Anaplerotic Reactions CH 339 K Citric Acid Cycle

Anaplerotic Reactions CH 339 K

Citric Acid Cycle in Anabolism

Anaplerotic Reactions ana: back; up; again; re. Plhrow: 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-Co. A • 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-Co. A + NAD+ + 2 H 2 O --> succinate + 2 Co. A + NADH + H+ Compare with Citric Acid Cycle Acetyl-Co. A + 3 NAD+ + FAD + GDP + Pi + 2 H 2 O --> 2 CO 2 + Co. A + 3 NADH + 3 H+ + 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