Citric Acid Cycle and Oxidative Phosphorylation

Acetyl-CoA Formation

Before the TCA cycle begins, pyruvate—the end product of glycolysis—must be converted to acetyl-CoA in the mitochondrial matrix by the pyruvate dehydrogenase complex (PDC). This irreversible reaction is a critical regulatory checkpoint: pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH + H⁺. The PDC is a massive multi-enzyme complex (E1: pyruvate dehydrogenase, E2: dihydrolipoyl transacetylase, E3: dihydrolipoyl dehydrogenase) requiring five cofactors derived from B-vitamins: thiamine pyrophosphate (TPP, from B₁), lipoic acid, CoA (from pantothenic acid, B₅), FAD (from riboflavin, B₂), and NAD⁺ (from niacin, B₃). Regulation is tight: the complex is inhibited by its products (acetyl-CoA, NADH) and by phosphorylation (PDH kinase inactivates E1; PDH phosphatase activates it). Kinase is stimulated by high ATP/NADH/acetyl-CoA; phosphatase is stimulated by Ca²⁺ and insulin. This is the link between glycolysis and the TCA cycle—once pyruvate is committed to acetyl-CoA, the carbons are destined for oxidation to CO₂ or for fatty-acid synthesis (citrate shuttle). Fatty acids also enter as acetyl-CoA via β-oxidation. The MCAT frequently asks about the PDC's cofactor-vitamin connections: thiamine deficiency (beriberi) impairs PDC, causing pyruvate to accumulate and shunt to lactate.

TCA Cycle Intermediates

The TCA cycle's eight steps oxidize acetyl-CoA (2 carbons) to two CO₂ molecules, regenerating oxaloacetate. Memorize the cycle in order with enzymes and outputs. Step 1: Citrate synthase condenses acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C). Irreversible; regulated by ATP, NADH, and citrate (feedback inhibition). Step 2: Aconitase isomerizes citrate → isocitrate via the intermediate cis-aconitate. Step 3: Isocitrate dehydrogenase (rate-limiting step) oxidatively decarboxylates isocitrate → α-ketoglutarate (5C), producing CO₂ and NADH. Irreversible; activated by ADP, Ca²⁺; inhibited by ATP, NADH. Step 4: α-Ketoglutarate dehydrogenase complex (analogous to PDC, with same five cofactors) converts α-ketoglutarate → succinyl-CoA (4C), producing CO₂ and NADH. Irreversible; inhibited by succinyl-CoA, NADH, ATP. Step 5: Succinyl-CoA synthetase (succinate thiokinase) cleaves the thioester bond to produce succinate + GTP (or ATP, depending on isoform). This is the only substrate-level phosphorylation in the TCA cycle. Step 6: Succinate dehydrogenase (Complex II of the ETC—the only TCA enzyme embedded in the inner mitochondrial membrane) oxidizes succinate → fumarate, producing FADH₂ (not NADH). FAD is covalently bound as a prosthetic group. Step 7: Fumarase (fumarate hydratase) hydrates fumarate → malate. Step 8: Malate dehydrogenase oxidizes malate → oxaloacetate, producing NADH, regenerating the starting 4C acceptor. Net per acetyl-CoA: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂. Per glucose (2 turns of the cycle): 6 NADH, 2 FADH₂, 2 GTP, 4 CO₂. The cycle is amphibolic—intermediates serve as precursors for amino-acid synthesis (α-ketoglutarate → glutamate; oxaloacetate → aspartate) and heme synthesis (succinyl-CoA + glycine → δ-aminolevulinic acid). Anaplerotic reactions replenish intermediates: pyruvate carboxylase converts pyruvate → oxaloacetate (biotin-dependent, activated by acetyl-CoA).

