Respiratory chain complexes
- Complex III (Cytochrome bc1 complex) transfers electrons from ubiquinone to two water-soluble cytochrome c located on the inner membrane of the mitochondria. Ubiquinone transfers 2 electrons, and cytochromes transfer one electron per cycle. At the same time, 2 protons of ubiquinone also pass there and are pumped through the complex.
NADPH + NAD+ ↔ NADP+ + NADH.
FeS -iron-sulfur centers.
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. Respiratory electron transport chain
The respiratory electron transport chain (ETC) is a system of structurally and functionally related transmembrane proteins and electron carriers. ETC allows you to store energy released during the oxidation of NADH and FADH2 with molecular oxygen (in the case of aerobic respiration) or other substances (in the case of anaerobic respiration) in the form of transmembrane proton potential due to the sequential transfer of electrons along the chain associated with the pumping of protons through membrane Components of the respiratory chain. The respiratory chain includes three protein complexes (complexes I, III and IV), embedded in the inner mitochondrial membrane, and two mobile carrier molecules - ubiquinone (coenzyme Q) and cytochrome c. Succinate dehydrogenase, which belongs to the citrate cycle itself, can also be considered as complex II of the respiratory chain. ATP synthase is sometimes called complex V, although it does not participate in electron transfer. The respiratory chain complexes are composed of many polypeptides and contain a number of different redox coenzymes associated with proteins. These include flavin [FMN (FMN) or FAD (FAD), in complexes I and II], iron-sulfur centers (in I, II and III) and heme groups (in II, III and IV). The detailed structure of most complexes has not yet been established. Electrons enter the respiratory chain in various ways. During the oxidation of NADH + H+, complex I transfers electrons through the FMN and Fe/S centers to ubiquinone. The electrons formed during the oxidation of succinate, acyl-CoA and other substrates are transferred to ubiquinone by complex II or another mitochondrial dehydrogenase through the enzyme-linked FADH2 or flavoprotein (see Fig.
Electron transport chain (ETC).
With. 166), In this case, the oxidized form of coenzyme Q is reduced to aromatic ubi-hydroquinone. The latter transfers electrons to complex III, which supplies them through two heme b, one Fe/S center and heme c1 to the small heme-containing protein cytochrome c. The latter transfers electrons to complex IV, cytochrome c oxidase. To carry out redox reactions, cytochrome c oxidase contains two copper-containing centers (CuA and CuB) and hemes a and a3, through which electrons finally reach oxygen. When O2 is reduced, a strong basic anion O2- is formed, which binds two protons and passes into water. The flow of electrons is associated with the proton gradient formed by complexes I, III and IV. Organization of the respiratory chain. Proton transfer by complexes I, III and IV proceeds vectorially from the matrix to the intermembrane space. When electrons are transferred in the respiratory chain, the concentration of H+ ions increases, i.e., the pH value decreases. In intact mitochondria, essentially only ATP synthase allows for the return movement of protons into the matrix. This is the basis for the regulatoryally important coupling of electron transfer with the formation of ATP. Ubiquinone, due to its nonpolar side chain, moves freely in the membrane. Water-soluble cytochrome c is found on outside inner membrane. Oxidation of NADH by complex I occurs on the inner side of the membrane as well as in the matrix, where the citrate cycle and β-oxidation, the most important sources of NADH, also occur. In addition, the reduction of O2 and the formation of ATP (ATP) occur in the matrix. The resulting ATP is transferred via the antiport mechanism (against ADP) into the intermembrane space (see p. 214), from where it penetrates into the cytoplasm through porins
Respiratory chain complexes
- Complex I (NADH dehydrogenase) oxidizes NAD-H, taking two electrons from it and transferring them to lipid-soluble ubiquinone, which diffuses inside the membrane to complex III. At the same time, complex I pumps 2 protons and 2 electrons from the matrix into the intermembrane space of the mitochondrion.
- Complex II (Succinate dehydrogenase) does not pump protons, but provides additional electrons into the chain due to the oxidation of succinate.
- Complex III (Cytochrome bc1 complex) transfers electrons from ubiquinone to two water-soluble cytochrome c located on the inner membrane of the mitochondria. Ubiquinone transfers 2 electrons, and cytochromes transfer one electron per cycle.
Mitochondria electron transport chain
At the same time, 2 protons of ubiquinone also pass there and are pumped through the complex.
- Complex IV (Cytochrome c oxidase) catalyzes the transfer of 4 electrons from 4 cytochrome molecules to O2 and pumps 4 protons into the intermembrane space. The complex consists of cytochromes a and a3, which, in addition to heme, contain copper ions.
Oxygen entering the mitochondria from the blood binds to the iron atom in the heme of cytochrome a3 in the form of an O2 molecule. Each oxygen atom adds two electrons and two protons and turns into a water molecule.
The substrate formed in the Krebs cycle undergoes dehydrogenation (hydrogen abstraction), as a result of which energy is released that goes into the formation of ATP, and the electrons and protons formed in the process combine with oxygen and form water. The reduction of the O2 molecule occurs as a result of the transfer of 4 electrons. With each addition of 2 electrons to oxygen, arriving through a chain of carriers, 2 protons are absorbed from the matrix, resulting in the formation of an H2O molecule.
Electrons are transferred through a chain of carriers that are located in the membrane itself. Carriers, when accepting electrons, are oxidized, and when they give them to the next carrier, they are reduced. At the end of the CPE, electrons are transferred to oxygen.
Protons are forced out of the mitochondrial membrane.
The displacement of protons occurs due to the energy of electron movement inside the membrane.
Protons cannot spontaneously return back into the membrane, so a positive charge accumulates on its outer side.
Protons at the end of the CPE again pass inward through a special protein - ATP synthetase (factor 5) and participate in the formation of water. When a proton passes through ATP synthetase, energy is released that goes into ATP synthesis.
As a result of ORR reactions of transporters, ATP is formed from ADP and inorganic phosphate.
Important: Without the presence of ADP, oxidation does not occur!
Substrates for NAD- and NADP-dependent dehydrogenases are found in the mitochondrial matrix and cytosol.
The main electron carriers are built into the inner membrane of mitochondria and are organized into 4 complexes located in a certain sequence (vectorally). In this sequence, their standard redox potentials become more positive as they approach oxygen
1. The substrate is first oxidized by dehydrogenase-NAD+, as a result the coenzyme NAD+ accepts a proton and turns into NADH.
Most dehydrogenases that supply electrons to the CPE contain NAD+. They catalyze reactions like:
R-CHOH-R1 + NAD+ ↔ R-CO-R1 + NADH + H+.
NADPH is not a direct electron donor in the CPE, but is used almost
exclusively in reductive biosyntheses. However, it is possible to incorporate electrons from NADPH into the CPE due to the action of pyridine nucleotide transhydrogenase, which catalyzes the reaction:
NADPH + NAD+ ↔ NADP+ + NADH.
Flavin dehydrogenases contain FAD or FMN as coenzymes.
FAD serves as an electron acceptor from many substrates in reactions such as:
R-CH2-CH2-R1 + E (FAD) ↔ R-CH=CH-R1 + E (FADH2),
where E is the protein part of the enzyme.
Most FAD-dependent dehydrogenases are soluble proteins localized in the mitochondrial matrix. The exception is succinate dehydrogenase, located in the inner membrane of mitochondria
Or the substrate is oxidized by dehydrogenase-FAD+, as a result of which the coenzyme FAD accepts a proton and becomes FADH2.
If succinate (succinic acid) is oxidized, then oxidation occurs immediately via FAD+ by succinate dehydrogenase.
FAD transfers electrons to Coenzyme Q (ubiquinone) through FES.
Important: ubiquinone is not a protein. All other carriers are proteins!
FeS -iron-sulfur centers.
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In general, the work of the respiratory chain is as follows:
Respiratory electron transport chain
NADH and FADH2 formed in catabolic reactions transfer hydrogen atoms (i.e., hydrogen protons and electrons) to the enzymes of the respiratory chain.
2. Electrons move through the enzymes of the respiratory chain and lose energy.
3. This energy is used to pump H+ protons from the matrix into the intermembrane space.
4. At the end of the respiratory chain, electrons strike oxygen and reduce it to water.
5. H+ protons rush back into the matrix and pass through ATP synthase.
6. At the same time, they lose energy, which is used for the synthesis of ATP.
General principle of oxidative phosphorylation
Reduced forms of NAD and FAD oxidize enzymes of the respiratory chain, due to this, phosphate is added to ADP, i.e. phosphorylation. Therefore, the entire process was called oxidative phosphorylation.
Respiratory chain
Total electron transport chain includes about 40 diverse proteins, which are organized into 4 large membrane-bound multienzyme complexes. There is also another complex that is not involved in electron transfer, but synthesizes ATP.
Block diagram of the respiratory chain
Electron carriers
1. Cytochromes c1, c, a, a3 (prosthetic group - heme) are located in various areas respiratory chain, cytochrome c is a mobile water-soluble protein that moves along the outer side of the membrane between the 3rd and 4th complexes. Cytochromes aa3 contain heme A. Instead of methyl (-CH3) and vinyl (-CH=CH2) groups, it contains a formyl (-CH) group and a hydrocarbon chain, respectively. The second feature is the presence of copper ions in special protein centers.
Cu+<->Сu2+ + e and Fe2+<->Fe3+ + e
2. Iron-sulfur proteins (FeS) – non-heme proteins, function together with flavin enzymes (1, 2, 3 complexes) 
3. FMN (complex 1): FMN + NADH + H+ ———FMNH2 + NAD+
(NAD+ + 2e + 2H+ ————- NADH + H+)
KoQ (ubiquinone) – non-protein transporter, complex 3.
