Lecture23-2009 - Citric Acid Cycle 1 Lecture 23 Chapter...

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1 Citric Acid Cycle 1 Lecture 23 Chapter 17-3 Figure 17-2 Citric Acid Cycle Other names: Krebs cycle, Tricarboxylic Acid Cycle (TCA). Circular pathway: all intermediates are “catalytic” (no net formation or consumption). Complete oxidation of Acetate to CO 2 . – 8 steps, 4 of which are oxidations. Electrons will be transferred into electron transport to generate ATP through Ox. Phos. Balanced equation: Acetyl-CoA + 3 NAD + + FAD + GDP + Pi --> 2 CO 2 + 3 NADH + FADH 2 + GTP + CoA Figure 17-2
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2 Reaction 1: Citrate Synthase Acetyl-CoA + OAA + H 2 O ---> Citrate + CoA Coupled reaction: Condensation of methyl group of Acetate to carbonyl of OAA (unfavorable). Hydrolysis of thioester (favorable). Overall reaction favorable. Irreversible in cells. Ordered binding of substrates (OAA first) prevents premature hydrolysis of thioester. Figure 17-9 Open form, binds OAA: Closed form, binds Acetyl-CoA: Ordered binding ensures Acetyl-CoA only binds with OAA is present. Hexokinase works the same way. Figure 17-10 Citrate Synthase mechanism (after both substrates are bound): Enol Enzyme-bound intermediate Page 578 Reaction 2: Aconitase Reversible isomerization of citrate and isocitrate. Enzyme bound intermediate: cis-Aconitate Contains an Iron-Sulfur cluster for binding substrate.
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3 Page 605 Iron-Sulfur Clusters (non-heme iron): Iron exists in ferrous and ferric state, so often function in electron transport.
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