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Biochemistry I - The Tricarboxylic Acid TCA Cycle

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    Study Guide Biochemistry I – The Tricarboxylic Acid TCA Cycle 1. First Phase of the TCA Cycle (Entry of Carbon into the Cycle) The first phase of the tricarboxylic acid (TCA) cycle focuses on how 2 - carbon units enter the cycle. This entry occurs through the coordinated action of two key enzymes: • Pyruvate dehydrogenase complex (PDC) • Citrate synthase Together, these enzymes link glycolysis to the TCA cycle . 1.1 Entry of Pyruvate into the TCA Cycle Pyruvate, produced during glycolysis, cannot enter the TCA cycle directly. Instead, it must first be converted into acetyl - CoA by the pyruvate dehydrogenase complex (PDC) . PDC is a large multienzyme complex that carries out three tightly linked reactions .

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    Study Guide Reaction 1: Removal of Carbon Dioxide (E1) • The E1 enzyme (pyruvate decarboxylase) removes CO ₂ from pyruvate. • This step uses the cofactor thiamine pyrophosphate (TPP) . • The carboxyl group of pyruvate is oxidized to CO ₂ . • Unlike yeast fermentation, the resulting 2 - carbon fragment is not released . • Instead, it remains bound within the enzyme complex as hydroxyethyl - TPP . This keeps the reactive intermediate safely controlled. Reaction 2: Transfer of the 2 - Carbon Unit to Lipoic Acid (E2) • The E2 enzyme (dihydrolipamide transacetylase) transfers the 2 - carbon fragment. • This transfer uses lipoic acid , an 8 - carbon molecule with a disulfide bond . • Lipoic acid is covalently attached to a lysine side chain , forming a long, flexible arm. • One sulfur accepts the acetyl group. • The other sulfur accepts hydrogen.

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    Study Guide As a result: • The disulfide bond is reduced • The acetyl group becomes attached to lipoic acid Reaction 3: Formation of Acetyl - CoA and Electron Transfer (E3) • The acetyl group is transferred from lipoic acid to Coenzyme A (CoA) . • This forms acetyl - CoA , a high - energy molecule. Next: • E3 (dihydrolipoamide dehydrogenase) re - oxidizes reduced lipoic acid. • The cofactor FAD accepts electrons, forming FADH ₂ . • FADH ₂ then transfers electrons to NAD ⁺ , producing NADH + H ⁺ . This step regenerates all cofactors so the complex can continue working.

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    Study Guide Figure 1 In summary, the reactions of the complex are:

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    Study Guide 1.2 Overall Reaction of the Pyruvate Dehydrogenase Complex After adding the reactions and canceling intermediates: This reaction permanently commits pyruvate to aerobic metabolism . 1.3 Entry into the TCA Cycle: Citrate Synthase Acetyl - CoA now enters the TCA cycle by reacting with oxaloacetate , a 4 - carbon dicarboxylic acid.

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    Study Guide Figure 2 • The enzyme citrate synthase catalyzes this reaction. • In organic chemistry terms, it is an aldol condensation . • The methyl group of acetyl - CoA loses a proton. • The resulting carbanion attacks the carbonyl carbon of oxaloacetate. • This forms citroyl - CoA , an enzyme - bound intermediate. • Hydrolysis releases free CoA and citrate . 1.4 Isomerization of Citrate Citrate cannot be directly decarboxylated. To prepare it: • Aconitase converts citrate into isocitrate . • This occurs via: o Dehydration (water removal) o Formation of aconitate o Rehydration in a different position This rearrangement makes the molecule suitable for oxidation.

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