Respiratory chain: functional enzymes

All biochemical reactions in the cells of any organism proceed with the expenditure of energy. The respiratory chain is a sequence of specific structures that are located on the inner membrane of the mitochondria and serve for the formation of ATP. Adenosine triphosphate is a universal source of energy and can accumulate in itself from 80 to 120 kJ.

The respiratory chain of electrons - what is it?

Electrons and protons play an important role in the formation of energy. They create a potential difference on opposite sides of the mitochondrial membrane, which generates a directed movement of particles - current. The respiratory chain (it is ETC, the electron transfer chain) is an intermediary in the transfer of positively charged particles into the intermembrane space and negatively charged particles in the thickness of the inner membrane of the mitochondria.

The main role in the formation of energy belongs to ATP synthase. This complex complex modifies the energy of the directed motion of protons into the energy of biochemical bonds. By the way, an almost identical complex is found in chloroplasts of plants.

Complexes and enzymes of the respiratory chain

The transfer of electrons is accompanied by biochemical reactions in the presence of an enzymatic apparatus. These biologically active substances, numerous copies of which form large complex structures, serve as intermediaries in the transfer of electrons.

The respiratory chain complexes are the central components of the transport of charged particles. In total, there are 4 such formations in the inner membrane of mitochondria, as well as ATP synthase. All these structures are united by a common goal - the transfer of electrons through the ETC, the transfer of hydrogen protons into the intermembrane space and, consequently, the synthesis of ATP.

The complex is a cluster of protein molecules, among which there are enzymes, structural and signaling proteins. Each of the 4 complexes fulfills its own function, which is peculiar to it. Let's see for what tasks in the ETC there are these structures.

I complex

In the transfer of electrons in the thickness of the mitochondrial membrane, the respiratory chain plays the main role. The reactions of the detachment of hydrogen protons and their accompanying electrons are one of the central reactions of the ETC. The first complex of the transport chain takes over the molecules NAD * H + (in animals) or NADP * H + (in plants), followed by the splitting off of four protons of hydrogen. Actually, because of this biochemical reaction, I complex is also called NADH dehydrogenase (by the name of the central enzyme).

The composition of the dehydrogenase complex includes iron-deficient proteins of 3 species, as well as flavin mononucleotides (FMN).

II complex

The work of this complex is not associated with the transfer of hydrogen protons into the intermembrane space. The main function of this structure is to supply additional electrons to the electron transport chain by oxidizing the succinate. The central enzyme of the complex is succinate-ubiquinone-oxidoreductase, which catalyzes the elimination of electrons from succinic acid and transfer to lipophilic ubiquinone.

The supplier of hydrogen and electron protons to the second complex is also FAD * H ₂ . However, the efficacy of flavin adenine dinucleotide is less than that of its analogs - NAD * H or NADP * H.

The composition of the II complex includes three types of iron-sulfur proteins and a central enzyme succinate-oxidoreductase.

III complex

The next component, the ETC, consists of cytochromes b ₅₅₆ , B ₅₆₀ and c ₁ , as well as iron-deficient protein Riske. The work of the third complex is associated with the transfer of two protons of hydrogen into the intermembrane space, and electrons from the lipophilic ubiquinone on cytochrome C.

A feature of the protein Risk is that it dissolves in fat. Other proteins of this group that were found in the respiratory chain complexes are water soluble. This feature affects the position of protein molecules in the thickness of the inner membrane of the mitochondria.

The third complex functions as ubiquinone-cytochrome c-oxidoreductase.

IV complex

It is also a cytochrome-oxidant complex, is the final point in the ETC. His work is to transfer the electron from cytochrome-c to oxygen atoms. Subsequently, negatively charged O atoms will react with hydrogen protons to form water. The main enzyme is cytochrome c-oxygen-oxidoreductase.

The fourth complex includes cytochromes a, a ₃ and two copper atoms. Cytochrome a ₃ plays a central role in the transfer of an electron to oxygen. The interaction of these structures is suppressed by nitrogen cyanide and carbon monoxide, which in a global sense leads to the cessation of the synthesis of ATP and death.

Ubihinon

Ubiquinone is a vitamin-like substance, a lipophilic compound that freely moves in the thickness of the membrane. The respiratory chain of mitochondria can not do without this structure, since it is responsible for transporting electrons from complexes I and II to complex III.

