
• NAD comes from essential vitamins and is present in low concentrations.
• Alcoholic fermentation (e.g., in yeast) also regenerates NAD.
• Pyruvate is fermented into acetaldehyde and then ethanol.

Alcohol Metabolism

• When alcohol is ingested, the liver converts ethanol to acetaldehyde and then to acetyl CoA.
• Acetaldehyde is toxic and contributes to hangovers.

ATP Production Comparison

• Glycolysis alone produces 2 ATP per glucose molecule.
• With oxygen present and mitochondria available, full breakdown yields approximately 32 ATP.
• Variations in ATP production depend on where NADH is made (matrix vs. cytoplasm).

Overview of the Entire Process

• Glucose is split into pyruvate, which is oxidized to acetyl CoA.
• Acetyl CoA enters the citric acid cycle, producing electron carriers.
• Electron carriers donate electrons to the electron transport system.
• This creates a proton gradient that drives ATP synthase.
• Carbon dioxide is released.
• Oxygen is converted to water.
• ATP is generated, primarily by ATP synthase.

Metabolic Regulation

• Metabolic processes are interconnected and share substances.
• Regulation is needed to control pathways and enzyme activity.
• Enzymes are typically regulated via enzyme inhibition (dimmer switch).

Interconnections and Regulation

• Intermediates in glycolysis can be used to make amino acids, polysaccharides, or glycerol.
• Acetyl CoA is used to make fatty acids.
• Intermediates in the citric acid cycle can become amino acids or nucleic acids.
• Proteins can be broken down for energy production.

Feedback Mechanisms and Regulation

• Feedback mechanisms regulate pathways to balance energy production and other cellular needs.
• The first step in a branch point is often regulated by the end product.
• Negative feedback turns down enzyme activity.
• Positive feedback switches on another part of the process.

Examples of Regulation

• Citrate and ATP regulate glycolysis.
• Citrate is the first molecule generated in the citric acid cycle.
• High levels of citrate and ATP indicate abundant energy.
• Phosphofructokinase (PFK-1) is the key enzyme regulated in glycolysis.
• ADP activates PFK-1 when ATP levels drop.

Regulation of Citric Acid Cycle

• ATP and NADH inhibit citrate synthase, slowing down the cycle.
• Isocitrate to alpha-ketoglutarate conversion is also regulated.
• ATP and NADH inhibit this enzyme, while ADP activates it.
• Intermediates can be used for other processes, like fatty acid or amino acid synthesis.

Summary and Review

• Understand carbohydrates, lipids, and ATP.
• Know how to extract energy from these molecules.
• Understand electron transport and ATP synthesis coupling.
• Follow the story of glucose metabolism.
• Know the difference between aerobic and anaerobic conditions.
• Understand how these processes are regulated.
• Avoid rote memorization; focus on understanding the "why" behind each step.