The Importance of the Krebs Cycle

The Krebs Cycle (discovered by Sir Hans Adolf Krebs) is a set of reactions that can completely oxidise acetyl-CoA to CO2 in order to release the energy to synthesise ATP, the energy currency of the cell, and it consists of a set of reactions through which the total degradation of the products of glycolysis is completed.

Processes that are important for life

The enzymes of the Krebs Cycle are present in the mitochondrial matrix; pyruvic acid crosses the mitochondrial membrane and enters the mitochondrion, where it is transformed into acetyl-CoA, which enters the Krebs Cycle.

The respiratory chain is a set of oxidation-reduction reactions that takes place in the mitochondria, culminating the respiratory processes, and it is formed by a set of molecules, called proton and electron carriers, which are present in the mitochondrial cristae.

These molecules, as they are reduced and oxidised, transfer protons and electrons from the substrate to molecular oxygen, which is reduced to form water; ATP is formed in this process, but for the various sequences of reactions in the chain to occur, the presence of several specific enzymes that catalyse the whole process is necessary.

• The energy balance of the aerobic respiration of one glucose molecule is 38 ATP molecules, equivalent to 266 kilocalories.
• The metabolism of a cell contains countless biochemical reactions; however, it is possible to identify a skeleton metabolism.

The Krebs Cycle, or tricarboxylic acid cycle, or citric acid cycle, takes place entirely in the mitochondrial matrix and its main function is to act as the common final pathway for the oxidation of organic molecules (carbohydrates, lipids, amino acids) through acetyl-CoA and its substrate.

Acetyl-CoA is mainly derived from glycolysis, beta-oxidation and amino acids and ketone bodies in a physiological event; the Krebs Cycle is a “metabolic crossroads”, where most of the oxidation and energy production of a cell is carried out.

Below are two videos representing the Krebs Cycle and ATP production; and although the second video is in Spanish, it shows exactly what we want to present, but with excellent 3D animation.

The Compounds and the Krebs Cycle

Oxaloacetate, a compound that helps the process and is present in very small quantities, can therefore be compared to a catalyst (which is used many times and is found at the end of the reaction).

The cycle produces the oxidation of carbon by integrating compounds (such as CO2), but above all, reduced coenzymes (NADH + H⁺ and FADH2) to be oxidised in the respiratory chain, leading to the production of large amounts of ATP by oxidative phosphorylation.

• Fats are of great importance as an organic fuel because of their high calorific value, and the main mechanism of energy production from lipids is the oxidation of fatty acids.
• The hydrolysis of lipids is carried out by specific lipases that release the fatty acids from glycerol; and once the fatty acids are produced, they undergo beta-oxidation in the cytoplasm; the acetyl-CoA thus formed can then enter the Krebs cycle; the energy yield of the beta-oxidation of a molecule with 16 carbon atoms is 130 ATP molecules, equivalent to 910 kilocalories.
• Proteins are hydrolysed in the digestive tract, and the amino acids enter the cells, where they can undergo different types of oxidation that will determine the entry of their derivatives into the Krebs cycle, the two main mechanisms of amino acid oxidation being transamination and oxidative deamination.

Transamination consists of the transfer of the amine group of an amino acid to a keto acid, so that one amino acid is degraded in order for another to be formed; these reactions are catalysed by enzymes called transaminases; in contrast, in oxidative deamination the amine groups of the amino acids are released in the form of NH3.

On the other hand, fermentation, which is an anaerobic process in which the respiratory chain is not involved, is not very profitable when compared with respiration; one glucose molecule yields only 2 ATP molecules, as it is an incomplete process in which the end product is an organic compound.