Tricarboxylic Acid (TCA) Cycle

Overview of the TCA Cycle

The Tricarboxylic Acid (TCA) Cycle, also known as the Citric Acid Cycle or Krebs Cycle, is a central metabolic pathway that oxidizes acetyl-CoA to CO2 and generates high-energy electron carriers (NADH, FADH2) for ATP production.

• Enzymes: Each step is catalyzed by a specific enzyme (e.g., citrate synthase, isocitrate dehydrogenase).

• Substrates and Products: Key substrates include acetyl-CoA and oxaloacetate; products include CO2, NADH, FADH2, and GTP/ATP.

• Structures: Students should be able to draw the structures of main intermediates (e.g., citrate, α-ketoglutarate, succinate).

Reactions Feeding Into and Out of the TCA Cycle

• Anaplerotic Reactions: Reactions that replenish TCA intermediates (e.g., pyruvate carboxylase forms oxaloacetate).

• Cataplerotic Reactions: Reactions that remove intermediates for biosynthesis (e.g., citrate for fatty acid synthesis).

• Transporters:

  • Phosphoenolpyruvate transporter: Moves PEP across the mitochondrial membrane.

  • Malate-aspartate shuttle: Transfers reducing equivalents (NADH) from cytosol to mitochondria.

  • Tricarboxylic acid transporter: Exchanges TCA intermediates between compartments.

Glyoxylate Cycle

The glyoxylate cycle is a variation of the TCA cycle in plants and some microorganisms, allowing net conversion of acetyl-CoA to succinate for gluconeogenesis.

• Key Enzymes: Isocitrate lyase and malate synthase.

• Bypasses: Steps that release CO2 are bypassed, conserving carbon.

Thermodynamics and Regulation

• Standard vs. Cellular Free Energies: Standard free energy change () is measured under standard conditions; actual cellular free energy () depends on metabolite concentrations.

• Equilibrium Constants: (equilibrium constant) and (reaction quotient) determine reaction direction: if , reaction proceeds forward; if , reverse.

• Regulation:

  • Substrate availability (e.g., acetyl-CoA, oxaloacetate, NAD+).

  • Energy charge (ATP/ADP ratio).

  • Allosteric regulation (e.g., inhibition by ATP, activation by ADP).

  • Phosphorylation of key enzymes (e.g., pyruvate dehydrogenase).

Electron Transport and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The Electron Transport Chain is a series of protein complexes (I-IV) in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to O2, generating a proton gradient.

• Complexes I-IV: Each complex has a specific role in electron transfer and proton pumping.

• Electron Carriers: NADH, FADH2, ubiquinone (Q), and cytochrome c.

• Proton Gradient: Electron transfer is coupled to proton translocation, creating an electrochemical gradient.

Oxidative Phosphorylation

• Substrate-Level vs. Oxidative Phosphorylation: Substrate-level phosphorylation directly forms ATP in metabolic reactions; oxidative phosphorylation uses the proton gradient to drive ATP synthesis.

• ATP Synthase (F0F1-ATPase): Enzyme complex that synthesizes ATP as protons flow back into the matrix.

• Regulation: Controlled by ADP availability and the proton gradient.

• Free Energy Calculations: The free energy change for proton movement is given by: where is the membrane potential.

Lipid Metabolism

Mobilization and Utilization of Fatty Acids

• Sources: Triacylglycerols from adipose tissue.

• Digestion and Transport: Fatty acids are released, transported in blood (bound to albumin), and taken up by cells.

• Storage and Utilization: Fatty acids are stored as triacylglycerols and mobilized for β-oxidation.

Fatty Acid Activation and β-Oxidation

• Acyl-CoA Synthetase: Activates fatty acids to acyl-CoA using ATP.

• β-Oxidation: Sequential removal of two-carbon units as acetyl-CoA.

  • Enzymes: Acyl-CoA dehydrogenase, enoyl-CoA hydratase, hydroxyacyl-CoA dehydrogenase, thiolase.

  • Substrates/Products: Fatty acyl-CoA, FADH2, NADH, acetyl-CoA.

