Citric Acid Cycle

A Citric Acid Cycle Schematic

The Citric Acid Cycle is a key metabolic pathway that connects carbohydrate, fat, and protein metabolism!!! See also [1].

• Krebs cycle importance
• Krebs cycle overview
• Krebs cycle reactions
• Major metabolic pathways converging on the citric acid cycle
• Citric acid cycle intermediates serve as substrates for biosynthetic processes

The Citric Acid Cycle (tricarboxylic acid cycle) is a series of enzyme catalyzed reactions which are critical in cellular respiration. Under oxidative conditions, pyruvate continues to be metabolized through the tricarboxylic acid cycle. In this cycle, Acetyl-CoA, a byproduct of glycolysis, along with various cofactors, are broken down into carbon dioxide, water, and energy in the form of GTP and NADH.

Pyruvate decarboxylation or pyruvate oxidation, also known as the link reaction (or oxidative decarboxylation of pyruvate), is the conversion of into by the enzyme complex pyruvate dehydrogenase complex (see Pyruvate dehydrogenase).

The multienzyme complex together catalyzes five distinct reactions in the conversion of pyruvate to Acetyl-CoA. The overall result is described by the following reaction:

Pyruvate + CoA + NAD+ ==> Acetyl-CoA + CO2 + NADH

However, pyruvate dehydrogenase (E1) is responsible for only the first two of the five reactions. The first of these is the decarboxylation of pyruvate and coupling of thiamine pyrophosphate (TPP) to form hydroxyethyl-TPP.

Pyruvate + TPP ==> Hydroxyethyl-TPP + CO2

Step 0 / 10 - Aldol condensation - Citrate synthase

• Citrate Synthase
• Step 0 / 10 = Krebs cycle step 1
• ATP-citrate synthase

4C to a 6C molecule

Step 1 - Dehydration - Aconitase

• Aconitase
• 1st step of Krebs cycle step 2

6C Citrate => 6C

Step 2 - Hydration - Aconitase

• Aconitase
• 2nd step of Krebs cycle step 2

6C cis-Aconitate => 6C

Step 3 - Oxidation - Isocitrate dehydrogenase

• Isocitrate dehydrogenase
• 1st step of Krebs cycle step 3

6C Isocitrate => 6C

Step 4 - Decarboxylation - Isocitrate dehydrogenase

• Isocitrate dehydrogenase
• 2nd step of Krebs cycle step 3

6C Oxalosuccinate => 5C

The α-ketoglutarate family of amino acid synthesis (synthesis of glutamate, glutamine, proline and arginine) begins with α-ketoglutarate, an intermediate in the Citric Acid Cycle. See Alpha-Ketoglutarates: glutamate, glutamine, proline, arginine

Step 5 - Oxidative decarboxylation - α-Ketoglutarate dehydrogenase (2-Oxoglutarate Dehydrogenase)

• 2-Oxoglutarate Dehydrogenase
• Step 5 = Krebs cycle step 4

5C α-Ketoglutarate => 4C chain (CoA excluded)

Step 6 - Substrate-level phosphorylation - Succinyl-CoA synthetase

• Succinyl-CoA synthetase
• Step 6 = Krebs cycle step 5

Succinyl-CoA =>

The succinyl-CoA formed during Beta oxidation can also enter the Citric Acid Cycle in this step.

Step 7 - Oxidation - Succinate dehydrogenase

• Succinate Dehydrogenase
• Step 7 = Krebs cycle step 6

Succinate =>

Step 8 - Hydration - Fumarase

• Fumarase
• Step 8 = Krebs cycle step 7

Fumarate =>

Step 9 - Oxidation - Malate dehydrogenase

• Malate dehydrogenase
• Step 9 = Krebs cycle step 8

L-Malate =>

Oxaloacetate takes part in gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis (Oxaloacetate/aspartate: lysine, asparagine, methionine, threonine, and isoleucine), fatty acid synthesis.

Step 10 / 0 - Aldol condensation - Citrate synthase

• Citrate Synthase
• Step 10 / 0 = Krebs cycle step 1

4C to a 6C molecule

To view automatically seeded indices concerning Citric Acid Cycle See:

• Citric Acid Cycle

See also:

• Biosynthesis of cholesterol
• Fatty acid synthesis
• Amino Acid Synthesis