Hans Krebs discovered citric acid cycle in 1937 and he got noble prize for it in 1953. This cycle is also named as Krebs cycle or Tricarboxylic acid cycle. This cycle occurs inside the matrix of mitochondria. This pathway is used for both catabolic reactions to generate energy as well as for anabolic reactions to generate metabolic intermediates for biosynthesis. In this cycle, there is oxidative as well as cyclic degradation of acetate that is derived from pyruvate. Citric acid cycle occurs in aerobic respiration only and it degrades pyruvate completely into inorganic substances; CO2 and H2O. In this cycle, two molecules of pyruvate are completely degraded and there is production of two molecules of ATP, eight molecules of NADH2 and two molecules FADH2. Krebs cycle is also known as amphibolic pathway as catabolic pathways converge upon it and anabolic processes diverge from it.

Steps of Citric Acid Cycle

Step 1 The acetic acid subunit of acetyl CoA is combined with oxaloacetate to form a molecule of citrate. The acetyl coenzyme A acts only as a transporter of acetic acid from one enzyme to another. After Step 1, the CoA is released by hydrolysis so that it may combine with another acetic acid molecule to begin the Krebs cycle again.

Step 2 The citric acid molecule undergoes an isomerization. A hydroxyl group and a hydrogen molecule are removed from the citrate structure in the form of water. The two carbons form a double bond until the water molecule is added back. Only now, the hydroxyl group and hydrogen molecule are reversed with respect to the original structure of the citrate molecule. Thus, isocitrate is formed.

Step 3 In this step, the isocitrate molecule is oxidized by a NAD molecule. The NAD molecule is reduced by the hydrogen atom and the hydroxyl group. The NAD binds with a hydrogen atom and carries off the other hydrogen atom leaving a carbonyl group. This structure is very unstable, so a molecule of CO2 is released creating alpha-ketoglutarate.

Step 4 In this step, coenzyme A, returns to oxidize the alpha-ketoglutarate molecule. A molecule of NAD is reduced again to form NADH and leaves with another hydrogen. This instability causes a carbonyl group to be released as carbon dioxide and a thioester bond is formed in its place between the former alpha-ketoglutarate and coenzyme A to create a molecule of succinyl-coenzyme A complex.

Step 5 A water molecule sheds its hydrogen atoms to coenzyme A. Then, a free-floating phosphate group displaces coenzyme A and forms a bond with the succinyl complex. The phosphate is then transferred to a molecule of GDP to produce an energy molecule of GTP. It leaves behind a molecule of succinate.

Step 6 In this step, succinate is oxidized by a molecule of FAD (Flavin Adenine Dinucleotide). The FAD removes two hydrogen atoms from the succinate and forces a double bond to form between the two carbon atoms, thus creating fumarate.

Step 7 An enzyme adds water to the fumarate molecule to form malate. The malate is created by adding one hydrogen atom to a carbon atom and then adding a hydroxyl group to a carbon next to a terminal carbonyl group.

Step 8 The malate molecule is oxidized by a NAD molecule. The carbon that carried the hydroxyl group is now converted into a carbonyl group. The end product of citric acid cycle is oxaloacetate. Oxaloacetate can then combines with acetyl-coenzyme A and begin the Krebs cycle all over again.

Products of the Krebs Cycle
Acetyl CoA + 3NAD+ + FAD + GDP + Pi + H20  2CO2 + 3NADH + FADH2 + GTP + CoA + 2H+

Control of the Krebs Cycle

There are three control points in theTCA Cycle:

1. Pyruvate dehydrogenase

• Inhibited by high levels of: ATP, acetyl CoA and NADH
• Encouraged by high levels of: ADP and pyruvate
• Acetyl CoA inhibits E2, the transacetylase component,
• NADH on the other hand inhibits E3, the dihydrolipoyl dehydrogenase

2. Isocitrate dehydrogenase

• Inhibited by high levels of: ATP and NADH
• Encouraged by high levels of: ADP
• ADP allosterically encourages the enzyme by increasing its affinity for isocitrate.

3. α-Ketoglutarate dehydrogenase

• Inhibited by high levels of: ATP, succinyl CoA and NADH

Significance of Citric Cycle

• Krebs cycle is the common pathway for oxidative breakdown of carbohydrate, fatty acids and amino acids.
• It is the major pathway for the synthesis of reduced coenzymes and controlled release of energy.
• α-Keto glutamate upon deamination produces glutamic acid, which forms arginine and proline and oxaloacetate produces aspartate.
• Succinyl CoA takes part in synthesis of pyrrole compounds like cytochrome, chlorophyll and phytochrome.
• Oxaloacetate, one of the component of citric acid cycle, forms pyrimidine and alkaloids.