NADH and FADH2

NADH and FADH₂ are the primary currency of reducing power produced by the TCA cycle. Each carries two high-energy electrons that will be donated to the electron transport chain. NADH is produced at three TCA steps—isocitrate dehydrogenase (step 3), α-ketoglutarate dehydrogenase (step 4), and malate dehydrogenase (step 8)—plus one from the pyruvate dehydrogenase complex, for a total of four NADH per turn of the cycle (eight per glucose when glycolysis NADH is included, depending on shuttle). FADH₂ is produced only at succinate dehydrogenase (step 6). The key difference: NADH donates electrons to Complex I of the ETC, while FADH₂ donates electrons to Complex II (succinate dehydrogenase itself). This has a critical energetic consequence—electrons from NADH enter the chain earlier and pump more protons (Complex I contributes to the gradient; Complex II does not), yielding ~2.5 ATP per NADH versus ~1.5 ATP per FADH₂. This difference is purely because FADH₂ bypasses Complex I, not because it carries less energy. The MCAT may ask you to calculate ATP yield given a tally of NADH and FADH₂ from different substrates. NADH from cytosolic glycolysis requires a shuttle to enter the mitochondria: the malate-aspartate shuttle (heart, liver—yields 2.5 ATP per NADH) delivers electrons to Complex I via mitochondrial NADH; the glycerol-3-phosphate shuttle (muscle, brain—yields 1.5 ATP per NADH) delivers electrons to Complex II via mitochondrial FADH₂. The choice of shuttle affects the ~30 vs. ~32 ATP tally.

Electron Transport Chain

The electron transport chain (ETC) is a series of four protein complexes (I–IV) embedded in the inner mitochondrial membrane, plus two mobile electron carriers (ubiquinone/CoQ and cytochrome c). Electrons flow from lower to higher reduction potential, sequentially through complexes with increasing E°′, culminating in the reduction of O₂ to H₂O—the terminal electron acceptor. Complex I (NADH dehydrogenase): accepts two electrons from NADH, oxidizing it to NAD⁺. Electrons pass through FMN and iron-sulfur (Fe-S) clusters to ubiquinone (CoQ), reducing it to CoQH₂ (ubiquinol). During this transfer, Complex I pumps 4 H⁺ from the matrix into the intermembrane space. Inhibitor: rotenone (a plant-derived pesticide) and amytal (barbiturate) block electron transfer from Fe-S clusters to ubiquinone. Complex II (succinate dehydrogenase): the only TCA cycle enzyme that is also an ETC component. It oxidizes FADH₂ (produced during succinate → fumarate) and passes electrons via Fe-S centers to ubiquinone. Complex II does NOT pump protons—this is why FADH₂ yields fewer ATP. Complex III (cytochrome bc1 complex): accepts electrons from ubiquinol and passes them to cytochrome c via the Q cycle. The Q cycle is a two-electron-to-one-electron adapter: ubiquinol (2 e⁻ carrier) reduces two molecules of cytochrome c (1 e⁻ carrier each) in sequence, pumping 4 H⁺ in the process. Inhibitor: antimycin A blocks electron transfer from cytochrome b to cytochrome c₁. Complex IV (cytochrome c oxidase): accepts electrons from cytochrome c and reduces O₂ to H₂O: 4 cyt c(reduced) + 8 H⁺(matrix) + O₂ → 4 cyt c(oxidized) + 4 H₂O + 4 H⁺(pumped). Complex IV pumps 2 H⁺ per 2 e⁻ (or 4 H⁺ when considering the full reaction). Inhibitors: cyanide (CN⁻), carbon monoxide (CO), and azide (N₃⁻) bind the heme a₃-CuB binuclear center with higher affinity than O₂, blocking electron transfer and halting the entire ETC. The mobile carriers: ubiquinone (CoQ) is a lipid-soluble quinone that shuttles electrons from Complex I and II to Complex III within the membrane. Cytochrome c is a small, water-soluble heme protein in the intermembrane space that shuttles electrons from Complex III to Complex IV. Cytochrome c release into the cytosol is a key apoptotic signal.

