Presentation Slide Bio Synthesis

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:

1. Biosynthesis of Glucose: Gluconeogenesis a) 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.

2. Biosynthesis of Fatty Acid • Biosynthesis of fatty acid requires acetyl-CoA, which is generated from citrate(the intermediate of TCA cycle) as mtmembrane 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 + AcetylCoA + 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.

Aspartate aminotransferase (glutamate-oxaloacetate transaminase) NH2

Oxaloacetate

Aspartate

HOOC-CO-CH2COOH

HOOC-CH-CH2COOH + -ketoglutarate

+ glutamate

Alanine aminotransferase (glutamate-pyruvate transaminase) NH2

Alanine

HOOC-CH-CH3 + -ketoglutarate

Pyruvate HOOC-CO-CH3 + glutamate

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