Anaplerotic (filling up) reactions replenish citric acid cycle intermediates We will look at 4 of these Replenish Oxa

1. The citric acid cycle is amphibolic(both catabolic and anabolic) • Some intermediates diverted to anabolic pathways • Anaplerotic (filling up) reactions replenish citric acid cycle intermediates • We will look at 4 of these • Replenish Oxaloacetate (3 ways) • Replenish Malate

2. PEP + HCO3- Oxaloacetate + Pi Pyruvate + CO2 + ATP + H2O Oxaloacetate + ADP + Pi 1. Pyruvate carboxylase is a major anaplerotic reaction in mammaliantissues (liver, kidney) • Enzyme is regulated by [AcetylCoA] • Inactive in absence of AcetylCoA • Active when excess AcetylCoA is present 2. Phosphoenolpyruvate carboxylase supplies oxaloacetate in plants, yeast and bacteria • PEP carboxykinase supplies oxloacetate in heart and skeletal muscle. • Malic Enzyme supplies malate in many organisms

4. Regulation of the Citric Acid Cycle • Pathway controlled by: • (1) Allosteric modulators • (2) Covalent modification of cycle enzymes • (3) Supply of acetyl CoA • (4) Regulation of pyruvate dehydrogenase complex controls acetyl CoA supply • Regulation of the PDH complex • - acetyl CoA, NADH, ATP and fatty acids allosterically inhibit  E2, E3 • - CoA, NAD+, AMP allosterically activate E2 and E3

5. Regulation of mammalian PDH complex by covalentmodification • Phosphorylation/dephosphorylation of E1 • Serine on E1 is site of (de)phosphorylation • ATP activates the kinase (deactivates E1) • Decreasing [ATP] allows phosphatase activity to dominate (activates E1) • Ca2+ (in contacting muscles) activates E1

6. Regulation of 3 exergonic steps of the cycle: citrate synthase(2), isocitrate dehydrogenase (ICDH)(3) and -ketoglutarate dehydrogenase complex(4) • Citrate synthase (step 1) • Activated by ADP • Inhibited by NADH,succinyl-CoA, citrate and ATP • 2. ICDH (step 3) • - Allostericically activated by Ca2+, ADP • - Allosterically inhibited by NADH • 3. -Ketoglutarate Dehydrogenase Complex (step 4) • - Activated by Ca2+ • - Inhibited by succinyl-CoA and NADH • Regulation of Glycolysis and citric acid cycle are integrated to produce only as much pyruvate as is needed.