The long hydrophobic tail of isoprene ensures the mobility of ubiquinone in the lipid bilayer.
KoQ and cytochrome c are mobile, all others are integral proteins.
Structure of enzymatic complexes of the respiratory chain
Complex. NADH-CoQ reductase
This complex also has a working title NADH dehydrogenase, contains 1FMN, 6 iron-sulfur proteins.
1. NADH + H+ + FMN ———2e + 2H+——— NAD+ + FMNH2
2. FMNH2 ————2e——— Fex Sx (Fe2+<->Fe3+ + e)
3. Fex Sx ————2e——— KoQ
Function
1. Accepts electrons from NADH and transfers them to coenzyme Q(ubiquinone).
2. Transfers 4H+ to the outer surface of the inner mitochondrial membrane.
Biological oxidation is a set of reactions of oxidation of substrates in living cells, the main function of which is to provide energy for metabolism.
The main functions of oxidative processes:
1) energy reserve in recyclable form,
2) energy dissipation in the form of heat,
3) formation of useful compounds,
4) breakdown of harmful substances.
Differences between biological oxidation and combustion
Biological oxidation is not a one-step exothermic reaction, but represents a chain of reactions during which energy is released, dissipated as heat and accumulated in ATP.
Biological oxidation is an enzymatic process.
Biological oxidation occurs at low temperatures and in the presence of water.
During the combustion of organic substances, energy is released due to the oxidation of carbon to carbon dioxide, and during biological oxidation due to the oxidation of hydrogen, the reduction of oxygen to water.
History of the development of the doctrine of biological oxidation.
Oxidase theory of A. N. Bach
The path of atmospheric oxygen to the substrate is through peroxide.
Activation of molecular oxygen:
a) oxygenase + O 2 oxygenase + peroxide
b) oxygenase + substrate oxygenase + oxidized substrate.
Theory of V. I. Palladin
Oxidation in a living organism occurs through dehydrogenation.
The hydrogen acceptor can be not only oxygen, but also another substance.
The essence of oxidation
Chemical reactions during which an electron is transferred from one molecule to another are called redox reactions.
Compounds that donate electrons, electron donors, or reducing agents.
Compounds that gain an electron
electron acceptors or oxidizing agents.
Oxidizing agents and reducing agents function as conjugate redox couples (redox couples).
Fe + ē Fe
oxidizing agent, reducing agent,
acceptor donor
Each redox pair is characterized by a standard potential (in volts)
Redox potential
The redox potential indicates the direction of electron transfer.
When comparing the redox potential of the system with a normal hydrogen electrode, the potential of which is zero, values are obtained that reflect the redox abilities of the substance.
Tissue respiration– a type of biological oxidation in which oxygen is the electron acceptor
Substrates of tissue respiration:
amino acids,
α-glycerophosphate,
fatty acid.
Krebs cycle acids (isocitrate, α-ketoglutarate, succinate, malate),
Tissue respiration is carried out with the help of enzymes of the respiratory chain.
Scheme of energy conversion in living cells: tissue respiration, ATP formation and ways of its use.
WITH 
ATP structure
ATP synthesis methods
The respiratory chain is a sequence of oxidoreductases in the inner membrane of mitochondria that transfer electrons and protons from the substrate to molecular oxygen.
Mitochondria
Transfer of electrons and protons with the participation of intermediate carriers.
SH2 is the initial donor of protons and electrons;
P1, P2, P3, P4 - intermediate carriers;
E1, E2, E3, E4 - enzymes of redox reactions
The respiratory chain is the main supplier of energy for the synthesis of high-energy bonds of ATP molecules in the process of oxidative phosphorylation.
Maintaining heat balance in the body. 57% of energy is released as heat.
Components of the respiratory chain
Hydrogen enters the respiratory chain in the form of NADH2, since most dehydrogenases inside mitochondria are NAD-dependent, as well as when acting on the substrate flavin dehydrogenase (coenzyme FAD).
NAD-dependent dehydrogenases
Accept electrons and protons directly from the substrate:
S -HH +NAD + S +NADH+H +
The collector function of NAD collects electrons and protons from the substrate.
Most dehydrogenases have NAD, but may also have NADP (G-6-PDG).
Some pyridine-dependent dehydrogenases are localized in mitochondria, and some are localized in the cytoplasm.
The cytosolic and mitochondrial pools of NAD and NADP are separated from each other by the mitochondrial membrane, which is impermeable to these coenzymes.
Shuttle mechanisms transport reduced nucleotides (NADH+H) from the cytoplasm to mitochondria/
In the cytoplasm, oxaloacetate is reduced to malate, which penetrates the mitochondria.
In mitochondria, under the influence of mitochondrial MDH, malate is converted into PAA, and NADH + H transfers electrons and protons to the respiratory chain.
