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 carboxylation 2) 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 + + 2H 2 O --> succinate + 2CoA + NADH + H + Compare with Citric Acid Cycle Acetyl-CoA + 3NAD + + FAD + GDP + P i + 2H 2 O --> 2CO 2 + CoA + 3NADH + 3H + + FADH 2 + 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