Lecture 7 Proton Motive Force: Making ATP

Class logistics and study approach

• Exam pickup: pick up exams from TAs at end of class; review to identify what didn’t go as hoped.
• Regrades: deadline next Wednesday; email with a picture/screenshot of the question(s) you want reviewed and a concise explanation of why points were not represented accurately.
• Communication for strategy: discuss exam strategies via email or attend office hours; regrades handled via email to keep things equitable.
• Study strategy recommended by instructor: start with class objectives, attempt prompts on your own, then compare with notes/slides to identify recall gaps; use this to generate flashcards or other study tools; problem sets follow.

Mitochondria and the Electron Transport Chain (ETC): overview

• Mitochondrial structure and compartments:
  • Outer membrane and inner mitochondrial membrane.
  • Matrix: inside the inner membrane.
  • Intermembrane space: space between outer and inner membranes.
• Key carriers delivering electrons to the ETC:
  • NADH and FADH2 (electron carriers).
  • NADH donates at Complex I; FADH2 donates at Complex II.
• Location of the ETC:
  • Electron transport occurs in the inner mitochondrial membrane.
  • Proton pumping into the intermembrane space creates the proton gradient used by ATP synthase.
• Integration of metabolism:
  • Anabolic and catabolic pathways feed substrates into glycolysis, the TCA cycle, and ETC.
  • NADH and FADH2 generated in glycolysis and TCA feed the ETC to ultimately produce ATP.

NADH entry, shuttle, and TCA yields

• NADH from glycolysis cannot cross the mitochondrial membrane directly; enters mitochondria via shuttles (e.g., malate–aspartate shuttle) to deliver reducing equivalents.
  • NADH transfers electrons to oxaloacetate to form malate, which can enter the matrix; malate is converted back to oxaloacetate, regenerating NADH in the matrix.
• TCA (Krebs cycle, also called the citric acid cycle) yields:
  • NADH and FADH2 as electron carriers.
  • GTP (which can be converted to ATP by a kinase).
  • The amount of NADH, FADH2, and GTP depends on whether we start from glucose or from acetyl‑CoA.
  • GTP ⇄ ATP interconversion: ext{GTP} + ext{ADP}
    ightarrow ext{GDP} + ext{ATP}.
• Overall idea: NADH, FADH2, and GTP produced feed the ETC to generate ATP.
• Theoretical ATP yield from glucose via oxidative phosphorylation is about 36\;\text{ATP}, but actual yield is lower due to inefficiencies and proton leak.

Electron transport chain: entry points and flow

• Four main mitochondrial protein complexes embedded in the inner membrane:
  • Complex I: NADH dehydrogenase (NADH donates at Complex I).
  • Complex II: Succinate dehydrogenase (FADH2 donates at Complex II); does not pump protons.
  • Complex III: Cytochrome bc1 complex; passes electrons to cytochrome c via the Q‑cycle.
  • Complex IV: Cytochrome c oxidase; reduces O2 to H2O.
• Electron carriers and shuttles:
  • Coenzyme Q (ubiquinone, Q) is lipid‑soluble and shuttles electrons between Complexes I/II and III; exists as a pool in the inner membrane.
  • Ubiquinol (QH2) is the reduced form that delivers electrons to Complex III.
  • Cytochrome C is a small, water‑soluble heme protein located in the intermembrane space; ferries electrons from Complex III to Complex IV.
• Entry points and yield differences:
  • NADH enters at Complex I; pumping of protons occurs at Complex I, III, and IV.
  • FADH2 enters at Complex II; Complex II does not pump protons, so fewer protons are pumped per FADH2 compared with NADH, yielding less ATP per FADH2.
• Important note on Q and cytochrome C:
  • Q transfers two electrons to Complex III; two successive electrons flow through Complex III and onto cytochrome C; cytochrome C toggles between single‑electron transfers because it can carry one electron at a time.

The chemiosmotic theory and foundational experiment

• Chemiosmotic theory (1961): Proton gradients across the inner mitochondrial membrane drive ATP synthesis via ATP synthase.
• Foundational demonstration: reconstituted vesicles with lipid membranes and a bacterial proton pump can generate a proton gradient; ATP synthase can produce ATP using this gradient even in the absence of an intact respiratory chain, supporting the idea that the gradient itself powers ATP synthesis.
• Orientation caution: in lab diagrams, the gradient can look reversed relative to the cellular context; in vivo, protons are pumped into the intermembrane space, not into the matrix.

