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• C H A P T E R • 12 • • • TCA CYCLE • • • • • • • • • • • to fat to fat AcCoA pyruvate citrate synthase citrate aconitase to glucose oxaloacetate isocitrate NADH isocitrate dehydrogenase malate dehydrogenase NADH malate α-ketoglutarate CoA fumarase α-KG dehydrogenase fumarate succinate dehydrogenase FADH2 succinyl CoA succinate NADH succinate thiokinase GTP Figure 12-1 The Tricarboxylic Acid (TCA) Cycle 153 BG McGraw-Hill: Gilbert, Basic Concepts in Biochemistry, JN 5036 • 154 • Basic Concepts in Biochemistry TCA CYCLE1 (see Fig. 12-1.) Function: To burn the acetyl-CoA made from fat, glucose, or protein in order to make ATP in cooperation with oxidative phosphorylation. Location: All cells with mitochondria. Connections: From glycolysis through acetyl-CoA. Pyruvate makes oxaloacetate and malate through the anaplerotic reactions. To b oxidation through acetyl-CoA. To amino acid degradation through acetyl-CoA and various intermediates of the cycle. Regulation: Supply and demand of TCA cycle. Availability of NAD and FAD as substrates. Inhibition by NADH. High-energy signals turn off. Low-energy signals turn on. ATP yield: Pyruvate ¡ 15ATP Acetyl-CoA ¡ 12ATP Equations: Pyruvate GDP Pi 3NAD FAD ¡ 3CO2 GTP 3NADH FADH2 3H Acetyl-CoA GDP Pi 2NAD FAD ¡ 2CO2 GTP 2NADH FADH2 2H 1 The tricarboxylic acid cycle is also known as the Krebs cycle or the citric acid cycle. Why give something so central to life only one name? BG McGraw-Hill: Gilbert, Basic Concepts in Biochemistry, JN 5036