Chemiosmosis

The chemiosmotic hypothesis, proposed by Peter Mitchell (Nobel Prize 1978), states that the energy of electron transport is stored as an electrochemical proton gradient across the inner mitochondrial membrane, and this proton-motive force (PMF) drives ATP synthesis. As electrons pass through Complexes I, III, and IV, protons are pumped from the matrix to the intermembrane space. The inner membrane is impermeable to protons, so the gradient builds. The PMF has two components: a chemical gradient (ΔpH—higher [H⁺] in intermembrane space, more alkaline matrix) and an electrical gradient (Δψ—matrix negative relative to intermembrane space, ~−150 to −180 mV). The total PMF = Δψ − (2.303RT/F)ΔpH. In mitochondria, Δψ is the dominant component. The energy stored in this gradient is the intermediate between electron transport and ATP synthesis—it couples oxidation (ETC) to phosphorylation (ATP synthase). This coupling is obligate: if protons cannot flow back through ATP synthase, the gradient builds until it opposes further proton pumping, and the ETC stalls (respiratory control). Uncouplers like 2,4-dinitrophenol (DNP) and FCCP (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) are lipid-soluble weak acids that carry protons directly across the inner membrane, dissipating the gradient as heat. With the gradient collapsed, the ETC runs unchecked (maximal O₂ consumption) but no ATP is produced. This is the mechanism of brown-fat thermogenesis: UCP1 (thermogenin) is a natural uncoupling protein in brown adipose tissue, activated by free fatty acids released via norepinephrine-stimulated lipolysis. The MCAT frequently asks for the consequences of uncoupling: increased O₂ consumption, increased NADH oxidation, increased heat production, and decreased ATP synthesis.

ATP Synthase

ATP synthase (Complex V, F₁F₀-ATPase) is a molecular rotary motor that converts the energy of the proton gradient into the chemical energy of ATP. The enzyme has two main structural domains: F₀ (integral membrane portion) and F₁ (peripheral matrix portion). F₀ is a proton channel: protons flow down their gradient through the c-ring (a rotor of 8–15 c-subunits, species-dependent) embedded in the inner membrane. Each proton that passes through rotates the c-ring by one c-subunit. The γ-subunit acts as a central stalk (axle) connecting the F₀ c-ring rotor to the F₁ catalytic head. As the γ-subunit rotates, it mechanically drives conformational changes in the three αβ-subunit pairs of F₁. The binding-change mechanism describes three conformational states cycling through each catalytic β-subunit: Open (O)—ADP and Pᵢ enter, ATP is released; Loose (L)—ADP and Pᵢ are loosely bound; Tight (T)—ADP and Pᵢ are forcefully condensed into ATP. Each 360° rotation of the γ-subunit produces 3 ATP (one per β-subunit). The number of c-subunits determines the H⁺/ATP ratio: with 8 c-subunits, ~2.7 H⁺ per ATP (~8 H⁺ per full rotation ÷ 3 ATP); with 10 c-subunits, ~3.3 H⁺ per ATP. Conservatively, the MCAT uses ~3–4 H⁺ per ATP as the cost of ATP synthesis (including transport costs). Inhibitor: oligomycin binds F₀ and blocks the proton channel, preventing proton flow and stopping ATP synthesis. Because the gradient cannot dissipate, the ETC also backs up—oligomycin indirectly inhibits electron transport (tight coupling). The MCAT may ask you to compare oxygen-consumption traces: after adding ADP (state 3, active respiration → O₂ consumption increases), after adding oligomycin (state 4, respiration slows), and after adding an uncoupler (uncoupled state, O₂ consumption maximal).