Proton motive force: two components and magnitude

• Proton motive force (PMF) has two components:
  • Chemical gradient: ΔpH (difference in proton concentration across the membrane).
  • Electrical potential: ΔΨ (membrane potential arising from charge separation).
• In human/mammalian mitochondria, the electrical component ΔΨ tends to be the dominant contributor to PMF, while the chemical ΔpH also contributes.
• Typical values (illustrative):
  • Electrical potential difference: ΔΨ ≈ -150 to -200 mV.
  • pH gradient: ΔpH ≈ 0.5 to 1.0 pH unit.
  • The combined PMF can be expressed (conceptually) as PMF = \Delta \Psi - \left(\dfrac{2.303RT}{F}\right)\Delta pH, which at physiological temperatures is often approximated by PMF \approx \Delta \Psi - 0.059 \cdot \Delta pH (mV and pH units).
• In different organisms, the relative contributions of ΔΨ and ΔpH vary (e.g., bacteria, chloroplasts, and human mitochondria can differ in the balance of chemical vs electrical components).
• Proton gradient drives ATP synthase to synthesize ATP from ADP and Pi.

ATP synthase: structure and mechanism

• ATP synthase has two main parts:
  • F0: the proton channel (the c‑ring) embedded in the membrane; proton binding and release drive rotation of the ring.
  • F1: the catalytic head (alpha3beta3 hexamer) responsible for ATP synthesis; sits on the matrix side.
• The c‑ring (F0) composition determines the number of protons required per rotation; each full rotation yields multiple ATP molecules; thus more subunits mean more protons per ATP and lower ATP-per-proton efficiency.
• The gamma subunit connects F0 to F1 and acts as a rotor; rotation of the gamma subunit induces conformational changes in the beta subunits, driving the catalytic cycle.
• Bind‑change mechanism (three states for each beta subunit):
  • Loose (ADP + Pi binding to form a substrate complex).
  • Tight (substrate converted to ATP within the catalytic site).
  • Open (ATP released, site resets for new substrates).
• As protons flow through the c‑ring, rotation of the rotor drives these conformational changes in the beta subunits, yielding ATP in the matrix.
• Rough efficiency and protons per ATP:
  • Roughly 3–4 protons are needed per ATP, depending on the number of c‑ring subunits.
  • Organism‑dependent variation in this ratio changes the ATP yield per glucose.
• Visualization and dynamic studies:
  • Single‑molecule fluorescence experiments have visualized rotation of the gamma subunit when a gradient is present, confirming the motor‑like action of ATP synthase.

ATP yield, efficiency, and the role of uncouplers

• Theoretical ATP yield from glucose via oxidative phosphorylation is about 30$-36\,\text{ATP}, but actual yields are typically lower due to proton leak and other losses.
• Overall efficiency of oxidative phosphorylation is around 34\%$$; a significant fraction of energy from the electrochemical gradient is dissipated as heat, partly via uncouplers.
• Uncouplers and thermogenesis:
  • Uncouplers (lipid‑soluble molecules) can shuttle protons across the membrane without ATP synthase, collapsing the PMF and releasing energy as heat.
  • A well‑known physiological example is UCP1 in brown adipose tissue, which contributes to non‑shivering thermogenesis by dissipating the gradient as heat.
  • Diet‑ or drug‑induced uncoupling (e.g., DNP) can dangerously raise body temperature and disrupt cellular homeostasis.
• In bacteria, the PMF can also drive processes other than ATP synthesis (e.g., nutrient uptake, flagellar rotation), illustrating the versatile use of PMF across life.

Oxygen’s essential role and metabolic consequences

• Oxygen is the final electron acceptor in the ETC (Complex IV reduces O2 to H2O).
• Absence of oxygen halts the ETC; electrons back up, NADH and other carriers remain reduced, and ATP production via oxidative phosphorylation ceases.
• Without ETC activity, cells increasingly rely on anaerobic pathways (e.g., lactate production) to regenerate NAD+ for glycolysis, which is less efficient for ATP production and can affect cellular pH.
• The mitochondrial ETC is present in many copies per cell; multiple parallel ETC “units” increase surface area and capacity for ATP production.
• Cyanide is a potent inhibitor that binds Complex IV, blocking electron transfer to O2 and collapsing the PMF, leading to rapid ATP depletion and fatal outcomes.

