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Transcript
Anaplerotic Reactions CH339K Citric Acid Cycle in Anabolism Anaplerotic Reactions ana: back; up; again; rePlhrow: I fill Anaplerotic reactions refill the pools of Krebs cycle intermediates that would otherwise become depleted. Important Anaplerotic (Replenishing) Reactions Pyruvate Carboxylase • Mitochondrial enzyme • Two roles • Gluconeogenesis • Anaplerosis • 4 identical subunits • Biotin cofactor Role of Biotin in the Pyruvate Carboxylase Reaction PEP Carboxykinase • Cytosolic enzyme active in gluconeogenesis • Mitochondrial enzyme active in anaplerosis PEP Carboxylase • Mechanism involves pyruvate enolate • Phosphate is transferred from PEP to bicarbonate • Pyruvate enolate then attacks the carboxyphosphate anhydride carbon • Sequesters carbon in some plants Malic Enzyme 1) Reductive carboxylation 2) Uses NADP+ as a cofactor instead of NAD+. 3) Pyruvate carboxyl reduced to hydroxyl Transaminations • Every a-amino acid has a conjugate a-keto acid. • Amino and keto conjugates are readily interconverted. Citric Acid Cycle in Anabolism Gyoxylate Cycle • Critters can make fat from carbohydrates • Plants, fungi, and some bacteria can make carbohydrates from fats via acetyl-CoA • Use a modified version of the Krebs Cycle • Plants use this to grow from stored oils in seeds • Bacteria use this to grow on simple carbon compounds when carbohydrates are unavailable • Critter exception – some nematodes in early stages can perform The triffids that ate England were apparently quite dependent on the glyoxylate cycle to assimilate there quivering human prey… The Glyoxylate Cycle --> gluconeogenesis The Glyoxylate Cycle 2 Acetyl-CoA + NAD+ + 2H2O --> succinate + 2CoA + NADH + H+ Compare with Citric Acid Cycle Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O --> 2CO2 + CoA + 3NADH + 3H+ + FADH2 + GTP Glyoxylate Cycle Takes Place in Glyoxysomes in Plants Connections Between Glyoxylate Cycle and Citric Acid Cycle Isocitrate: An Intermediate Common to the Glyoxylate and Citric Acid Cycles