III. A common catabolic pathway.

Acetyl-CoA undergoes oxidation in the cycle of di- and tricarboxylic acids (Krebs cycle). Oxidation is accompanied by the formation of reduced forms of NADH + H⁺ and FADH₂. These are the primary donors of hydrogen for the electron transport chain. Reactions of the common catabolic pathway occur in the matrix of mitochondria. Reduced coenzymes transfer hydrogen directly to the components of the respiratory chain which are located in the inner membrane of mitochondria, where ATP is produced. There is a transfer of electrons from reduced nucleotides to oxygen (through the respiratory chain). It is accompanied by the formation of water. This electron transport is associated with ATP synthesis during oxidative phosphorylation. At this stage, 2/3 energy of nutrients are released.

Thus, after the formation of pyruvic acid further path of breakdown of substances to CO₂ and H₂O occurs identically in the common catabolic pathway.

Metabolites of the common catabolic pathway are precursors in the synthesis of a number of substances in the body. For example, pyruvate is a precursor for glucose and acetyl-CoA is a precursor for fatty acids.

6.2. Bioenergetics

Metabolism is divided into two categories: catabolism and anabolism. One of the main functions of catabolism is receiving chemical energy contained in nutrients and the usage of this energy to provide necessary functions.

The energy of oxidized substances is used mainly for the synthesis of ATP from ADP. ATP is the universal source of energy in the body.

If the energy released during the hydrolysis reaction of the substance, exceeds 30 kJ/mole, the hydrolysable bond is called a high-energy (energy-rich) bond. The energy of ATP hydrolysis is about 50 kJ/mole. In formula the energy-rich bond is denoted by ~ (tilde).

Types of energy-rich compounds: pyrophosphates (ATP), phosphoguanidines (creatine phosphate), acyl phosphate (1,3 bisphosphoglycerate), enol phosphates (phosphoenol pyruvate), thiol esters (acetyl-CoA).

Cell energy supply can occur in anaerobic conditions:

C₆H₁₂O₆ = 2 C₃H₆O₃ + 65 kJ/mole

One way of ATP synthesis from ADP is substrate level phosphorylation. It is ATP formation at the expense of other high-energy compound:

1,3-bisphosphoglycerate 3-phosphoglycerate

The use of oxygen by cells opens the possibility for a more complete Tissue respiration is a set of oxidation reactions of substrates in living cells, accompanied by the consumption of molecular oxygen and leading to the releasing of carbon dioxide and water and formation of biological energy.

For the first time the essence of breath was explained by Antoine Laurent Lavoisier (1777); he drew attention to the similarity between the combustion of organic substances outside the body and the respiration of animals. In the body, oxidation occurs at relatively low temperature in the presence of water, and its velocity is regulated by metabolism.

Let’s consider the reaction of glucose oxidation:

In the body, this process is multistage. Carbon is converted into carbon dioxide by the oxygen of the oxidized substance and the oxygen of water. This reaction involves hydrogen acceptors, which carry hydrogen on oxygen. Oxygen is used for the synthesis of water from the hydrogen of oxidized substrates.

The other substances are oxidized in the same way. Kidneys, brain, liver are characterized by the highest rate of tissue respiration; and the skin, muscles (at rest) - the smallest one.

The main way of ATP synthesis from ADP is oxidative phosphorylation. This is the synthesis of ATP from ADP and inorganic phosphate, which occurs due to the energy released by oxidation of organic substances in the process of cellular respiration. It is coupling of respiration and phosphorylation.

ADP + H₃PO₄ + energy  ATP + H₂O