Uncoupling and Inhibition

The MCAT tests inhibitors and uncouplers as a way to assess whether you understand the coupling between electron transport and ATP synthesis. Four classic ETC inhibitors with their targets: Rotenone blocks Complex I (NADH → CoQ); electrons from NADH cannot enter the chain, but FADH₂ (Complex II) entry is unaffected. Antimycin A blocks Complex III (CoQH₂ → cytochrome c); both NADH and FADH₂ electron flow is halted because both feed into ubiquinone upstream. Cyanide (CN⁻), carbon monoxide (CO), azide (N₃⁻): block Complex IV (cytochrome c → O₂); electrons back up through the entire chain, and O₂ reduction to H₂O ceases. Oligomycin: blocks F₀ proton channel of ATP synthase; ATP synthesis stops, and the proton gradient builds, back-inhibiting the ETC via respiratory control. Uncouplers (DNP, FCCP, thermogenin): dissipate the gradient without making ATP; ETC runs at maximum velocity, O₂ consumption is high, but no ATP is produced—all energy is released as heat. A critical distinction: ETC inhibitors (rotenone, antimycin A, cyanide) decrease O₂ consumption because electron flow is blocked. Uncouplers (DNP) increase O₂ consumption because the ETC is unshackled from ATP demand. Oligomycin decreases O₂ consumption because the backed-up gradient prevents further proton pumping. The MCAT may present traces of O₂ consumption over time with sequential additions: (1) Addition of ADP to mitochondria in the presence of substrate (e.g., succinate) → state 3, rapid O₂ consumption. (2) Addition of oligomycin → state 4, O₂ consumption drops. (3) Addition of DNP (uncoupler) → O₂ consumption surges to maximal rate. (4) Addition of cyanide → O₂ consumption falls to zero. This classic experiment (Chance and Williams) tests whether you can distinguish respiratory states and pinpoint the site of action of each agent. In a patient context: cyanide poisoning causes histotoxic hypoxia—cells cannot use O₂ even though blood O₂ levels are normal; venous blood appears bright red (high O₂ content because tissues cannot extract it). Rotenone is used as a piscicide and insecticide; antimycin A is a Streptomyces antibiotic. Oligomycin is an experimental tool, not a clinical drug. DNP was used as a weight-loss drug in the 1930s but caused fatal hyperthermia.

How it works

The logic of aerobic respiration is elegantly economic. The TCA cycle strips every electron from acetyl-CoA, capturing them as NADH and FADH₂ while discarding the carbon skeleton as CO₂. The cycle is catalytic—oxaloacetate is regenerated—so one molecule of oxaloacetate can oxidize countless acetyl-CoA molecules. The electron transport chain is a controlled cascade: electrons are passed downhill in free energy, and the energy released at three coupling sites (Complexes I, III, and IV) is used to pump protons. The inner membrane's impermeability to protons creates a battery—the proton-motive force—and ATP synthase is the turbine that discharges it to make ATP. The MCAT rewards students who can trace an electron from NADH through Complex I → CoQ → Complex III → cytochrome c → Complex IV → O₂, naming the inhibitor at each step, and then calculate the ATP yield. The pathway is stoichiometrically precise: every NADH yields ~2.5 ATP; every FADH₂ yields ~1.5 ATP. The ~30–32 ATP per glucose is an integrated accounting problem: glycolysis (2 ATP, 2 NADH), PDC (2 NADH), TCA (2 GTP/ATP, 6 NADH, 2 FADH₂), minus the transport cost of cytosolic NADH depending on shuttle. When you can reproduce that tally, you have mastered the highest-yield biochemistry pathway on the MCAT.