Redox system of the respiratory chain
D 
The respiratory chain includes 4 enzyme complexes that catalyze the oxidation of NADH + H by oxygen.
NADH-KoQ reductase catalyzes the transfer of electrons from NADH to KoQ.
NADH dehydrogenase,
non-heme FeS – clusters,
NADH dehydrogenase
flavoprotein,
located in the inner membrane of mitochondria.
The coenzyme is FMN, which accepts electrons from NADH+H.
FMN + NADH + H FMN 2 + NAD
In FeS proteins, iron is bound to a sulfur residue.
Succinate-KoQ reductase catalyzes electron transfer from succinate to KoQ
This complex includes:
non-heme Fe,
SDH - flavoprotein,
tightly bound to the inner mitochondrial membrane.
The coenzyme is FAD.
KoQ (ubiquinone)
Sources of ubiquinone are vitamins K and E.
KoQ is located in the respiratory chain between flavin enzymes and cytochromes.
KoQ + FMNN 2 KoQН 2 + FMN
Ubiquinone is a collector, as it collects reduced
equivalents not only from NADH-DG, but also from SDH
and other components.
KoQН2 – cytochrome C reductase catalyzes the transfer of electrons from KoQН2 to cytochrome
The complex includes:
cytochrome B,
cytochrome C1,
non-heme Fe,
Cytochromes are complex iron-containing proteins, colored red.
The coenzyme is similar to heme, but the iron in the cytochromes changes its valency.
First described by McMunn and studied by Keilin.
Cytochromes transport electrons.
There are 25-30 different cytochromes known, which differ:
redox potential,
absorption spectrum,
molecular weight,
solubility in water.
P 
growth group of heme in the structure of cytochromes.
Binding of heme to the protein part of cytochrome C
Cytochrome oxidase catalyzes the transfer of electrons from cytochrome C to oxygen.
The complex includes:
cytochrome a,
cytochrome a3,
non-heme Fe,
Cytochrome oxidase differs from other cytochromes:
presence of copper,
reacts with oxygen
proton pump.
This enzyme has 4 redox centers:
Cytochrome C CuA heme A heme a 3 CuB O 2
Cu + e Cu
When one electron is transported, two hydrogen ions are transferred, one of which is used in the reduction of oxygen to water, and the other crosses the membrane.
Oxygen entering the mitochondria from the blood binds to the iron atom in the heme of cytochrome a.
Then each of the atoms of the oxygen molecule
adds 2 electrons and 2 protons,
turning into a water molecule.
Protons come from the aqueous environment.
4ē + 4H + O 2 2H 2 0
200 - 400 ml of water is synthesized per day - endogenous water.
The entire process of NADH+H oxidation in the respiratory chain is associated with the transfer of 10H from the inside of the membrane to the outside.
Complexes I, III, IV are involved in this process.
Complex II transfers hydrogen from succinate to KoQ. This complex does not directly participate in the formation of energy.
Respiratory chain disorders
A condition of fatal childhood mitochondrial myopathy and kidney dysfunction.
Associated with decreased activity or complete absence of most respiratory chain oxidoreductases.
The order of distribution of enzymes in the respiratory chain is determined by the redox potential.
The redox potential changes in the circuit as electrons lose free energy passing through the circuit and move to a lower energy level.
The substrate must have a more negative potential than the carrier enzyme:
Glucose (-0.5 V) is switched on at the very beginning of the respiratory chain.
Ascorbic acid (+ 0.2 V) is included with cytochrome C1.
Electrons can pass through all carriers from the substrate to oxygen.
Shortened chains
Succinate donates electrons to FAD CoQ cytochromes O 2. The redox potential of succinate is 0.13.
Amino acids flavin enzymes (amino acid oxidases) O 2 H 2 O 2 .
Respiratory inhibitors
The insecticide rotenone blocks NADH-DH. Barbiturates block the transition from AF to ubiquinone.
Antimycin A blocks the stage: cytochrome B cytochrome C.
Cyanides and carbon monoxide are cytochrome oxidase inhibitors. Hydrocyanic acid reacts with Fe, carbon monoxide with Fe.
Cascade release of energy in the respiratory chain
The passage of an electron through the circuit is accompanied by a stepwise, stage-by-stage, fractional release of energy.
The total energy difference in the respiratory chain from -0.32 to +0.82 is 1.14 V.
The energy released in a cascade can be utilized.
The transfer of one pair of electrons from NADH + H to oxygen gives 52.6 kcal.
Since the energy of electrons cannot be “stored”, it is converted into the energy of chemical bonds of ATP.
There are 2 types of respiratory chains:
associated with energy transport,
not associated with energy transport.
Tissue respiration includes:
removal of hydrogen from the substrate,
multi-step process of electron transfer to oxygen.
