Krebs cycle

Krebs cycle is also known as Citric acid cycle OR Tricarboxylic acid cycle OR Szent-Gyorgi-Krebs cycle. Acety CO-A produced from pyruvate via link reaction enters the Citric acid cycle, where it gets oxidised to carbon dioxide producing a pool of chemical energy(ATP, NADH and FADH2).

• Proposed by Sir Hans Krebs(1937)
• Happens in the mitochondrial matrix
• Complete oxidation of Acetyl CO-A into CO2
• As one glucose molecule produces two pyruvate molecule which in turn produces two Acetyl CO-A molecules, so there are two Krebs cycle for a single glucose molecule
• Enzymes for Krebs cycle present in mitochondrial matrix except for Succinate dehydrogenase, which is present in Inner mitochondrial membrane
• The acceptor of Acetyl CO-A is Oxaloacetate(OAA)

Pathway

Krebs cycle occurs in mitochondria when Acetyl CO-A enters into mitochondria. It is an amphibolic pathway as its intermediates are utilised in many anabolic pathways.

The citric acid cycle comprises of 8 steps-

Step- 1: Formation of Citrate

Acetyl CO-A reacts with Oxaloacetate in the presence of water to form Citric acid. The enzyme Citrate synthetase catalyses the reaction, in which Coenzyme A is regenerated.

Step- 2: Isomerisation of Citrate to Isocitrate

The enzyme Aconitase(Fe++ acts as a prosthetic group) removes a water molecule from Citric acid to form Cis-Aconitic acid, which is a dehydration reaction.

Cis-Aconitic acid is hydrated in the presence of Aconitase(Fe++ acts as a prosthetic group) to form Iso-Citric acid.

Step- 3: Formation of Alpha-ketoglutarate

The enzyme Isocitrate dehydrogenase reduces NAD+ to NADH thereby converting Iso-Citric acid to Oxalosuccinic acid.

The enzyme Decarboxylase removes a carbon dioxide molecule from Oxalosuccinic acid thus converting it into Alpha-ketoglutaric acid, which is a five-carbon compound.

Step- 4: Conversion of Alpha-ketoglutarate to Succinyl CO-A

Alpha-ketoglutarate Dehydrogenase reduces NAD+ to NADH and also removes carbon dioxide from Alpha-ketoglutaric acid to give Succinyl CO-A.

Step- 5: Formation of Succinate

On hydrolysis of Succinyl CO-A, Succinic acid and Coenzyme A are formed. The energy which is liberated is used to convert Guanine diphosphate to Guanine triphosphate(GDP-GTP). This one molecule of GTP generates one equivalent molecule ATP.

Step- 6: Conversion of Succinate to Fumarate

The enzyme Succinate dehydrogenase(found in Inner mitochondrial membrane) reduces FAD to FADH2 thereby producing Fumarate out of Succinate.

Step- 7: Formation of Malate

Fumarate hydrolyses Fumarate to produce Malate.

Step- 8: Conversion of Malate to Oxaloacetate

The enzyme Malate Dehydrogenase reduces NAD+ to NADH through the conversion of Malate to Oxaloacetate.

Summary of Citric acid cycle

Acetyl CO-A + 3 NAD+ +FAD +GDP +Inorganic phosphate+ 2 water = 2 Carbon dioxide + COA-SH + GTP + 3 NADH + 3 Hydrogen + FADH2

The fate of NADH and FADH2

These NADH and FADH2 molecules stores a large amount of energy stored in glucose. The energy stored in them needs to be extracted. Thus NADH and FADH2 produced in mitochondrial matrix travels to Inner mitochondrial membrane, where it enters the Electron Transport Chain(ETC). ETC is a series of complexes that transfer electrons from electron donors to electron acceptors via redox reactions and couples this electron transfer with the transfer of proton(H+ ions) across a membrane, which creates a proton gradient. This proton gradient helps in the production of ATP. Hence the energy stored in NADH and FADH2 extracted in the form of small packets called ATP.

Energetics

Oxidation of Acetyl CO-A in the Citric acid cycle is expressed by the following equation-

Acetyl CO-A + 2 O2 + GDP + Inorganic phosphate + 3 NAD+ + FAD = 2 Co2 + COA-SH + GTP + 3 NADH + 3 H+ + FADH2

Total ATP produced during the TCA cycle

Number of ATP generated by oxidation of 3 NADH = 9 ATP

Number of ATP generated by oxidation of FADH2 = 2 ATP

Number of ATP generated from GTP = 1 ATP

Total ATP production = 12 ATP

In Electron Transport Chain(ETC) each molecule of NADH, FADH2 produces 3 ATP and 2 ATP respectively. 1 molecule of GTP is equivalent to 1 ATP molecule.

Significance

1. It plays a double significant role in oxidation, as well as in the synthetic process.
2. The citric acid cycle is the final common metabolic pathway in the oxidation of carbohydrates, fats and proteins.A few amino acids degrade to form an intermediate of Citric acid cycle. Acetyl CO-A is produced from fat metabolism.
3. Tricarboxylic acid(in the form of ATP) is the major source of energy formed in the Citric acid cycle.