Ubiquinone is a benzoquinone derivative. This structure on the schemes can be indicated by the letter Q or abbreviated LU (lipophilic ubiquinone). Oxidation of the molecule leads to the formation of seven-quinone - a strong oxidizer, which is potentially dangerous for the cell.

ATP synthase

The main role in the formation of energy belongs to ATP synthase. This mushroom-like structure uses the energy of the directed motion of particles (protons) to transform it into the energy of chemical bonds.

The main process that occurs throughout the ETC is oxidation. The respiratory chain is responsible for the transfer of electrons into the thickness of the mitochondrial membrane and their accumulation in the matrix. Simultaneously, complexes I, III and IV pump protons of hydrogen into the intermembrane space. The difference in charges on the sides of the membrane leads to directional motion of the protons through ATP synthase. So H + fall into the matrix, meet electrons (which are associated with oxygen) and form a neutral substance for the cell - water.

The ATP synthase consists of F0 And F1 subunits, which together form a molecule router. F1 consists of three alpha and three beta subunits, which together form a channel. This channel has exactly the same diameter as the protons of hydrogen have. When positively charged particles pass through the ATP synthase, the molecule's head F ₀ rotates 360 degrees around its axis. During this time, phosphorus residues are attached to AMP or ADP (adenosine and diphosphate) with the help of macroergic bonds, in which a large amount of energy is enclosed.

ATP synthases are found in the body not only in mitochondria. In plants, these complexes are also located on the membrane of vacuoles (tonoplast), as well as on the thylakoids of chloroplast.

Also in the cells of animals and plants there are ATP-ase. They have a similar structure, as in ATP synthases, but their action is directed to the cleavage of phosphorus residues with the expenditure of energy.

Biological meaning of the respiratory chain

First, the end product of ETC reactions is the so-called metabolic water (300-400 ml per day). Secondly, ATP synthesis occurs and energy is stored in the biochemical bonds of this molecule. A day is synthesized 40-60 kg of adenosine triphosphate and the same amount is used in enzymatic reactions of the cell. The lifetime of one molecule of ATP is 1 minute, so the respiratory chain should work smoothly, clearly and without errors. Otherwise, the cell will die.

Mitochondria are considered to be the power stations of any cell. Their number depends on the energy costs that are required for certain functions. For example, in neurons, up to 1,000 mitochondria can be counted, which often form a cluster in the so-called synaptic plaque.

Differences in the respiratory chain in plants and animals

In plants, the additional "energy station" of the cell is the chloroplast. ATP synthases have also been found on the inner membrane of these organelles, and this is an advantage over animal cells.

Also, plants can survive in conditions of high concentration of carbon monoxide, nitrogen and cyanides due to a cyanide-resistant path in the ETC. The respiratory chain, therefore, ends in ubiquinone, the electrons from which are immediately transferred to oxygen atoms. As a result, less ATP is synthesized, but the plant can survive adverse conditions. Animals in such cases with prolonged exposure die.

It is possible to compare the efficiency of NAD, FAD and cyanide-stable pathway by means of the ATP production index for the transfer of 1 electron.

• With NAD or NADP formed 3 molecules of ATP;
• With FAD two molecules of ATP are formed;
• One molecule of ATP is formed along the cyanide-stable pathway.

Evolutionary value of ETC

For all eukaryotic organisms, one of the main sources of energy is the respiratory chain. Biochemistry of ATP synthesis in a cell is divided into two types: substrate phosphorylation and oxidative phosphorylation. ETC is used in the synthesis of energy of the second type, that is, due to oxidation-reduction reactions.

In prokaryotic organisms, ATP is formed only in the process of substrate phosphorylation in the glycolysis stage. Six-carbon sugars (mainly glucose) are involved in a cycle of reactions, and at the exit the cell receives 2 molecules of ATP. This type of energy synthesis is considered to be the most primitive, since in the eukaryotic process 36 ATP molecules are formed in the process of oxidative phosphorylation.

However, this does not mean that modern plants and animals have lost the ability to substrate phosphorylation. Simply this type of ATP synthesis has become only one of the three stages of obtaining energy in the cell.

Glycolysis in eukaryotes passes in the cytoplasm of the cell. There are all the necessary enzymes that can split glucose up to two molecules of pyruvic acid with the formation of 2 molecules of ATP. All subsequent stages pass in the matrix of mitochondria. The Krebs cycle, or cycle of tricarboxylic acids, also occurs in the mitochondria. This is a closed chain of reactions, as a result of which NAD * H and FAD * H2 are synthesized. These molecules will go as an expendable material in the ETC.