Electron transfer is accompanied by a decrease in free energy.
Part of the energy is dissipated as heat, and 40% is used for ATP synthesis.
14.1.1. In the pyruvate dehydrogenase reaction and in the Krebs cycle, dehydrogenation (oxidation) of substrates (pyruvate, isocitrate, α-ketoglutarate, succinate, malate) occurs. As a result of these reactions, NADH and FADH2 are formed. These reduced forms of coenzymes are oxidized in the mitochondrial respiratory chain. The oxidation of NADH and FADH2, which occurs in conjunction with the synthesis of ATP from ADP and H3 PO4, is called oxidative phosphorylation.
A diagram of the structure of mitochondria is shown in Figure 14.1. Mitochondria are intracellular organelles with two membranes: outer (1) and inner (2). The inner mitochondrial membrane forms numerous folds - cristae (3). The space bounded by the inner mitochondrial membrane is called the matrix (4), the space bounded by the outer and inner membranes is the intermembrane space (5).
Figure 14.1. Scheme of the structure of mitochondria.
14.1.2. Respiratory chain- a sequential chain of enzymes that transfers hydrogen ions and electrons from oxidized substrates to molecular oxygen - the final hydrogen acceptor. During these reactions, energy is released gradually, in small portions, and it can be accumulated in the form of ATP. Localization of respiratory chain enzymes is the inner mitochondrial membrane.
The respiratory chain includes four multienzyme complexes (Figure 14.2).
Figure 14.2. Enzyme complexes of the respiratory chain (sites of interface between oxidation and phosphorylation are indicated):
I. NADH-KoQ reductase(contains intermediate hydrogen acceptors: flavin mononucleotide and iron-sulfur proteins). II. Succinate-KoQ reductase(contains intermediate hydrogen acceptors: FAD and iron-sulfur proteins). III. KoQН 2-cytochrome c reductase(contains electron acceptors: cytochromes b and c1, iron-sulfur proteins). IV. Cytochrome c oxidase(contains electron acceptors: cytochromes a and a3, copper ions Cu2+).14.1.3. Ubiquinone (coenzyme Q) and cytochrome c act as intermediate electron carriers.
Ubiquinone (KoQ)- a fat-soluble vitamin-like substance that can easily diffuse in the hydrophobic phase of the inner mitochondrial membrane. The biological role of coenzyme Q is the transfer of electrons in the respiratory chain from flavoproteins (complexes I and II) to cytochromes (complex III).
Cytochrome c- a complex protein, chromoprotein, the prosthetic group of which - heme - contains iron with variable valence (Fe3+ in oxidized form and Fe2+ in reduced form). Cytochrome c is a water-soluble compound and is located at the periphery of the inner mitochondrial membrane in the hydrophilic phase. The biological role of cytochrome c is the transfer of electrons in the respiratory chain from complex III to complex IV.14.1.4. Intermediate electron carriers in the respiratory chain are arranged according to their redox potentials. In this sequence, the ability to donate electrons (oxidize) decreases, and the ability to gain electrons (reduce) increases. NADH has the greatest ability to donate electrons, and molecular oxygen has the greatest ability to gain electrons.
Figure 14.3 shows the structure of the reactive site of some intermediate proton and electron carriers in oxidized and reduced forms and their interconversion.
Figure 14.3. Interconversions of oxidized and reduced forms of intermediate carriers of electrons and protons.
14.1.5. The mechanism of ATP synthesis describes chemiosmotic theory(author - P. Mitchell). According to this theory, components of the respiratory chain located in the inner mitochondrial membrane, during electron transfer, can “capture” protons from the mitochondrial matrix and transfer them to the intermembrane space. In this case, the outer surface of the inner membrane acquires a positive charge, and the inner one - a negative one, i.e. a proton concentration gradient is created with a more acidic pH value outside. This is how the transmembrane potential arises (ΔµH+). There are three sections of the respiratory chain where it is formed. These regions correspond to complexes I, III and IV of the electron transport chain (Figure 14.4).
Figure 14.4. Location of respiratory chain enzymes and ATP synthetase in the inner mitochondrial membrane.
Protons released into the intermembrane space due to the energy of electron transfer again pass into the mitochondrial matrix. This process is carried out by the enzyme H+ -dependent ATP synthetase (H+ -ATPase). The enzyme consists of two parts (see Figure 10.4): a water-soluble catalytic part (F1) and a proton channel immersed in the membrane (F0). The transition of H+ ions from an area with a higher to an area with a lower concentration is accompanied by the release of free energy, due to which ATP is synthesized.
14.1.6. Energy accumulated in the form of ATP is used in the body to power a variety of biochemical and physiological processes. Remember the main examples of the use of ATP energy:
1) synthesis of complex chemical substances from simpler ones (anabolic reactions); 2) muscle contraction (mechanical work); 3) formation of transmembrane biopotentials; 4) active transport of substances through biological membranes.All biochemical reactions in the cells of any organism occur with the expenditure of energy. The respiratory chain is a sequence of specific structures that are located on the inner membrane of mitochondria and serve to produce ATP. Adenosine triphosphate is a universal source of energy and is capable of accumulating from 80 to 120 kJ.
Electron breathing chain - 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 the directed movement of particles - a current. The respiratory chain (aka ETC, electron transport 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 mitochondrial membrane.
The main role in energy production belongs to ATP synthase. This complex complex modifies the energy of the directed movement of protons into the energy of biochemical bonds. By the way, an almost identical complex is found in plant chloroplasts.
Complexes and enzymes of the respiratory chain
Electron transfer is accompanied by biochemical reactions in the presence of an enzymatic apparatus. These multiple copies form large complex structures that mediate electron transfer.
Respiratory chain complexes are central components of charged particle transport. 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 along the ETC, the transfer of hydrogen protons into the intermembrane space and, as a consequence,
The complex is a collection of protein molecules, among which are enzymes, structural and signaling proteins. Each of the 4 complexes performs its own, unique function. Let's figure out what tasks these structures are present in the ETC.
Complex I
The respiratory chain plays a major role in the transfer of electrons throughout the mitochondrial membrane. The reactions of abstraction of hydrogen protons and accompanying electrons are one of the central reactions of ETC. The first complex of the transport chain accepts molecules of NAD*H+ (in animals) or NADP*H+ (in plants), followed by the elimination of four hydrogen protons. Actually, because of this biochemical reaction, complex I is also called NADH dehydrogenase (after the name of the central enzyme).
The composition includes iron-sulfur proteins of 3 types, as well as flavin mononucleotides (FMN).
II complex
Job 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 through the oxidation of succinate. The central enzyme of the complex is succinate-ubiquinone oxidoreductase, which catalyzes the removal of electrons from succinic acid and transfer to the lipophilic ubiquinone.
The supplier of hydrogen protons and electrons to the second complex is also FAD*H 2. However, the efficiency of flavin adenine dinucleotide is less than that of its analogues - NAD*H or NADP*H.
Complex II includes three types of iron-sulfur proteins and the central enzyme succinate oxidoreductase.
III complex
The next component, ETC, consists of cytochromes b 556, b 560 and c 1, as well as Rieske iron-sulfur protein. The work of the third complex involves the transfer of two hydrogen protons into the intermembrane space, and electrons from the lipophilic ubiquinone to cytochrome C.
The peculiarity of Rieske protein is that it dissolves in fat. Other proteins in this group, which were found in the respiratory chain complexes, are water soluble. This feature affects the position of protein molecules in the thickness of the inner mitochondrial membrane.
The third complex functions as a ubiquinone-cytochrome c oxidoreductase.
IV complex
It is also the cytochrome-oxidant complex, which is the final point in the ETC. Its job is to transfer electrons from cytochrome c to oxygen atoms. Subsequently, the 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 electron transfer 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 ATP synthesis and death.
Ubiquinone
Ubiquinone is a vitamin-like substance, a lipophilic compound that moves freely throughout the membrane. The mitochondrial respiratory chain cannot do without this structure, since it is responsible for transporting electrons from complexes I and II to complex III.
Ubiquinone is a derivative of benzoquinone. This structure can be designated in diagrams by the letter Q or abbreviated as LU (lipophilic ubiquinone). Oxidation of the molecule leads to the formation of semiquinone, a strong oxidizing agent that is potentially dangerous to the cell.
ATP synthase
The main role in energy production belongs to ATP synthase. This mushroom-like structure uses the energy of the directed movement of particles (protons) to convert it into the energy of chemical bonds.
The main process that occurs throughout the ETC is the respiratory chain, which is responsible for the transfer of electrons throughout the mitochondrial membrane and their accumulation in the matrix. At the same time, complexes I, III and IV pump hydrogen protons into the intermembrane space. The difference in charges on the sides of the membrane leads to the directional movement of protons through ATP synthase. So H+ enters the matrix, meets electrons (which are associated with oxygen) and forms a substance neutral for the cell - water.
ATP synthase consists of F0 and F1 subunits, which together form the router molecule. F1 consists of three alpha and three beta subunits, which together form a channel. This channel has exactly the same diameter as the hydrogen protons. When positively charged particles pass through ATP synthase, the F 0 head of the molecule rotates 360 degrees around its axis. During this time, phosphorus residues are added to AMP or ADP (adenosine mono- and diphosphate), which contain a large amount of energy.
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 the chloroplast.
ATPases are also present in animal and plant cells. They have a similar structure as ATP synthases, but their action is aimed at the cleavage of phosphorus residues with the expenditure of energy.
