Anaplerotic Reactions CH339K. Citric Acid Cycle in Anabolism

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