anaplerotic reactions ch339k. citric acid cycle in anabolism
TRANSCRIPT
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Anaplerotic Reactions
CH339K
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Citric Acid Cycle in Anabolism
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Anaplerotic Reactions
: back; up; again; re-
: I fill
Anaplerotic reactions refill the pools of Krebs cycle intermediates that would otherwise become depleted.
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Important Anaplerotic (Replenishing) Reactions
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Pyruvate Carboxylase
• Mitochondrial enzyme• Two roles
• Gluconeogenesis• Anaplerosis
• 4 identical subunits• Biotin cofactor
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Role of Biotin in the Pyruvate Carboxylase
Reaction
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PEP Carboxykinase
• Cytosolic enzyme active in gluconeogenesis• Mitochondrial enzyme active in anaplerosis
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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
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Malic Enzyme
1) Reductive carboxylation2) Uses NADP+ as a cofactor instead of NAD+.3) Pyruvate carboxyl reduced to hydroxyl
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Transaminations
• Every -amino acid has a conjugate -keto acid.
• Amino and keto conjugates are readily interconverted.
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Citric Acid Cycle in Anabolism
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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
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The triffids that ate England were apparently quite dependent on the glyoxylate cycle to assimilate there quivering human prey…
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The Glyoxylate Cycle
--> gluconeogenesis
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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
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Glyoxylate Cycle Takes Place in Glyoxysomes in Plants
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Connections Between Glyoxylate Cycle and Citric Acid Cycle
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Isocitrate: An Intermediate Common to
the Glyoxylate
and Citric Acid Cycles