General concepts of energy metabolism

The energy supply of the brain and the body, in general, is due to the breakdown of high molecular weight substances. The process of decomposition of complex compounds to simpler was called catabolism . During the catabolism of carbohydrates, fats and proteins, energy is released, which is partially accumulated in the form of macroergic compounds – adenosine triphosphoric acid (ATP), creatine phosphate, and partially consumed in the form of heat. Under macroergic understand compounds having chemical bonds, the splitting of which is accompanied by the release of a large amount of energy. These compounds are called ” cell energy currency. “. Their energy is subsequently used to provide various types of vital functions (biosynthetic processes, creation and maintenance of electrical potentials of cell membranes, muscle contraction, etc.). The most important metabolic pathways of catabolism are glycolysis (for carbohydrates), beta oxidation of fatty acids and the pathways of the breakdown of amino acids and the Krebs cycle. Carbohydrate catabolism includes the following main processes: glycolysis, glycogenolysis, and the Krebs cycle (tricarboxylic acid cycle).

Glycolysis and glycogenolysis can occur anaerobically, mainly in the cytoplasm, and lead to the breakdown of glucose or a reserve substrate of glycogen to pyruvic or lactic (lactate) acids. Glycolysis releases energy that is used to synthesize ATP. From one glucose molecule, 2 ATP molecules are formed.

Both anaerobic and aerobic glucose conversion processes have the same mechanisms up to the formation of pyruvic acid. Subsequently, pyruvate is transported to mitochondria, where it is decarboxylated to acetyl-CoA. In mitochondria, through the Krebs cycle, acetyl-CoA is oxidized to carbon dioxide and water. During aerobic oxidation, energy is released from one glucose molecule, which goes to the formation of ATP (up to 38 molecules).

Lipid catabolism includes their hydrolysis, resulting in the formation of glycerol and fatty acids. Free fatty acids are transported to mitochondria, where they are broken down during beta oxidation. Finally, acetyl CoA formed during beta oxidation is catabolized in the Krebs cycle.

Proteins undergo hydrolytic oxidation, mainly in lysosomes, with the formation of amino acids. Further catabolism of many amino acids begins with the cleavage of the amino group, which is ultimately excreted as urea. Carbon fragments of amino acids are metabolized in the Krebs cycle through acetyl-CoA.

In the Krebs cycle, carbon dioxide is formed and hydrogen atoms are split off, which are transferred to the respiratory chain, which provides a sequence of reactions of the transfer of hydrogen and electrons to oxygen. This chain includes NADH dehydrogenases, flavoproteins, non-heme iron-containing proteins, coenzyme Q (ubiquinone) and cytochrome b, c 1 , c, a and a 3 . Respiratory processes are associated with oxidative phosphorylation, during which ATP is synthesized from adenosine diphosphoric acid (ADP) and phosphate. The Krebs cycle takes place inside the mitochondria, the sequence of reactions of the transfer of hydrogen and electrons to oxygen (respiratory chain) is localized on the inner mitochondrial membrane. The energy released during the transfer of hydrogen and electrons through the respiratory chain is used to form ATP. According to chemo osmotic theory , the transfer of electrons and protons along the respiratory chain creates a proton gradient on both sides of the inner mitochondrial membrane. The energy of this gradient is the driving force of the ATP synthesis process.

The respiratory chain can be inhibited in several places by barbiturates and antibiotics. Intracellular acidification, which occurs, for example, with increased glycolysis, also has a damaging effect on the respiratory chain. In these cases, the escape of electrons of the respiratory chain at the level of ubiquinone and interact directly with oxygen to generate free radicals kislor ode . Oxygen free radicals, having increased reactivity, can cause so -called oxidative stress, which has a damaging effect and various cellular structures of oxidative phosphorylation are, as a rule, incompatible with life, which is observed, for example, under the action of cyanide.

If, as a result of the complete oxidation of one glucose molecule, 38 ATP molecules are formed, as a result of the oxidation of one fatty acid molecule, for example palmitate, up to 129 ATP molecules are formed, and the ATP yield during amino acid oxidation approximately corresponds to the ATP yield during glucose oxidation.

local_offerevent_note September 22, 2019

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