Rotenone effect and pathway inhibition (end‑of‑course practice question)

• Rotenone inhibits Complex I (NADH dehydrogenase):
  • NADH cannot donate electrons through Complex I; NADH accumulates because it cannot be oxidized.
  • FADH2 (which donates at Complex II) can still feed electrons into the chain via Complex II and downstream complexes, but overall ATP production drops because NADH‑driven proton pumping is impaired.
• Outcome: NADH accumulates; FADH2 is used normally via Complex II; ATP production is markedly reduced.

ATP synthase stoichiometry: impact of c‑ring subunit number

• The number of subunits in the c‑ring (F0) determines how many protons are required per rotation and thus per ATP.
• If the C‑ring has more subunits (e.g., 12 vs 8), more protons are required per full rotation to synthesize the same amount of ATP.
• Consequence for ATP production per proton:
  • More subunits (more protons per ATP) -> ATP produced per proton decreases.
  • In other words, increasing the number of c‑ring subunits lowers the efficiency of ATP production per proton pumped.

Mitochondrial diseases and clinical relevance

• Mitochondrial diseases: disruptions can arise from nuclear or mitochondrial genes affecting ETC components or assembly.
• Common clinical manifestations: neurological issues (strokes, seizures), muscle weakness/degeneration, cardiac and endocrine dysfunction, hearing loss.
• Three brief examples discussed:
  • Kearns–Sayre syndrome (referred to as “knee loss” in lecture): mitochondrial disease with multi‑system effects.
  • LHON: Leber hereditary optic neuropathy, affecting vision.
  • MERRF (listed as MIRF in lecture): myoclonus epilepsy with ragged‑red fibers; characteristic muscle pathology.
• Diagnosis and management:
  • Muscle biopsies, genetic testing, and biochemical assays.
  • Supportive treatments and nutritional supplementation (e.g., coenzyme Q) as needed.
  • Experimental approaches: gene therapy and mitochondrial replacement strategies.
• Practical takeaway: mitochondrial diseases impact energy production in muscles, brain, and heart; there is no universal cure, but supportive care can help manage symptoms.

End‑of‑lecture practice questions (concept checks)

• A patient is exposed to rotenone, a Complex I inhibitor. Which changes would you expect in the redox carriers and ATP production?
  • NADH is unable to be oxidized, so NADH accumulates; FADH2 can still donate via Complex II, but overall ATP production declines because NADH oxidation is blocked.
• If the C‑ring of ATP synthase had 12 subunits instead of 8, how would this affect ATP production per proton?
  • More protons would be required per ATP, so ATP production per proton would decrease (lower efficiency per proton).

Quick reality checks and real‑world connections

• Without oxygen, the ETC cannot operate, and cells switch to anaerobic metabolism, producing lactate and lowering pH if accumulation occurs.
• The ETC is not a single chain; multiple ETC units exist in the inner mitochondrial membrane, increasing capacity and efficiency depending on tissue energy demands.
• Heat production via uncouplers and brown fat is an intentional physiological mechanism to regulate body temperature, illustrating that not all energy from metabolism is used for ATP production.

Summary of key concepts

• The ETC transfers electrons from NADH and FADH2 through Complex I–IV, pumping protons to generate a proton motive force across the inner mitochondrial membrane.
• Proton motive force consists of both electrical (ΔΨ) and chemical (ΔpH) components; the overall gradient drives ATP synthase to make ATP from ADP and Pi.
• Complex II does not pump protons, so NADH yields more ATP per molecule than FADH2 because NADH contributes more extensively to the gradient via Complex I.
• ATP synthase converts the rotational energy of the c‑ring/gamma rotor into chemical energy by producing ATP in the F1 head; the number of protons per ATP is organism‑dependent (roughly 3–4 protons per ATP).
• Oxidative phosphorylation is surprisingly inefficient (about 34%), with energy lost as heat via uncoupling processes and other cellular pathways.
• Cyanide, rotenone, and other inhibitors illustrate how delicate ETC function is; disruption can rapidly impair ATP production and cellular viability.
• Mitochondrial diseases illustrate the crucial role of oxidative metabolism in neurological, muscular, and cardiac health, with diverse etiologies and limited cures.