How it works

The logic of aerobic respiration is elegantly economic. The TCA cycle strips every electron from acetyl-CoA, capturing them as NADH and FADH₂ while discarding the carbon skeleton as CO₂. The cycle is catalytic—oxaloacetate is regenerated—so one molecule of oxaloacetate can oxidize countless acetyl-CoA molecules. The electron transport chain is a controlled cascade: electrons are passed downhill in free energy, and the energy released at three coupling sites (Complexes I, III, and IV) is used to pump protons. The inner membrane's impermeability to protons creates a battery—the proton-motive force—and ATP synthase is the turbine that discharges it to make ATP. The MCAT rewards students who can trace an electron from NADH through Complex I → CoQ → Complex III → cytochrome c → Complex IV → O₂, naming the inhibitor at each step, and then calculate the ATP yield. The pathway is stoichiometrically precise: every NADH yields ~2.5 ATP; every FADH₂ yields ~1.5 ATP. The ~30–32 ATP per glucose is an integrated accounting problem: glycolysis (2 ATP, 2 NADH), PDC (2 NADH), TCA (2 GTP/ATP, 6 NADH, 2 FADH₂), minus the transport cost of cytosolic NADH depending on shuttle. When you can reproduce that tally, you have mastered the highest-yield biochemistry pathway on the MCAT.

Comparisons

• B/B (TCA cycle): Know the 8 steps, enzymes, and outputs. Isocitrate dehydrogenase is rate-limiting. Succinate dehydrogenase is Complex II—the only shared enzyme between TCA and ETC.
• B/B (PDC and vitamins): The five cofactors (TPP, lipoic acid, CoA, FAD, NAD⁺) are B-vitamin derived. Thiamine deficiency (beriberi) → impaired PDC → lactic acidosis and neurological symptoms.
• B/B (ETC inhibitors): Rotenone (C-I), antimycin A (C-III), cyanide/CO (C-IV), oligomycin (C-V/F₀). Know the site, mechanism, and effect on O₂ consumption for each.
• B/B (Uncoupling and thermogenesis): DNP, UCP1/thermogenin in brown fat. Norepinephrine activates lipolysis → free fatty acids activate UCP1, especially important in newborns and hibernating animals.
• C/P (Redox and electrochemistry): The ETC is a biological galvanic cell. Electrons flow from low to high reduction potential. The PMF = Δψ − (2.303RT/F)ΔpH. The free energy of NADH oxidation (ΔG°' ≈ −220 kJ/mol) is conserved as ~2.5 ATP.
• C/P (Bioenergetics): The ~30–32 ATP/glucose tally connects to free-energy calculations. The efficiency of aerobic respiration vs. glycolysis (2 ATP) illustrates why O₂ is the terminal electron acceptor.
• B/B (Regulation): PDC by phosphorylation; citrate synthase by ATP/NADH; isocitrate dehydrogenase by ADP/NADH. These integrate the TCA cycle with energy charge. Pyruvate carboxylase (anaplerotic) is activated by acetyl-CoA.

