Transcript Citric Acid Cycle in Anabolism

Anaplerotic Reactions
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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