Cataplerotic reactions: Pathways that utilize TCA cycle intermediates• Cataplerotic reactions utilize the intermediates of TCA cycle to

biosynthesize important products such as glucose, fatty acids and amino acids.

• Cataplerotic reactions also required to prevent inappropriate accumulation of TCA cycle intermediates in mitochondrion.

• Cataplerotic reactions occurs in 3 major pathways:

Cataplerotic Reactions

Glucose Biosynthesis

Fatty Acid Biosynthesis

Amino Acid Biosynthesis

1. Biosynthesis of Glucose: Gluconeogenesisa) Transport of oxaloacetate out of mitochondrion:• Gluconeogenesis uses oxaloacetate in TCA cycle.• Gluconeogenesis is cytosolic process, but oxaloacetate cannot transported put

of mitochondrion, hence must be converted to malate or aspartate by malate dehydrogenase and aspartate dehydrogenase respectively.

• Malate or aspartate are converted back to oxaloacetate after entering cytosol.• Oxaloacetate-malate-oxaloacetate conversion also transfers NADH reducing

equivalent from mitochondrion to cytosol.• The cytosolic NADH produced through the reduction of NAD+ in conversion of

malate back to oxaloacetate in cytosol.• Cytosolic NADH is required for gluconeogenesis.

b) PEPCK Mechanism:• Oxaloacetate is then decarboxylated and phosphorylated to PEP by PEP

caboxykinase(PEPCK), PEP is then converted to pyruvate.• The next steps are in reverse of glycolysis, which will finally yield glucose.

2. Biosynthesis of Fatty Acid• Biosynthesis of fatty acid requires acetyl-CoA, which is generated from

citrate(the intermediate of TCA cycle) as mt-membrane is impermeable to acetyl-CoA.

• Citrate can across the mt-membrane into cytosol via Tricarboxylate transport system.

• In cytosol, citrate is converted to oxaloacetate by ATP-citrate lyase: Citrate + CoA-SH + ATP Oxaloacetate + Acetyl-CoA + ADP + Pi• Oxaloacetate is then reduced to malate via oxidation of NADH by malate

dehydrogenase: Oxaloacetate + NADH + H+ Malate + NAD+• Malate is then oxidatively decarboxylated to pyruvate, via reduction of NADP+,

by malic enzyme: Malate + NADP+ Pyruvate + NADPH + CO2

• Pyruvate returning back to TCA cycle in mt.• The NADPH produced is also required in reductive reactions in fatty acid

biosynthesis.

Overview of Biosynthesis of Fatty Acid from Intermediate of TCA Cycle:

3. Amino Acid Biosynthesis• In biosynthesis of amino acid from the intermediates of TCA cycle, α-

ketoglutarate and oxaloacetate are used as the intermediates for starting materials via transamination.

• α-ketoglutarate can be converted to glutamate via reductive amination by glutamate dehydrogenase:

α-ketoglutarate + NADH + H+ + NH4+ Glutamate + NAD+ + H2O• α-ketoglutarate can also receiving –NH3+ group from another amino acid,

producing glutamate and a α-keto acid derived from the reactant amino acid.• Oxaloacetate can also receives the –NH3+ from glutamate, producing α-

ketoglutarate and aspartate.• Oxaloacetate can transaminated with alanine, forming aspartate and pyruvate in

which the –COOH is transferred to alanine.

+ -ketoglutarate+ glutamate

Aspartate aminotransferase (glutamate-oxaloacetate transaminase)

NH2 Aspartate

HOOC-CH-CH2COOH

Oxaloacetate

HOOC-CO-CH2COOH

Alanine aminotransferase (glutamate-pyruvate transaminase)

+ -ketoglutarate+ glutamate

NH2 Alanine

HOOC-CH-CH3

Pyruvate

HOOC-CO-CH3

Overview of Intermediates of TCA Cycle as Precursor of Biosynthesis of Other Products: