ATP synthesis is a process. ATP synthesis in cell mitochondria
3. Coefficient of oxidative phosphorylation
Oxidation of the NADH molecule in CPE is accompanied by the formation of 3 ATP molecules; electrons from FAD-dependent dehydrogenases enter the CPE on KoQ, bypassing the first conjugation point. Therefore, only 2 ATP molecules are formed. The ratio of the amount of phosphoric acid (P) used for phosphorylation of ADP to the oxygen atom (O) absorbed during respiration is called the oxidative phosphorylation coefficient and is denoted by P / O. Therefore, for NADH Р / О \u003d 3, for succinate Р / О - 2. These values \u200b\u200breflect the theoretical maximum of ATP synthesis, in fact, this value is less.
49. Regulation of the electron transport chain (respiratory control). Dissociation of tissue respiration and oxidative phosphorylation. Thermoregulatory function of tissue respiration. Thermogenic function of energy metabolism in brown adipose tissue.
4. Respiratory control
Oxidation of substrates and phosphorylation of ADP in mitochondria are tightly coupled. The rate of use of ATP regulates the rate of electron flow in the CPE. If ATP is not used and its concentration in cells increases, then the flow of electrons to oxygen stops. On the other hand, the consumption of ATP and its conversion to ADP increases the oxidation of substrates and oxygen uptake. The dependence of the mitochondrial respiration intensity on the ADP concentration is called respiratory control. The respiratory control mechanism is characterized by high accuracy and is important, since as a result of its action, the rate of ATP synthesis corresponds to the cell's energy requirements. ATP reserves in the cell do not exist. The relative concentrations of ATP / ADP in tissues vary within narrow limits, while energy consumption by the cell, i.e. the frequency of revolutions of the cycle of ATP and ADP can vary tens of times.
B. Transport of ATP and ADP across mitochondrial membranes
In most eukaryotic cells, the synthesis of the main amount of ATP occurs inside the mitochondria, and the main consumers of ATP are located outside it. On the other hand, a sufficient concentration of ADP must be maintained in the mitochondrial matrix. These charged molecules cannot independently pass through the lipid layer of membranes. The inner membrane is impermeable to charged and hydrophilic substances, but it contains a certain amount of transporters that selectively transfer such molecules from the cytosol to the matrix and from the matrix to the cytosol.
The membrane contains an antiporter ATP / ADP protein that carries out the transport of these metabolites across the membrane (Fig. 6-16). The ADP molecule enters the mitochondrial matrix only if the ATP molecule is released from the matrix.
The driving force of this exchange is the membrane potential of electron transfer along the CPE. Calculations show that about a quarter of the free energy of the proton potential is spent on the transport of ATP and ADP. Other transporters can also use the energy of the electrochemical gradient. This is how the inorganic phosphate necessary for the synthesis of ATP is transferred into the mitochondria. The direct source of free energy for the transport of Ca 2+ into the matrix is \u200b\u200balso the proton potential, rather than the energy of ATP.
B. Uncoupling of respiration and phosphorylation
Some chemicals (protonophores) can transfer protons or other ions (ionophores) from the intermembrane space through the membrane to the matrix, bypassing the proton channels of ATP synthase. As a result, the electrochemical potential disappears and the synthesis of ATP stops. This phenomenon is called dissociation of respiration and phosphorylation. As a result of uncoupling, the amount of ATP decreases, and ADP increases. In this case, the rate of oxidation of NADH and FADH 2 increases, and the amount of absorbed oxygen also increases, but energy is released in the form of heat, and the P / O ratio sharply decreases. As a rule, uncouplers are lipophilic substances that easily pass through the lipid layer of the membrane. One of these substances is 2,4-dinitrophenol, which easily passes from the ionized form to the non-ionized one, attaching a proton in the intermembrane space and transferring it to the matrix.
Examples of uncouplers can also be some drugs, for example, dicumarol - an anticoagulant or metabolites that are formed in the body, bilirubin - a catabolic product, thyroxin - a hormone thyroid gland... All these substances show an uncoupling effect only at their high concentration.
D. Thermoregulatory function of CPE
The synthesis of ATP molecules consumes about 40-45% of the total energy of electrons transferred along the CPE, approximately 25% is spent on the work of transferring substances through the membrane. The rest of the energy is dissipated as heat and is used by warm-blooded animals to maintain body temperature. In addition, additional heat formation can occur when respiration and phosphorylation are disconnected. Uncoupling oxidative phosphorylation can be biologically beneficial. It generates heat to maintain body temperature in newborns, hibernating animals, and all mammals in the process of adapting to cold. In newborns, as well as in hibernating animals, there is a special tissue that specializes in heat production by uncoupling respiration and phosphorylation - brown fat. Brown fat contains many mitochondria. In the mitochondrial membrane, there is a large excess of respiratory enzymes in comparison with ATP synthase. About 10% of all proteins are the so-called uncoupling protein (RB-1) - thermogenin. Brown fat is present in newborns, but it is practically absent in an adult. IN last years Facts appeared indicating the existence of uncoupling proteins in the mitochondria of various organs and tissues of mammals, similar in structure to RB-1 of brown adipose tissue. In its structure, thermogenin is close to the ATP / ADP antiporter, but it is not capable of transporting nucleotides, although it retained the ability to transport fatty acid anions that serve as uncouplers.
On the outside of the membrane, the fatty acid anion attaches a proton and in this form crosses the membrane; on the inner side of the membrane, it dissociates, giving up a proton to the matrix and thereby reducing the proton gradient. The resulting anion is returned to the outside of the membrane by means of an ATP / ADP antiporter.
Cooling stimulates the release of norepinephrine from the endings of the sympathetic nerves. As a result, lipase is activated in adipose tissue and fat is mobilized from fat depots. The resulting free fatty acids serve not only as a "fuel", but also as an important regulator of uncoupling of respiration and phosphorylation.
H + -translocating ATP synthase consists of two parts: a proton channel (F 0) built into the membrane of at least 13 subunits and catalytic subunit (F 1) serving in the matrix. The “head” of the catalytic part is formed by three α- and three β-subunits, between which there are three active centers. The "trunk" of the structure is formed by polypeptides F 0 -part and γ-, δ- and ε-subunits of the head.
The catalytic cycle is subdivided into three phases, each of which takes place alternately in three active sites. First, there is a binding of ADP (ADP) and P 1 (1), then a phosphoanhydride bond (2) is formed, and, finally, the final product of reaction (3) is released. With each transfer of a proton through the F 0 protein channel into the matrix, all three active centers catalyze the next stage of the reaction. It is assumed that the energy of proton transport is primarily spent on the rotation of the γ-subunit, as a result of which the conformations of the α- and β-subunits change cyclically.
Articles of the section "ATP synthesis":
- B. ATP synthase
2012-2019. Visual biochemistry. Molecular biology. Ammonia. Enzymes and their characteristics.
The reference book in a visual form - in the form of color schemes - describes all biochemical processes. Biochemically important chemical compounds, their structure and properties, the main processes with their participation, as well as mechanisms and biochemistry are considered critical processes in wildlife. For students and teachers of chemical, biological and medical universities, biochemists, biologists, physicians, as well as all those interested in life processes.
The work of respiratory enzymes is regulated by an effect that is called respiratory control.
Is the direct effect of the electrochemical gradient on the speed of movement of electrons along the respiratory chain (i.e., on the amount of respiration). In turn, the magnitude of the gradient directly depends on aTP / ADP ratio, the quantitative sum of which in the cell is practically constant ([ATP] + [ADP] \u003d const). Catabolic reactions aim to maintain consistently high ATP and low ADP levels.
An increase in the proton gradient occurs with a decrease in the amount of ADP and accumulation of ATP ( resting state), i.e. when ATP synthase is deprived of its substrate and H + ions do not penetrate into the mitochondrial matrix... In this case, the inhibitory effect of the gradient is enhanced and the movement of electrons along the chain slows down... Enzyme complexes remain in a reduced state. The consequence is a decrease in the oxidation of NADH and FADH 2 on complexes I and II, inhibition of the CTX enzymes with the participation of NADH and slowing down catabolism in a cage.
Electrochemical gradient versus electron velocity
A decrease in the proton gradient occurs when ATP reserves are depleted and ADP is in excess, i.e. when the cell is working... In this case aTP synthase is actively working and H + ions pass through the F o channel into the matrix... In this case, the proton gradient naturally decreases, the flow of electrons along the chain increases, and as a result, the pumping of H + ions into the intermembrane space increases and again their rapid "sinking" through the ATP synthase into the mitochondria with the synthesis of ATP. Enzyme complexes I and II enhance the oxidation of NADH and FADH 2 (as electron sources) and the inhibitory effect of NADH is removed for citric acid cycle and pyruvate dehydrogenase complex. As a result - catabolic reactions are activated carbohydrates and fats.
Adenosine triphosphoric acid-ATP - an obligatory energy component of any living cell. ATP is also a nucleotide, consisting of a nitrogenous base of adenine, a ribose sugar and three residues of a phosphoric acid molecule. This is an unstable structure. IN metabolic processes residues of phosphoric acid are sequentially cleaved from it by breaking with an energy-rich but fragile bond between the second and third residues of phosphoric acid. The detachment of one molecule of phosphoric acid is accompanied by the release of about 40 kJ of energy. In this case, ATP passes into adenosine diphosphoric acid (ADP), and with further cleavage of the phosphoric acid residue from ADP, adenosine monophosphoric acid (AMP) is formed.
Scheme of the structure of ATP and its transformation into ADP (T.A. Kozlova, V.S. Kuchmenko. Biology in tables. M., 2000 )
Consequently, ATP is a kind of energy accumulator in the cell, which is "discharged" during its breakdown. The breakdown of ATP occurs in the course of reactions for the synthesis of proteins, fats, carbohydrates and any other vital functions of cells. These reactions take place with the absorption of energy, which is extracted during the breakdown of substances.
ATP is synthesized in mitochondria in several stages. The first one is preparatory - proceeds stepwise, with the involvement of specific enzymes at each step. In this case, complex organic compounds are split into monomers: proteins - to amino acids, carbohydrates - to glucose, nucleic acids - to nucleotides, etc. The breaking of bonds in these substances is accompanied by the release of a small amount of energy. The resulting monomers under the action of other enzymes can undergo further decomposition with the formation of simpler substances up to carbon dioxide and water.
Scheme ATP synthesis in the autochondria of the cell
EXPLANATION TO THE SCHEME CONVERSION OF SUBSTANCES AND ENERGY IN THE PROCESS OF DISSIMILATION
Stage I - preparatory: complex organic substances under the action of digestive enzymes break down into simple ones, while only thermal energy is released.
Proteins -\u003e amino acids
Fats- >
glycerin and fatty acids
Starch -\u003e glucose
Stage II - glycolysis (oxygen-free): carried out in the hyaloplasm, not associated with membranes; enzymes are involved in it; glucose undergoes splitting:
In yeast fungi, the glucose molecule, without the participation of oxygen, is converted into ethanol and carbon dioxide (alcoholic fermentation):
In other microorganisms, glycolysis can end with the formation of acetone, acetic acid, etc. In all cases, the decomposition of one glucose molecule is accompanied by the formation of two ATP molecules. During the anoxic breakdown of glucose, 40% of the energy is retained in the ATP molecule in the form of a chemical bond, and the rest is dissipated in the form of heat.
Stage III - hydrolysis (oxygen): it is carried out in the mitochondria, is associated with the mitochondrial matrix and the inner membrane, enzymes are involved in it, lactic acid undergoes cleavage: C3H6O3 + 3H20 -\u003e 3CO2 + 12H. CO2 (carbon dioxide) is released from the mitochondria into the environment. The hydrogen atom is included in a chain of reactions, the end result of which is the synthesis of ATP. These reactions proceed in the following sequence:
1. The hydrogen atom H, with the help of carrier enzymes, enters the inner membrane of mitochondria, which forms cristae, where it is oxidized: H-e -\u003e H +
2. Proton of hydrogen H + (cation) is carried by carriers to the outer surface of the membrane of the cristae. For protons, this membrane is impermeable, so they accumulate in the intermembrane space, forming a proton reservoir.
3. Electrons of hydrogen e transferred to the inner surface of the membrane of the cristae and immediately attached to oxygen using the oxidase enzyme, forming a negatively charged active oxygen (anion): O2 + e -\u003e O2-
4. Cations and anions on both sides of the membrane create oppositely charged electric field, and when the potential difference reaches 200 mV, the proton channel begins to operate. It occurs in the molecules of ATP synthetase enzymes, which are embedded in the inner membrane that forms the cristae.
5. Through the proton channel, hydrogen protons H +rush into the mitochondria, creating high level energy, most of which goes to the synthesis of ATP from ADP and F (ADP + F -\u003e ATP), and protons H + interact with active oxygen to form water and molecular 02:
(4H ++ 202- -\u003e 2H20 + 02)
Thus, O2, which enters the mitochondria during the respiration of the organism, is necessary for the attachment of hydrogen protons H. In its absence, the entire process in mitochondria stops, since the electron transport chain ceases to function. General reaction Stage III:
(2CzNboz + 6Oz + 36ADP + 36F ---\u003e 6CO2 + 36ATP + + 42H20)
As a result of the cleavage of one glucose molecule, 38 ATP molecules are formed: at stage II - 2 ATP and at stage III - 36 ATP. The formed ATP molecules go beyond the mitochondria and participate in all cell processes where energy is needed. Splitting, ATP gives up energy (one phosphate bond contains 40 kJ) and returns to mitochondria in the form of ADP and F (phosphate).
ATP synthase (H + -ATP-ase) is an integral protein of the inner mitochondrial membrane. It is located in close proximity to the respiratory chain. ATP synthase consists of 2 protein complexes, designated as F 0 and F 1 (Fig. 6-15).
Figure: 6-15. The structure and mechanism of action of ATP synthase. A - F 0 and F 1 - complexes of ATP synthase, F 0 contains polypeptide chains that form a channel that penetrates the membrane through and through. Through this channel, protons return to the matrix from the intermembrane space; protein F 1 protrudes into the matrix from the inner side of the membrane and contains 9 subunits, 6 of which form 3 pairs of α and β ("head"), covering the core part, which consists of 3 subunits γ, δ and ε. γ and ε are movable and form a rod rotating inside the fixed head and connected with the complex F0. In active centers formed by pairs of subunits α and β, binding of ADP, inorganic phosphate (P i) and ATP occurs. B - The catalytic cycle of ATP synthesis includes 3 phases, each of which takes place alternately in 3 active centers: 1 - binding of ADP and H 3 PO 4; 2 - the formation of a phosphoanhydride bond of ATP; 3 - release of the final product. With each transfer of protons through the F 0 channel into the matrix, all 3 active centers catalyze the next phase of the cycle. The energy of the electrochemical potential is spent on turning the rod, as a result of which the conformation of the α- and β-subunits changes cyclically and ATP is synthesized.
3.The coefficient of oxidative
phosphorylation
Oxidation of the NADH molecule in CPE is accompanied by the formation of 3 ATP molecules; electrons from FAD-dependent dehydrogenases enter the CPE on KoQ, bypassing the first conjugation point. Therefore, only 2 ATP molecules are formed. The ratio of the amount of phosphoric acid (P) used for phosphorylation of ADP to the oxygen atom (O) absorbed during respiration is called the oxidative phosphorylation coefficient and is denoted by P / O. Therefore, for NADH Р / О \u003d 3, for succinate Р / О - 2. These values \u200b\u200breflect the theoretical maximum of ATP synthesis, in fact, this value is less.
Regulation of the electron transport chain (respiratory control). Dissociation of tissue respiration and oxidative phosphorylation. Thermoregulatory function of tissue respiration. Thermogenic function of energy metabolism in brown adipose tissue.
Respiratory control
Oxidation of substrates and phosphorylation of ADP in mitochondria are tightly coupled. The rate of use of ATP regulates the rate of electron flow in the CPE. If ATP is not used and its concentration in cells increases, then the flow of electrons to oxygen stops. On the other hand, the consumption of ATP and its conversion to ADP increases the oxidation of substrates and oxygen uptake. The dependence of the mitochondrial respiration intensity on the ADP concentration is called respiratory control. The respiratory control mechanism is characterized by high accuracy and is important, since as a result of its action, the rate of ATP synthesis corresponds to the cell's energy requirements. ATP reserves in the cell do not exist. The relative concentrations of ATP / ADP in tissues vary within narrow limits, while energy consumption by the cell, i.e. the frequency of revolutions of the cycle of ATP and ADP can vary tens of times.