Common confusions

• Confusing the ATP yield of NADH vs. FADH₂. NADH → Complex I → ~2.5 ATP. FADH₂ → Complex II → ~1.5 ATP. The difference is because FADH₂ bypasses Complex I and its proton-pumping step, not because it carries less energy.
• Thinking succinate dehydrogenase is only a TCA enzyme. It is ALSO Complex II of the ETC—embedded in the inner mitochondrial membrane with FAD as a covalently bound prosthetic group. This dual role is high-yield.
• Mixing up the four ETC inhibitors. Rotenone (Complex I), antimycin A (Complex III), cyanide/CO (Complex IV), oligomycin (ATP synthase/F₀). Know the order: I → CoQ → III → cyt c → IV → O₂.
• Forgetting the Q cycle. Complex III accepts a 2-electron carrier (CoQH₂) and reduces two 1-electron carriers (cytochrome c). The Q cycle explains how this 2:1 conversion pumps additional protons.
• Assuming uncouplers stop the ETC. Uncouplers (DNP, FCCP) DISSIPATE the gradient—the ETC runs FASTER because there is no back-pressure. O₂ consumption increases; ATP production falls to zero. ETC inhibitors (rotenone, cyanide) STOP the ETC—O₂ consumption decreases.
• Miscounting ATP yield because of the shuttle. Cytosolic NADH from glycolysis yields 2.5 ATP (malate-aspartate shuttle, heart/liver) or 1.5 ATP (glycerol-3-phosphate shuttle, muscle/brain). The ~30 vs. ~32 ATP difference is entirely this shuttle variance.
• Confusing substrate-level vs. oxidative phosphorylation in the TCA cycle. Only ONE step produces GTP directly: succinyl-CoA synthetase. Every other ATP-equivalent comes from NADH/FADH₂ feeding the ETC.
• Overlooking anaplerotic reactions. Pyruvate carboxylase (pyruvate → oxaloacetate) replenishes TCA intermediates consumed for biosynthesis. Acetyl-CoA activates it—when acetyl-CoA is high, the cycle needs more oxaloacetate to accept it.
• Mistaking the number of CO₂ produced per turn. Two CO₂ per acetyl-CoA: one at isocitrate dehydrogenase, one at α-ketoglutarate dehydrogenase. The carbon atoms in the CO₂ come from oxaloacetate, NOT from the acetyl-CoA that entered that turn (they come from prior acetyl-CoA after multiple turns—the MCAT may test this isotope-labeling logic).
• Forgetting that PDC is irreversible in humans. Pyruvate → acetyl-CoA cannot be reversed. Fatty acids cannot be converted to glucose in animals (no net conversion of acetyl-CoA to pyruvate). This is why the TCA cycle 'burns' acetyl-CoA completely.

Quick review

• PDC: pyruvate → acetyl-CoA (+CO₂, NADH). Five cofactors: TPP (B₁), lipoic acid, CoA (B₅), FAD (B₂), NAD⁺ (B₃). Inhibited by ATP, NADH, acetyl-CoA; activated by Ca²⁺, insulin.
• TCA cycle (8 steps): Citrate → Isocitrate → α-KG → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate. Key mnemonic: Can I Keep Studying Succinct Science For Med School?
• Outputs per turn: 3 NADH (isocitrate DH, α-KG DH, malate DH), 1 FADH₂ (succinate DH), 1 GTP (succinyl-CoA synthetase), 2 CO₂ (isocitrate DH, α-KG DH).
• Rate-limiting step: isocitrate dehydrogenase. Activated by ADP, Ca²⁺; inhibited by ATP, NADH.
• Succinate dehydrogenase = Complex II. FAD is covalently bound. Only TCA enzyme in the inner mitochondrial membrane.
• Anaplerotic: pyruvate carboxylase (pyruvate → OAA, requires biotin, activated by acetyl-CoA).
• ETC: Complex I (NADH → CoQ, 4 H⁺ pumped). Complex II (FADH₂ → CoQ, 0 H⁺ pumped). Complex III (CoQ → cyt c, 4 H⁺ pumped via Q cycle). Complex IV (cyt c → O₂ → H₂O, 2-4 H⁺ pumped). Mobile carriers: CoQ (lipid), cyt c (intermembrane space).
• Inhibitors: Rotenone (C-I), Antimycin A (C-III), Cyanide/CO/Azide (C-IV), Oligomycin (ATP synthase/F₀).
• Uncouplers: DNP, FCCP, UCP1/thermogenin (brown fat). Dissipate proton gradient → max O₂ consumption, zero ATP, heat produced.
• ATP synthase: F₀ proton channel (c-ring rotor), F₁ catalytic head (αβ subunits). Binding-change mechanism: O (open) → L (loose) → T (tight). ~3 H⁺ per ATP.
• ATP yield per glucose: Glycolysis (2 ATP + 2 NADH), PDC (2 NADH), TCA (2 GTP + 6 NADH + 2 FADH₂). ~30–32 ATP total (shuttle-dependent).
• NADH → Complex I → ~2.5 ATP. FADH₂ → Complex II → ~1.5 ATP. Malate-aspartate shuttle (~2.5/NADH) vs. glycerol-3-phosphate shuttle (~1.5/NADH).