Biochemistry I - The Tricarboxylic Acid TCA Cycle - Quick Guides | Biochemistry

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Study GuideBiochemistry I–The Tricarboxylic Acid TCA Cycle1.First Phase of the TCA Cycle(Entry of Carbon into the Cycle)The first phase of thetricarboxylic acid (TCA) cyclefocuses on how2-carbon unitsenter thecycle. This entry occurs through the coordinated action of two key enzymes:•Pyruvate dehydrogenase complex (PDC)•Citrate synthaseTogether, these enzymes linkglycolysisto theTCA cycle.1.1Entry of Pyruvate into the TCA CyclePyruvate, produced during glycolysis, cannot enter the TCA cycle directly. Instead, it must first beconverted intoacetyl-CoAby thepyruvate dehydrogenase complex (PDC).PDC is alarge multienzyme complexthat carries outthree tightly linked reactions.

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Study GuideReaction 1: Removal of Carbon Dioxide (E1)•TheE1 enzyme (pyruvate decarboxylase)removesCO₂from pyruvate.•This step uses the cofactorthiamine pyrophosphate (TPP).•The carboxyl group of pyruvate is oxidized to CO₂.•Unlike yeast fermentation, the resulting2-carbon fragment is not released.•Instead, it remains bound within the enzyme complex ashydroxyethyl-TPP.This keeps the reactive intermediate safely controlled.Reaction 2: Transfer of the 2-Carbon Unit to Lipoic Acid (E2)•TheE2 enzyme (dihydrolipamide transacetylase)transfers the 2-carbon fragment.•This transfer useslipoic acid, an 8-carbon molecule with adisulfide bond.•Lipoic acid is covalently attached to alysine side chain, forming a long, flexible arm.•One sulfur accepts the acetyl group.•The other sulfur accepts hydrogen.

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Study GuideAs a result:•The disulfide bond isreduced•The acetyl group becomes attached to lipoic acidReaction 3: Formation of Acetyl-CoA and Electron Transfer (E3)•The acetyl group is transferred from lipoic acid toCoenzyme A (CoA).•This formsacetyl-CoA, a high-energy molecule.Next:•E3 (dihydrolipoamide dehydrogenase)re-oxidizes reduced lipoic acid.•The cofactorFADaccepts electrons, formingFADH₂.•FADH₂then transfers electrons toNAD⁺, producingNADH + H⁺.This step regenerates all cofactors so the complex can continue working.

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Study GuideFigure 1In summary, the reactions of the complex are:

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Study Guide1.2Overall Reaction of the Pyruvate Dehydrogenase ComplexAfter adding the reactions and canceling intermediates:This reaction permanently commits pyruvate toaerobic metabolism.1.3Entry into the TCA Cycle: Citrate SynthaseAcetyl-CoA now enters the TCA cycle by reacting withoxaloacetate, a 4-carbon dicarboxylic acid.

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Study GuideFigure 2•The enzymecitrate synthasecatalyzes this reaction.•In organic chemistry terms, it is analdol condensation.•The methyl group of acetyl-CoA loses a proton.•The resulting carbanion attacks the carbonyl carbon of oxaloacetate.•This formscitroyl-CoA, an enzyme-bound intermediate.•Hydrolysis releasesfree CoAandcitrate.1.4Isomerization of CitrateCitrate cannot be directly decarboxylated. To prepare it:•Aconitaseconverts citrate intoisocitrate.•This occurs via:oDehydration (water removal)oFormation ofaconitateoRehydration in a different positionThis rearrangement makes the molecule suitable for oxidation.

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