The Citric Acid Cycle or TCA Cycle

The Citric Acid Cycle (TCA cycle) is one of our most important metabolic pathways for the generation of energy and the production of various substrates for other products. It is without understatement a central metabolic pathway and the final common oxidative pathways for fats, amino acids and carbohydrates.

TCA is short for tricarboxylic acid which in most cases is citric acid which has three carboxyl groups. It’s also known as the Krebs cycle.

The TCA cycle supplies energy in the form of ATP but also intermediates and substrates for the synthesis of glucose, heme, amino acids etc. The cycle is a connecting pathway between various individual metabolic pathways.

All the reactions occur in the mitochondrial matrix and in close proximity to the ETC. The matrix is also home to mitochondrial beta-oxidation of fatty acids.

One of the most important connections is the link between glycolysis which is the breakdown of carbohydrates and the TCA cycle. Ultimately the main substrate entering the TCA cycle is pyruvate. This substrate is oxidatively decraboxylated to acetyl CoA using the PDH complex (pyruvate dehydrogenase: PDH). It then involves oxidation of acetyl CoA to carbon dioxide and water.

The substrate acetyl CoA not only comes from both pyruvate but through the oxidation of fatty acids. Pyruvate is often marketed as a sports supplement.

The Key Reactions Of The TCA Cycle

Step 1: The formation of citrate.

Oxaloacetate which is a key substrate in the cycle condenses acetyl CoA to citrate. This is catalysed by the enzyme citrate synthase. This particular enzyme is inhibited by ATP, NADH and by citrate itself. The citrate molecule is a competitive inhibitor with oxaloacetate. Such a high level of inhibition of an enzyme signifies it has an important and pivotal role in controlling the TCA cycle especially when levels of various ‘energy’ molecules are high.

Step 2 & 3: Citrate is isomerized to isocitrate

Citrate is isomerized to isocitrate using the enzyme aconitase. This reaction is over two stages which involves a dehydration followed by a hydration. The intermediate formed after dehydration is cis-aconitate.

Steps 4 & 5: The formation of alpha-ketoglutarate

The enzyme isocitrate dehydrogenase (ICDH) catalyses the conversion of isocitrate to oxalosuccinate by oxidative decarboxylation. Oxalosuccinate is converted to alpha-ketoglutarate.

At this step, a molecule of NADH is formed and a molecule of carbon dioxide is generated.

The enzyme, ICDH is also an important one because it is heavily regulated. it is activated in a cooperative manner by its substrate isocitrate and the cofactors, NAD+, ADP and magnesium ions Mg²⁺. It is also activated by the presence of calcium ions when its is linked with muscle contraction.

It is inhibited by NADH and ATP. Again when ‘energy’ levels are high, the TCA cycle is restricted but if any molecules needed for energy are low, then they ‘activate’ the enzyme.

The molecule alpha-ketoglutarate is also an important substrate used in the urea cycle for producing urea.

Step 6: The conversion of alpha-ketoglutarate to succinyl CoA.

This conversion occurs through oxidative decarboxylation which is catalysed by the alpha-ketoglutarate dehydrogenase complex. This is a particular complex enzyme which is known as a multienzyme complex.

At this stage of the cycle, a second NADH molecule is generated along with a second carbon dioxide molecule being liberated.

Step 7: The formation of succinate

Succinyl CoA is converted to succinate by succinate thiokinase. This reaction is coupled with the phosphorylation of GDP to GTP (not ATP this time). This is an example of substrate level phosphorylation. The GTP is then converted to ATP via the enzyme nucleoside diphosphate kinase.

Step 8: Conversion of succinate to fumarate

Succinate is oxidised by succinate dehydrogenase (SDH) to fumarate. The reaction produces FADH2.

Step 9: Formation of malate

The enzyme fumarase catalyses the conversion of fumarate to malate with the addition of a water molecule.

Step 10: Conversion of malate to oxaloacetate

Malate is oxidised to oxaloacetate by malate dehydrogenase (MDH). This reaction is the third and final reaction producing NADH. The oxaloacetate is regenerated which is combined with another molecule of acetyl CoA and so the cycle continues.

The regeneration of oxaloacetate is an important point. The cycle involves the oxidation of acetyl CoA to 2 molecules of carbon dioxide but the oxaloacetate is simultaneously regenerated. There is no net consumption of oxaloacetate or indeed any other intermediate in the cycle.

What Is The Significance Of the TCA Cycle?

1. It is cycle for ATP production
2. There is complete oxidation of acetyl CoA
3. It is the final common oxidative pathway.
4. It involves the integration of major metabolic pathways
5. Fat is metabolised via carbohydrates
6. Any excess carbohydrate is converted into neutral fat
7. There is no net synthesis of carbohydrates from fat
8. The carbon skeleton of amino acids finally enters the TCA cycle.