Citric acid cycle

The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. In aerobic organisms, the citric acid cycle is involved in the chemical conversion of carbohydrates, fats and proteins into carbon dioxide and water to generate a form of usable energy. The citric acid cycle also provides precursors for many compounds such as certain amino acids, and some of its reactions are therefore important even in cells performing fermentation.

Reactions

Molecule	Enzyme	Reaction type	Reactants/ Coenzymes	Products/ Coenzymes
I. Citrate	1. Aconitase	Dehydration		H2O
II. cis-Aconitate	2. Aconitase	Hydration	H2O
III. Isocitrate	3. Isocitrate dehydrogenase	Oxidation	NAD+	NADH + H+
IV. Oxalosuccinate	4. Isocitrate dehydrogenase	Decarboxylation
V. α-Ketoglutarate	5. α-Ketoglutarate dehydrogenase	Oxidative decarboxylation	NAD+ + CoA-SH	NADH + H+ + CO2
VI. Succinyl-CoA	6. Succinyl-CoA synthetase	Hydrolysis	GDP + Pi	GTP + CoA-SH
VII. Succinate	7. Succinate dehydrogenase	Oxidation	FAD	FADH2
VIII. Fumarate	8. Fumarase	Addition (H2O)	H2O
IX. L-Malate	9. Malate dehydrogenase	Oxidation	NAD+	NADH + H+
X. Oxaloacetate	10. Citrate synthase	Condensation

The sum of all reactions in the citric acid cycle is:

Acetyl-CoA + 3 NAD⁺ + FAD + GDP + P_i + 3 H₂O →
CoA-SH + 3 NADH + H⁺ + FADH₂ + GTP + 2 CO₂ + 3 H⁺

Two carbon atoms are oxidized to CO₂, and the energy from these reactions is stored in GTP , NADH and FADH₂. NADH and FADH₂ are coenzymes that accept the electrons released in oxidation reactions, and are utilized in oxidative phosphorylation.

A simplified view of the process:

• The process begins with the oxidation of pyruvate, producing one CO₂, and one acetyl-CoA.
• Acetyl-CoA reacts with the four carbon carboxylic acid, oxaloacetate--to form the six carbon carboxylic acid, citrate.
• Through a series of reactions citrate is converted back to oxaloacetate. This cycle produces:
  • 2 CO₂
  • 3 NADH and 3 H⁺.
  • one FADH₂.
  • one GTP

Regulation

Many of the enzymes in the TCA cycle are regulated by negative feedback from ATP when the energy charge of the cell is high. Such enzymes include the pyruvate dehydrogenase complex that synthesises the acetyl-CoA needed for the first reaction of the TCA cycle. Also the enzymes citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase, that regulate the first three steps of the TCA cycle, are inhibited by high concentrations of ATP. This regulation ensures that the TCA cycle will not oxidise excessive amounts of pyruvate and acetyl-CoA when ATP in the cell is plentiful. This type of negative regulation by ATP is by an allosteric mechanism.

Several enzymes are also negatively regulated when the level of reducing equivalents in a cell are high (high ratio of NADH/NAD+). This mechanism for regulation is due to substrate inhibition by NADH of the enzymes that use NAD+ as a substrate. This includes both the entry point enzymes pyruvate dehydrogenase and citrate synthase.

Major metabolic pathways converging on the TCA cycle

Most of the body's catabolic pathways converge on the TCA cycle, as the diagram shows. Reactions that form intermediates of the cycle are called anaplerotic reactions.

The citric acid cycle is the second step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA and enters the citric acid cycle.

In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. These amino acids are brought into the cells and can be a source of energy by being funnelled into the citric acid cycle.

In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart tissue, fatty acids are broken down through a process known as beta oxidation which results in acetyl-CoA which can be used in the citric acid cycle. Sometimes beta oxidation can yield propionyl CoA which can result in further glucose production by gluconeogenesis in liver.

The citric acid cycle is always followed by oxidative phosphorylation. This process extracts the energy from NADH and FADH₂, recreating NAD⁺ and FAD, so that the cycle can continue. The citric acid cycle itself does not use oxygen, but oxidative phosphorylation does.

The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the citric acid cycle and oxidative phosphorylation equals about 36 ATP molecules. The citric acid cycle is called an amphibolic pathway because it participates in both catabolism and anabolism.

See also

• Oxidative decarboxylation
• Citric acid
• Reverse (Reductive) Krebs cycle

Reference(s)

• Campbell, Reece: Biology (seventh edition). Benjamin Cummings, 2005.

External links

• An animation of the citric acid cycle
• A more detailed tutorial animation
• A citric-acid cycle self quiz flash applet
• The chemical logic behind the citric acid cycle