Biological meaning of the respiratory chain
Firstly, the end product of ETC reactions is the so-called metabolic water (300-400 ml per day). Secondly, ATP is synthesized and energy is stored in the biochemical bonds of this molecule. 40-60 kg of adenosine triphosphate is synthesized per day and the same amount is used in enzymatic reactions of the cell. The lifespan of one ATP molecule is 1 minute, so the respiratory chain must work smoothly, clearly and without errors. Otherwise, the cell will die.
Mitochondria are considered the energy stations of any cell. Their number depends on the energy consumption required for certain functions. For example, neurons can contain up to 1000 mitochondria, which often form a cluster in the so-called synaptic plaque.
Differences in the respiratory chain between plants and animals
In plants, the additional “energy station” of the cell is the chloroplast. ATP synthases are also found on the inner membrane of these organelles, and this is an advantage over animal cells.
Plants can also survive in conditions of high concentrations of carbon monoxide, nitrogen and cyanide due to the cyanide-resistant pathway in the ETC. The respiratory chain thus ends at ubiquinone, electrons from which are immediately transferred to oxygen atoms. As a result, less ATP is synthesized, but the plant can survive unfavorable conditions. Animals in such cases die after prolonged exposure.
The efficiency of NAD, FAD, and the cyanide-resistant pathway can be compared using the rate of ATP production per electron transfer.
- 3 ATP molecules are formed with NAD or NADP;
- 2 ATP molecules are formed with FAD;
- the cyanide-resistant pathway produces 1 molecule of ATP.
Evolutionary significance of ETC
For all eukaryotic organisms, one of the main sources of energy is the respiratory chain. The biochemistry of ATP synthesis in the cell is divided into two types: substrate phosphorylation and oxidative phosphorylation. ETC is used in the synthesis of energy of the second type, i.e. due to redox reactions.
In prokaryotic organisms, ATP is formed only during the process of substrate phosphorylation at the stage of glycolysis. Six-carbon sugars (mainly glucose) are involved in the reaction cycle, and as a result the cell receives 2 ATP molecules. This type of energy synthesis is considered the most primitive, since in eukaryotes, 36 ATP molecules are formed during the process of oxidative phosphorylation.
However, this does not mean that modern plants and animals have lost the ability to substrate phosphorylation. It’s just that this type of ATP synthesis has become only one of the three stages of energy production in the cell.
Glycolysis in eukaryotes takes place in the cytoplasm of the cell. There are all the necessary enzymes that can break down glucose into two molecules with the formation of 2 molecules of ATP. All subsequent stages take place in the mitochondrial matrix. The Krebs cycle, or tricarboxylic acid cycle, also occurs in 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 consumables to the ETC.
The oxidation of substrates during respiration can be thought of as the transfer of electrons and protons (i.e., hydrogen atoms) from organic substances to oxygen. This process involves a number of intermediate carriers that form respiratory chain.
Respiratory chain (electron transport chain, electron transport chain) - a system of transmembrane proteins and electron carriers that transfer electrons from substrates to oxygen. In eukaryotic cells, the respiratory chain is located in the inner membrane of the mitochondria.
When NAD + and NADP + interact with hydrogen atoms, a reversible addition of hydrogen atoms occurs.
The NAD + (NADP +) molecule includes 2 electrons and one proton, the second proton remains in the medium:
Another primary source of hydrogen atoms and electrons is reduced flavoprotein (FAD or FMN):
Reduced forms of these cofactors are capable of transporting hydrogen and electrons to the mitochondrial respiratory chain.
The components of the respiratory chain are embedded in the mitochondrial membrane in the form of 4 protein-lipid complexes (Fig. 33).
Complex I (NADH dehydrogenase) includes FMN And iron sulfur protein FeS (non-heme iron). Iron-sulfur protein is involved in the redox process. Complex I oxidizes NADH, transferring 2 electrons from it to coenzyme Q (KоQ) and pumps 4 protons from the matrix into the intermembrane space of the mitochondria.
KoQ(ubiquinone) is a benzoquinone derivative. It is a small lipophilic molecule. Moving in the lipid layer of the membrane, ubiquinone ensures the transfer of electrons between complexes I - III and II - III.
Complex II (succinate dehydrogenase) includes FAD And iron sulfur protein. Provides entry into the chain of additional electrons due to the oxidation of succinate.
Complex III (QH 2 dehydrogenase) includes cytochromes b And from 1 And iron sulfur protein. Cytochromes- hemoproteins in which the prosthetic heme group is close to the heme of hemoglobin (in cytochrome b it is identical). Complex III transfers electrons from ubiquinone to cytochrome c and pumps
2 protons into the intermembrane space.
Complex IV (cytochrome c oxidase) comprises cytochromes a And a 3, which, in addition to heme, contain copper ions. Complex IV catalyzes the transfer of electrons from cytochrome molecules to O2 and pumps 4 protons into the intermembrane space.
