TCA Cycle & Oxidative Phosphorylation

Pyruvate dehydrogenase: the bridge into the cycle

Reaction. In the mitochondrial matrix, the pyruvate dehydrogenase (PDH) complex oxidatively decarboxylates pyruvate to acetyl-CoA, releasing CO2 and generating NADH. The step is irreversible — the one-way bridge from glycolysis to the TCA cycle, and the reason acetyl-CoA can never be turned back into glucose.

Regulation. PDH is activated by insulin, ADP, and Ca2+ (and a high NAD+/NADH ratio) and inhibited by its products acetyl-CoA and NADH, as well as by ATP.

Five cofactors. B1 thiamine (as TPP), B2 riboflavin (FAD), B3 niacin (NAD+), B5 pantothenate (CoA), and lipoic acid. α-Ketoglutarate dehydrogenase uses the same five. Mnemonic: Tender Loving Care For Nancy (TPP, Lipoic acid, CoA, FAD, NAD+).

Because the two dehydrogenases share cofactors, thiamine deficiency (e.g., malnutrition) impairs both at once, so pyruvate and α-ketoglutarate accumulate. Thiamine is also needed by transketolase and branched-chain ketoacid dehydrogenase.

PDH deficiency. Often X-linked (E1-α subunit). Pyruvate backs up into lactate and alanine, causing lactic acidosis and neurologic defects from birth. Treatment is a ketogenic diet — high fat plus lysine and leucine, the only purely ketogenic amino acids — which supplies acetyl-CoA without passing through PDH.

The TCA cycle

Sequence. In the matrix, citrate synthase joins acetyl-CoA to oxaloacetate, and the cycle proceeds: citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate. Mnemonic: Citrate Is Krebs' Starting Substrate For Making Oxaloacetate.

Control points. Isocitrate dehydrogenase is the rate-limiting enzyme (stimulated by ADP and Ca2+, inhibited by ATP and NADH). α-Ketoglutarate dehydrogenase converts α-KG to succinyl-CoA, releasing NADH and CO2; it mirrors PDH, using the same five cofactors, so arsenic inhibits it too. Succinate dehydrogenase doubles as Complex II of the electron transport chain.

Per glucose. One glucose yields two acetyl-CoA, so the cycle turns twice: 6 NADH, 2 FADH2, and 2 GTP.

Beyond energy. Intermediates are drawn off for biosynthesis (citrate for fatty acids, succinyl-CoA for heme, α-ketoglutarate and oxaloacetate for amino acids) and replenished by anaplerotic reactions such as pyruvate carboxylase.

Electron transport and ATP synthesis

Electron path. NADH donates electrons to Complex I; FADH2 electrons enter through succinate dehydrogenase (Complex II) at coenzyme Q, bypassing Complex I. Electrons then pass to Complex III, cytochrome c, Complex IV (cytochrome c oxidase), and finally O2, the terminal acceptor, forming water. Complexes I, III, and IV pump H+ from the matrix into the intermembrane space; Complex II pumps none, which is why FADH2 yields less ATP.

Chemiosmosis. The H+ gradient drives protons back through ATP synthase, which makes ATP. Each NADH yields about 2.5 ATP and each FADH2 about 1.5 ATP.

Inhibitors, uncouplers, and poisons

Electron transport inhibitors. Rotenone (Complex I), antimycin A (Complex III), and cyanide, CO, and azide (Complex IV) halt electron flow: the proton gradient collapses, ATP synthesis stops, and O2 consumption falls.

ATP synthase inhibitors. Oligomycin blocks Complex V directly. Protons cannot flow back, so the gradient builds until electron transport stalls; ATP production and O2 consumption both fall.

Uncouplers. 2,4-Dinitrophenol (an illicit weight-loss drug), aspirin/salicylates in overdose, and thermogenin (UCP1) in brown fat carry H+ back across the inner membrane, bypassing ATP synthase. The chain runs faster and O2 consumption rises, but ATP falls and the energy is lost as heat — hence hyperthermia.

Cyanide. Blocks cytochrome c oxidase, so tissues cannot use the O2 delivered to them: severe lactic acidosis, elevated venous O2 saturation, almond breath, cherry-red skin, and tachypnea progressing to bradycardia and collapse. Sources include smoke inhalation and nitroprusside. Antidotes: hydroxocobalamin (binds cyanide as cyanocobalamin), sodium thiosulfate (sulfur donor that converts cyanide to renally excreted thiocyanate), or nitrites (induce methemoglobin, which sequesters cyanide).

Carbon monoxide. Also inhibits Complex IV, but its main effect is binding hemoglobin far more avidly than O2, which cuts O2 delivery and left-shifts the dissociation curve. It too can give cherry-red skin. Treat with 100% or hyperbaric O2.

Arsenic. Binds lipoic acid, crippling both PDH and α-ketoglutarate dehydrogenase. Classic presentation: vomiting, rice-water stools, and garlic breath.

A fire victim can have both CO and cyanide poisoning; profound lactic acidosis with high venous O2 saturation points to cyanide.