Anaplerotic Reactions
CH339K
Citric Acid Cycle in Anabolism
Anaplerotic Reactions
: back; up; again; re-
: 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 carboxylation2) Uses NADP+ as a cofactor instead of NAD+.3) Pyruvate carboxyl reduced to hydroxyl
Transaminations
• Every -amino acid has a conjugate -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