Study guides › B/B › Biochemistry
Citric Acid Cycle & Oxidative Phosphorylation
Aerobic cells finish burning fuel in the mitochondrion. You will follow pyruvate into acetyl-CoA, around the citric acid cycle, and through the electron transport chain to ATP, then predict what poisons, uncouplers, and shuttles do to the yield.
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What this chapter covers
- Pyruvate Dehydrogenase: The Link Reaction. PDH is the one-way door between glycolysis and the cycle, and its cofactors and regulation are favorite exam material.
- The Citric Acid Cycle Step by Step. You must know the order of intermediates, which enzymes make NADH, FADH₂, GTP, and CO₂, and where the carbons go.
- Regulation and the Role of the Cycle in Metabolism. The cycle is tuned by energy state, and its intermediates feed biosynthesis, so it must also be refilled.
- The Electron Transport Chain and Chemiosmosis. Electrons fall from NADH to O₂ through four complexes, and the energy pumps protons that drive ATP synthase.
- Coupling, Uncouplers, Shuttles, and ATP Yield. Predicting oxygen consumption and ATP output under a poison is a classic MCAT passage, and the yield arithmetic must be automatic.
Key points to know
- PDH: pyruvate + NAD⁺ + CoA → acetyl-CoA + NADH + CO₂ (irreversible)
- Cofactors: TPP, lipoate, FAD, NAD⁺, CoA
- Order: OAA, citrate, isocitrate, α-KG, succinyl-CoA, succinate, fumarate, malate, OAA
- NADH is made by isocitrate DH, α-KG DH, and malate DH
- Isocitrate DH is the main control point: ADP and Ca²⁺ activate; ATP and NADH inhibit
- The cycle is amphibolic: it supplies precursors for fatty acids, amino acids and heme
- Electrons: NADH to I to Q to III to cyt c to IV to O₂; succinate enters at II
- H⁺ pumped by I, III, IV only (4, 4, 2 per NADH)
Try a practice question
Which pairing of PDH cofactor and vitamin precursor is correct?
- FAD and niacin
- NAD⁺ and riboflavin
- TPP and thiamine (B1)
- CoA and biotin
Show the answer
C. TPP is derived from thiamine. FAD comes from riboflavin, NAD⁺ from niacin, and CoA from pantothenate. Biotin belongs to carboxylases, not PDH.
Common questions
What does this chapter cover?
Citric Acid Cycle & Oxidative Phosphorylation covers Pyruvate Dehydrogenase: The Link Reaction, The Citric Acid Cycle Step by Step, Regulation and the Role of the Cycle in Metabolism, The Electron Transport Chain and Chemiosmosis, Coupling, Uncouplers, Shuttles, and ATP Yield.
What should I know first?
PDH: pyruvate + NAD⁺ + CoA → acetyl-CoA + NADH + CO₂ (irreversible) Cofactors: TPP, lipoate, FAD, NAD⁺, CoA Order: OAA, citrate, isocitrate, α-KG, succinyl-CoA, succinate, fumarate, malate, OAA
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