Cytochrome a 3 is the terminal portion of the respiratory chain ( cytochrome oxidase): cytochrome oxidation occurs With and water formation. In the human body, the mitochondrial respiratory chain produces 300-400 ml of water per day (metabolic water).
The components of the mitochondrial respiratory chain are arranged in descending order of redox potential. The movement of electrons in the respiratory chain occurs along a gradient of redox potential and is a source of energy for the transfer of protons. The transfer of two electrons through each complex allows for the pumping of four protons. As a result, a difference in proton concentrations appears on the sides of the membrane and, at the same time, a difference in electrical potentials with a plus sign on the outer surface. The electrochemical potential forces protons to move in the opposite direction - from the outer surface to the inside. However, the membrane is impermeable to them, with the exception of areas where the enzyme is located proton ATP synthase(Fig. 34).
ATP synthase consists of two parts - a stator and a rotor.
Stator consists of three α-subunits and three β-subunits - they are directly involved in the synthesis of ATP from ADP and phosphate. They are adjacent to the δ subunit, and together they form the F1 subunit.
Rotor consists of g- and e-subunits.
The stator is held in the membrane, and the rotor rotates due to the energy of protons.
IN stator there is a proton channel (F0). It consists of two half-channels, which are offset from one another. The proton passes through one half of the channel, then, on a rotating rotor, enters the second half of the channel.
| Rice. 34. Structure of proton ATP synthase |
The driving force for ATP synthase, which catalyzes the reaction
ADP + H 3 PO 4 = ATP + H 2 O,
is the electrochemical potential difference created when protons move through the channel.
P. Mitchell to explain the molecular mechanism of coupling electron transport and ATP formation in the respiratory chain in 1960, he proposed chemiosmotic concept: in the respiratory chain there are only 3 sections (complexes I, III, IV), where the transfer of electrons is associated with the accumulation of energy sufficient for the formation of ATP.
Phosphorylation coefficient- the ratio of the amount of ATP formed to the absorbed oxygen: ATP/O or R/O. Maximum phosphorylation coefficient value 3 , if the oxidation reaction involves NADH+H +, And 2 , if the oxidation of the substrate occurs through FADN 2. The actual values obtained are less (2.5 and 1.5), i.e. the respiration process is not completely associated with phosphorylation. The degree of coupling depends mainly on the integrity of the mitochondrial membrane.
The resulting ATP with the participation ADP-ATP translocase transported from the matrix to the outside of the membrane and enters the cytosol. At the same time, the same translocase transfers ADP in the opposite direction, from the cytosol to the mitochondrial matrix.
For each contraction of the heart muscle, about 2% of the ATP present in it is consumed. All ATP would be consumed in 1 minute if there were no regeneration. When a blood clot forms in a coronary artery, the supply of oxygen to the cells stops, accordingly, the regeneration of ATP stops, and the cells die ( myocardial infarction).
An increase in ADP concentration leads to an acceleration of respiration and phosphorylation. The dependence of the intensity of mitochondrial respiration on the concentration of ADP is called respiratory control.
To assess the effect of adenyl nucleotides on metabolic processes, use energy charge of the cell (ECC):
Normally, EZK = 0.7-0.8: the rate of ATP formation is equal to the rate of its use, the adenyl system is saturated with energy.
For ECD< 0,7 ускоряется образование АТФ путем увеличения скорости реакций общего пути катаболизма.
If EZK = 1, then the processes of ATP synthesis are inhibited and its use is accelerated.
The respiratory control mechanism is characterized by high precision. The relative concentrations of ATP and ADP in tissues vary within narrow limits, while energy consumption by a cell can vary tens of times.
Thus, the energy of nutrients in the cell is first transformed into ATP energy, and then ATP serves as a direct source of energy for biochemical and physiological processes. These energy transformations are energy metabolism.
Hypoenergetic states are divided into:
1. Nutritional(fasting, vitamin deficiency).
2. Hypoxic. Connected:
With disruption of oxygen supply to the blood. Exogenous hypoxia - lack of oxygen in the inhaled air, pulmonary (respiratory)-impaired pulmonary ventilation;
With impaired oxygen transport in the blood. Hemodynamic hypoxia associated with circulatory disorders (generalized - heart defects, blood loss; local - vasospasm, thrombosis); causes hemoglobin hypoxia– hypohemoglobinemia, hemoglobinopathies, blocking of hemoglobin by poisons.
3. Mitochondrial. The use of oxygen in cells is hampered as a result of disruption of mitochondrial functions by inhibitors of respiratory chain enzymes, oxidation and phosphorylation uncouplers, and membranotropic substances.
With complete fasting, the body's food reserves last for several weeks. When the body is deprived of oxygen, death occurs within 2-3 minutes. Therefore, hypoxia is the most common reason hypoenergetic states, and brain hypoxia is the direct cause of death. Among resuscitation procedures, the leading place is occupied by measures aimed at restoring the supply of oxygen to organs.