  • Students should be able to draw structures and determine carbon oxidation states.

Fatty Acid Synthesis vs. Degradation

• Synthesis: Occurs in cytosol, uses malonyl-CoA, NADPH, and involves fatty acid synthase.

• Degradation: Occurs in mitochondria, uses acyl-CoA, NAD+, FAD, and involves β-oxidation enzymes.

Ketone Bodies

• Formation: Acetyl-CoA is converted to acetoacetate, β-hydroxybutyrate, and acetone in the liver.

• Function: Alternative energy source during fasting or diabetes.

Regulation of Lipid Metabolism

• Malonyl-CoA: Inhibits carnitine shuttle, preventing simultaneous synthesis and degradation.

• Carnitine Shuttle: Transports fatty acyl-CoA into mitochondria for β-oxidation.

• Acetyl-CoA Carboxylase (ACC): Key regulatory enzyme in fatty acid synthesis.

Photosynthesis: Light and Dark Reactions

Capture of Light Energy

• Pigments: Chlorophylls and accessory pigments absorb light energy.

• Energy Absorption: Light excites electrons, which are transferred through photosystems.

Photosystems and Electron Transfer

• Photosystem II (PSII): Splits water, releases O2, and donates electrons to the electron transport chain.

• Photosystem I (PSI): Receives electrons and reduces NADP+ to NADPH.

• Cyclic vs. Non-Cyclic Electron Flow:

  • Cyclic: Electrons cycle back to PSI, generating ATP only.

  • Non-Cyclic: Electrons flow from water to NADP+, producing both ATP and NADPH.

  • Source of Electrons: Water (non-cyclic), PSI (cyclic).

  • Ultimate Electron Acceptor: NADP+ (non-cyclic).

• Organization: Photosystems are embedded in thylakoid membranes.

Free Energy in Photosynthesis

• Light Energy: Drives electron transfer and proton gradient formation.

• Free Energy Change: Calculated similarly to mitochondria, considering light input and membrane gradients.

Calvin Cycle

• Stages: CO2 fixation, reduction, and regeneration of ribulose-1,5-bisphosphate.

• Rubisco: Catalyzes CO2 fixation; regulated by pH, Mg2+, and activator proteins.

Nitrogen and Amino Acid Metabolism

Nitrogen Fixation

• Process: Conversion of atmospheric N2 to NH3 by nitrogenase.

• Symbiosis: Rhizobium bacteria form nodules on plant roots to fix nitrogen.

Amino Acid Biosynthesis

• Biosynthetic Families: Grouped by precursor intermediates (e.g., α-ketoglutarate, oxaloacetate).

• Transamination: Amino group transfer between amino acids and α-keto acids.

  • General Reaction:

  • Enzyme: Aminotransferase (requires pyridoxal phosphate, PLP).

• Essential vs. Non-Essential Amino Acids: Essential must be obtained from diet; non-essential can be synthesized by the body.

Amino Acid Degradation

• Ketogenic Amino Acids: Degraded to acetyl-CoA or acetoacetate (e.g., leucine, lysine).

• Glucogenic Amino Acids: Degraded to pyruvate or TCA intermediates (e.g., alanine, glutamine).

• Classification Table:

Nitrogen Transport and the Urea Cycle

• Nitrogen Transport: Nitrogen is transported as glutamine or alanine in the blood.

• Nitrogen Assimilation: Incorporation of ammonia into amino acids (e.g., via glutamine synthetase).

• Oxidative Degradation: Amino acids are deaminated, producing ammonia and carbon skeletons.

• Glutamate and Glutamine: Central roles in nitrogen metabolism and transport.

• Glucose-Alanine Cycle: Transfers amino groups from muscle to liver for urea synthesis.

• Urea Cycle:

  • Location: Occurs in hepatocyte mitochondria and cytosol.

  • Inputs: Ammonia, CO2, aspartate.

  • Outputs: Urea, fumarate.

  • Key Intermediates: Carbamoyl phosphate, citrulline, argininosuccinate, arginine, ornithine.

  • General Reaction: