Lecture 6 Mitochondrion and Aerobic Resperation

Mitochondria and Metabolism — Key Concepts and Exam Prep

• Context and goals

  • Mitochondria are central to cellular energy production (ATP) and also play roles in apoptosis and heat generation.
  • Historical prelude: discovery of mitochondria’s role in energy is relatively recent (roughly around 1946 the link between mitochondria and ATP was established).
  • Over the last ~50–60 years, deepened understanding of mitochondrial dynamics, metabolism, and disease connections has grown substantially.
  • The mitochondria are the primary site of aerobic respiration and ATP generation, but they also have important ancillary roles beyond ATP production.

• Mitochondrial structure and compartments

  • Mitochondrion is a double-membrane organelle found in nearly all eukaryotic cells.
  • Outer membrane: permeable to small molecules.
  • Inner membrane: highly selective and impermeable; contains transport proteins.
  • Two distinct spaces:
  • Intermembrane space (between outer and inner membranes).
  • Matrix (inside the inner membrane).
  • Cristae: folds of the inner membrane that increase surface area for the electron transport chain (ETC) and ATP synthesis.
  • Functional implication: more cristae surface area -> more ETC capacity -> more ATP production; variability in cristae density across tissues (e.g., heart muscle vs liver).

• Endosymbiotic theory and mitochondrial autonomy

  • Mitochondria likely originated from engulfed bacteria (endosymbiotic theory).
  • Features that support semi-autonomy:
  • Circular DNA within mitochondria.
  • Mitochondria have their own ribosomes and can synthesize some proteins.
  • They still rely on many nuclear-encoded proteins imported from the cytosol.
  • Mitochondria replicate by fission, independently of the cell cycle, allowing dynamic responses to cellular energy needs (fission vs fusion dynamics).

• Mitochondrial dynamics: fission and fusion

  • Fission: splitting of mitochondria, involves DRP1 (dynamin-related protein 1) and related factors.
  • Fusion: joining of mitochondria, involves mitofusins (MFN1/2) and related proteins.
  • Dynamics regulate shape, number, and function; allow adaptation to energy demands and stress.
  • Visual/experimental note: mitochondria undergo visible cycles of fission and fusion; these processes can affect energy output and heat production.

• Tissue energy demands and mitochondrial morphology

  • Muscle (cardiac and skeletal) and neurons typically have high energy demands and more densely packed mitochondria with extensive cristae.
  • Liver and red blood cells show different mitochondrial demands and cristae morphology.
  • Practical question from lecture: why do cardiac muscles require mitochondria with more densely packed cristae than liver cells? Answer: constant contractions and repair/recovery needs demand constant, high ATP turnover; dense cristae increase ETC capacity and ATP synthesis in muscle.

• Mitochondrial transport and entry of substrates

  • Entry points into mitochondrial matrix from cytosol:
  • Pyruvate pathway:
    • Pyruvate must cross the inner membrane via the mitochondrial pyruvate carrier (MPC).
    • Once inside the matrix, pyruvate is converted by the pyruvate dehydrogenase complex (PDH) to acetyl-CoA (and CO2) and NADH, or it can be carboxylated to oxaloacetate by pyruvate carboxylase.
    • Key concept: pyruvate enters mitochondria for use in the TCA cycle; glycolysis occurs in the cytosol outside the mitochondria.
  • Fatty acid pathway:
    • Fatty acids must be activated to fatty acyl-CoA and then transported into the matrix via the carnitine shuttle.
    • Outer membrane transport and activation: CPT1 (carnitine palmitoyltransferase 1) transfers the acyl group to carnitine, forming acylcarnitine.
    • Translocation: CACT (carnitine/acylcarnitine translocase) shuttles acylcarnitine across the inner membrane in exchange for free carnitine.
    • Inside the matrix: CPT2 (carnitine palmitoyltransferase 2) transfers the acyl group back to CoA to form acyl-CoA for β-oxidation.
    • The cycle is energy-dependent; carnitine availability is crucial. Carnitine deficiency leads to energy failure and symptoms like weakness and hypoglycemia; tissue-specific effects depend on where transport is impaired (e.g., liver vs muscle).
  • Important concept: transport and activation steps are tightly regulated and require ATP in the activation step and proper transport across membranes.

• Pyruvate fate and metabolic branching

  • If MPC is functioning well, pyruvate enters the mitochondria and is converted to acetyl-CoA by PDH, feeding the TCA cycle.
  • Pyruvate can also be converted to oxaloacetate via pyruvate carboxylase, providing an anaplerotic input to replenish TCA intermediates.
  • If mitochondrial entry is blocked or impaired, pyruvate can accumulate in the cytosol and be converted to lactate by lactate dehydrogenase, regenerating NAD+ to sustain glycolysis (cytoplasmic NAD+ recycling).
  • Consequence of impaired mitochondrial entry: decreased ATP production from the TCA cycle and oxidative phosphorylation; increased lactate production can lead to lactic acidosis if severe.
  • Real-world analogy: if the main energy factory (mitochondria) can’t accept raw fuel (pyruvate), the cell switches to a faster but less efficient backup energy pathway (lactate production) to meet immediate demands.

• Linking glycolysis, TCA, and oxidative phosphorylation

  • Glycolysis yields pyruvate in the cytosol; pyruvate enters mitochondria to feed the TCA cycle via PDH (plus the citrate/oxaloacetate pool).
  • The TCA cycle oxidizes acetyl-CoA to CO2 and reduces NAD+ and FAD to NADH and FADH2, respectively.
  • NADH and FADH2 feed electrons into the electron transport chain (ETC) to generate a proton gradient, which drives ATP synthase to produce ATP.
  • Important reminders from lecture:
  • The TCA cycle is a central hub, providing reducing equivalents (NADH, FADH2) and metabolic intermediates for biosynthesis.
  • The cycle is regulated and interconnected with numerous other pathways; dysfunction can propagate through metabolism.

• The Citric Acid Cycle (TCA) — overview and energetics

  • Location: mitochondrial matrix.
  • Overall entry: acetyl-CoA + oxaloacetate → citrate (6 carbons) as the committed step.
  • Stepwise progression (briefly):
  • Citrate synthase: acetyl-CoA + oxaloacetate → citrate (6C) [committed step]
  • Aconitase/isomerization: citrate ↔ isocitrate (6C)
  • Isocitrate dehydrogenase: isocitrate → α-ketoglutarate + CO2 + NADH
  • α-Ketoglutarate dehydrogenase complex: α-KG → succinyl-CoA + CO2 + NADH
  • Succinyl-CoA synthetase: succinyl-CoA → succinate + GTP (substrate-level phosphorylation; GTP can be converted to ATP by nucleoside diphosphate kinase)
  • Succinate dehydrogenase: succinate → fumarate + FADH2
  • Fumarase: fumarate → malate
  • Malate dehydrogenase: malate → oxaloacetate + NADH
  • Net yields per acetyl-CoA:
  • 3\ \mathrm{NADH},\ 1\ \mathrm{FADH2},\ 1\ \mathrm{GTP},\ 2\ \mathrm{CO2}
  • Important note on stoichiometry:
  • Each glucose molecule yields two pyruvate → two acetyl-CoA, so the cycle is effectively per acetyl-CoA. Therefore, per glucose you get roughly: 6\ \mathrm{NADH},\ 2\ \mathrm{FADH2},\ 2\ \mathrm{GTP},\ 4\ \mathrm{CO2}.
  • Regulation and kinetics:
  • Key regulatory steps are largely irreversible: citrate synthase (step 1), isocitrate dehydrogenase (step 3), and α-ketoglutarate dehydrogenase (step 4).
  • Allosteric control: ATP and NADH inhibit the cycle; ADP and Ca2+ activate it.
  • The cycle is both anabolic and catabolic: it supplies biosynthetic precursors and energy-yielding reactions.
  • Warburg effect (context): cancer cells may rely on aerobic glycolysis (lactate production in the presence of oxygen) rather than oxidative phosphorylation, enabling rapid biomass production and proliferation.
  • CO2 as a byproduct: CO2 is released during decarboxylation steps and serves as a regulatory signal in metabolism but is not directly used for ATP production.

• Electron carriers and redox energetics

  • NADH and FADH2 are the principal reducing equivalents feeding the ETC.
  • Electron transfer and free energy concepts:
  • Reduction/oxidation: NADH formation involves gaining electrons; NAD+ formation involves losing electrons.
  • General redox potential concept: stronger electron donors have more negative reduction potentials.
  • Redox thermodynamics can be summarized by: \Delta G = -nF\Delta E, where ΔE is the difference in reduction potential.
  • The Nernst equation for redox couples: E = E^{\circ} - \frac{RT}{nF} \ln Q, where Q is the reaction quotient and n is the number of electrons transferred.
  • NADH NAD+/NADH potentials and the ETC:
  • NADH feeds electrons into Complex I (NADH dehydrogenase).
  • FADH2 feeds electrons into Complex II (succinate dehydrogenase).
  • Stoichiometry of ATP production (typical teaching, varies by shuttle and efficiency):
  • Approx. 2.5\ \text{ATP per NADH} when electrons enter at Complex I via NADH.
  • Approx. 1.5\ \text{ATP per FADH_2} when electrons enter at Complex II via FADH2.
  • NADH shuttling between cytosol and mitochondria:
  • NADH cannot directly cross the inner mitochondrial membrane.
  • Malate–aspartate shuttle transfers reducing equivalents as malate across the membrane, regenerating NADH in the matrix.
  • Glycerol-3-phosphate shuttle transfers electrons to FAD, forming FADH2 that feeds Complex II.
  • Specific shuttle details mentioned in lecture:
  • Malate–aspartate shuttle: transfers electrons to NAD+ to form NADH in the matrix; involves malate ↔ oxaloacetate cycling.
  • Glycerol-3-phosphate shuttle: transfers electrons to FAD, producing FADH2 that enters Complex II.
  • A note about FADH2 origin in the cycle:
  • FADH2 can also be generated directly in the TCA cycle at succinate dehydrogenase (Complex II), which interfaces with the ETC and contributes to the overall ATP yield.

• The role of the Warburg effect and metabolic flexibility

  • Warburg effect concept: some cancer cells favor aerobic glycolysis (lactate production) even in the presence of oxygen, reducing reliance on oxidative phosphorylation.
  • Consequences: rapid provision of metabolic intermediates for biosynthesis supports proliferation; energy yield per glucose is different from oxidative phosphorylation, but overall metabolic wiring is reprogrammed.

• Practical implications and clinical connections

  • Fatty acid transport defects (carnitine shuttle): energy deficiency symptoms vary by tissue; liver impairment affects ketone regulation; muscle impairment affects energy supply for contraction and repair.
  • Pyruvate transport defects or PDH dysfunction: reduced acetyl-CoA entry into the TCA cycle; potential buildup of pyruvate and lactate; decreased ATP production; risk of lactic acidosis if severe.
  • Mitochondrial dynamics and disease: alterations in fission/fusion balance can affect energy production and cellular health; potential links to neurodegenerative and cardiac diseases (conceptual linkage, not detailed in lecture).

• Exam review and study strategy highlights

  • Exam feedback and process
  • Exams are returned at end of class; scores posted on Canvas alongside the return.
  • Distribution of scores discussed (e.g., high score 97, average 67, concern about low end of distribution).
  • Regrading policy: one-week window to request regrading with a written explanation; instructor and TAs review and may adjust points.
  • Short-answer strategy tips
  • Read the question carefully and answer all parts.
  • More words does not guarantee more points; avoid contradictions and avoid simply regurgitating definitions.
  • Focus on application of concepts, not just definitions.
  • Use precise scientific language and proper verbs; write as if explaining to a fellow scientist.
  • If unsure, ask during the exam or raise a hand for clarification.
  • Always read answers aloud after writing to check coherence and flow.
  • Study structure and approach
  • Use learning objectives as study guides; attempt to answer them without notes, then fill gaps with slides.
  • Create flashcards from objectives to reinforce key concepts.
  • Practice questions first without notes; identify gaps; then discuss answers in groups to reinforce understanding.
  • Use review sessions to ask thoughtful questions (not just request answers).
  • TA support and recitations
  • TA office hours are transitioning to recitation-style sessions with example and practice problems.
  • Reach out to TAs by email if times don’t work for you.
  • Exam logistics and seating
  • Exams distributed by last name groups; seating arrangement details provided for a specific class session.

• Summary of key takeaways to memorize (conceptual anchors)

  • Mitochondria: double-membrane, inner membrane permeability, cristae surface area, matrix compartment, endosymbiotic origin, own DNA and ribosomes, self-replication by fission.
  • Transport and metabolism initiation: MPC for pyruvate; CPT1/CACT/CPT2 for fatty acid entry; PDH converts pyruvate to acetyl-CoA; pyruvate carboxylase can make oxaloacetate.
  • TCA cycle yields per acetyl-CoA: 3\ \mathrm{NADH},\ 1\ \mathrm{FADH2},\ 1\ \mathrm{GTP},\ 2\ \mathrm{CO2}; per glucose (2 acetyl-CoA): double that.
  • ETC and ATP yield: NADH ~2.5 ATP; FADH2 ~1.5 ATP; two main shuttles connect cytosolic NADH to the mitochondrial ETC.
  • Redox and regulation: ATP/NADH inhibit TCA; ADP/Ca2+ activate; multiple allosteric sites; Warburg effect introduces metabolic reprogramming in cancer cells.
  • Clinical correlations: carnitine transport defects, lactic acidosis, tissue-specific metabolic needs, energy vs heat production in mitochondria.

• Connections to earlier and broader concepts

  • Membrane transport basics (from prior lectures) underpin the inner mitochondrial membrane’s selectivity and transporters (MPC, CPT1/2, CACT).
  • Bioenergetics foundations (Glycolysis, TCA, ETC) tie to redox chemistry, ATP synthesis, and metabolic integration across pathways.
  • TheWarburg effect links metabolism to disease and cellular growth strategies, illustrating how energy pathways adapt to cellular demands.

• Ethical/philosophical/practical implications mentioned or implied

  • Practical: understanding metabolism informs disease comprehension and treatment (e.g., metabolic deficiencies, lactate handling, energy supply in muscle and heart).
  • Practical: knowledge of metabolic regulation highlights the importance of balanced energy sources and caution around misguided diets (e.g., excessive ketogenic approaches can impact liver and overall metabolism).
  • Ethical/philosophical: ongoing research into metabolism and cancer (e.g., Warburg effect) drives discussions about experimental design, interpretation, and clinical translation.

• Quick reference formulas and constants to remember

  • Net TCA yield per acetyl-CoA: 3\ \mathrm{NADH},\ 1\ \mathrm{FADH2},\ 1\ \mathrm{GTP},\ 2\ \mathrm{CO2}
  • ATP yields (rough, shuttle-dependent): ext{ATP per NADH} \approx 2.5, ext{ATP per } \mathrm{FADH_2} \approx 1.5
  • Overall redox energetics: \Delta G = -nF\Delta E and E = E^{\circ} - \frac{RT}{nF}\ln Q

• Links to practical exam prep

  • Focus your study on the three key regulatory steps in the TCA: citrate synthase (step 1), isocitrate dehydrogenase (step 3), and α-ketoglutarate dehydrogenase (step 4).
  • Be able to explain how disruptions in transport (MPC or carnitine shuttle) affect ATP production and potentially lead to lactate buildup.
  • Be prepared to discuss how mitochondrial dynamics (fission/fusion) relate to energy demand and tissue-specific needs.
  • Practice applying concepts to hypothetical scenarios (e.g., what happens if MPC is blocked? how would a cell compensate? what would be the systemic effects?).