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Table 6 Redox potential (RP)

In the living cells oxygen rarely takes part in direct oxidation of the substrates. In most cases, oxidation involves many steps. This is because molecular oxygen and most cell substrates differ too widely in energy level for these to react directly. This difference is decreased by means of various intermediate substances which can transfer hydrogen atoms or electron (look table of RP).

From this table it is clearly that the main respiratory chain in mitochondria proceeds from the NAD-linked dehydrogenase systems on the other hand, through FP and Cyt, to molecular oxygen on the other. Not all substrates are linked to the respiratory chain through NAD-specific dehydrogenases: some, because their redox potentials are more positive (eg, fumarate/succinate), are linked directly to FP dehydrogenases, which in turn are linked to the cytochromes of the respiratory chain.

Stages of biologic oxidation by the respiratory chain

1. formation of NADH2 in dehydrogenase reactions, f.e. malate + NAD à oxaloacetic acid + NADH2 (in CAC)

2. formation of FADH2. FAD is a coenzyme for several enzymes termed flavoproteins (FP), f.e. succinic acid + FAD à fumarate + FPH2 (FADH2)

3. transfer of 2 H atoms from NADH2 to FMN takes up these H atoms is itself converted to FMNH2 – NADH2 + FP (FMN) à NAD + FPH2 (FMNH2), in this reaction enough energy is released to produce one mole of ATP from ADP and Pi;

4. in the next step FPH2 transfer 2H to coenzyme Q – FPH2 + CoQ à FP + CoQH2

5. oxidation of CoQH2. Reduced CoQ can be converted to its oxidized form by losing the 2 H atoms but the released H atoms now splits into protons and electrons: CoQH2à CoQ + 2H⁺+2ē. The 2 protons enter the mitochondrial pool of the H+ ions, while electrons are taken up by the respiratory pigments called cytochromes (Cyt).

6. The 1-st Cyt take part in the process is Cytb, 2 molecules of which are reduced by the 2 electrons released from CoQ: 2Cytb⁺⁺⁺ + 2ē à2Cytb⁺⁺

7. In the next stage the reduced Cytb passes its electron to the next member of the Cyt system to Cytc1. Sufficient energy is evolved in this reaction to produce 1 ATP from ADP and Pi:

2Cytb⁺⁺ + 2Cytc1⁺⁺⁺ à 2Cytb⁺⁺⁺ + 2Cytc1⁺⁺ (~ATP). In this way Cytb gets ixidized, while Cytc1 gets reduced.

8. in the next stage Cytc1⁺⁺ passes its lectron to Cytc⁺⁺⁺ : 2Cytc1⁺⁺ + 2Cytc⁺⁺⁺ à2Cytc1⁺⁺⁺ + 2Cytc⁺⁺

9. Cytc⁺⁺ now passes its electron to Cyta⁺⁺⁺ converting it Cyta⁺⁺: 2Cytc⁺⁺ + 2Cyta⁺⁺⁺ à2Cytc⁺⁺⁺ + 2Cyta⁺⁺ . Then Cyta⁺⁺ passes its electron to the final member of Cyt system – to Cyta3⁺⁺⁺ . on this stage sufficient energy is evolved to produce 1 ATP from ADP and Pi: 2Cyta⁺⁺ + 2Cyta3⁺⁺⁺ à2Cyta⁺⁺⁺ + 2Cyta3⁺⁺ (~ATP). It is showed that Cyta and Cyta3 occur as a complex termed Cyta-a3 complex or cytochromeoxidase, which is attached to a protein and ions Cu

10. Cyta3 is autooxidizable. 2 molecules of reduced Cyta3 take part in the final reaction in which the 2 hydrogen ions liberated in the body fluids from CoQH2 also participate: 2Cytaa3⁺⁺ + 2H⁺ + ½ O₂ à 2Cytaa3⁺⁺⁺ + H2O. Functionally and structurally the components of the respiratory chain present in the inner mitochondrial membrane as 4 protein lipid respiratory chain complexes. These complexes have a definite spatial orientation in the membranes. Cytc is the only soluble Cyt and together with CoQ, seems to be a more mobile component of the respiratory chain connecting the fixed complexes

Nonphysiologic substrates can include in the different stages of biologic oxidation: succinate à Cytb; ascorbate à Cytb and Cytc; soluble Cytc (exogenous) à Cytc

Oxidative phosphorylation

ADP is a molecule that captures in the form of high energy phosphate, some of the free energy released by catabolic processes. The resulting ATP passes on this free energy to drive those processes requiring energy. Thus, ATP has been called the energy “currency” of the cell.

Examination of intact mitochondria reveals: that when substrates are oxidized via a NAD-linked dehydrogenases and the respiratory chain, 3 mol of ADP to form 3 mol of ATP per ½ molof O2 consumed, i.e. the P/O ratio is 3. On the other hand, when a substrate is oxidized via a FP only 2 mol of ATP are formed, i.e. P/O=2. These reactions are known as oxidative phosphorylation at the respiratory chain level. Dehydrogenations in the pathway of catabolism of glucose in both glycolysis and CAC, plus phosphorulations at the substrate level, can now account for nearly 42% of the free energy resulting from the combustion of glucose captured in the form of high-energy phosphates. It is evident that the respiratory chain is responsible for a large proportion of total ATP formation. The concept of P/O coefficient, respiratory control

Coefficient P/O is a ratio of quantity of molecules of inorganic phosphate used on formation of ATP on each engaged oxygen atom. This coefficient may be equal 3 or 2. P/O is equal 3 when the BO beginns from NADH2. Then 3 points of conjugation of BO and OP. P/O is equal 2 when BO beginns from FPH2. Then 2 points of conjugation of BO and OP.

Respiratory control: OP controls the velocity of BO. This is property of undamaged mitochondrial membranes. In the presence of all components of BO, but in absence of ADP the velocity of BO is decreased. In a large amount of ADP and small content of ATP the velocity of BO is increased.

The relation between BO and OP can be proved due to usage of disconnectors, e.g. nitro-and galogenoderivatives of Phenols, some antibiotics, cyanides, CO, thyroxine, progesterone, these compounds disconnect relation between BO and OP. It is results in unusage of energy of BO on ATP formation (i.e., OP doesn't occur). All energy of BO is discharged as heat through perspiration.

Inhibition of oxidative phosphorylation.

Various substances are known to inhibit formation of ATP without interrupting transport of hydrogen or electrons through the respiratory chain. This phenomenon is known as “uncoupling” of oxidative phosphorylation. The important examples of such uncouples are dinitrophenol, dicoumaral, v.K antagonists, a glycoside (atractyloside), thyroxine.

Normally the process of oxidation and phosphorylation are coupled in such a way, that about 42% of the energy released in oxidation-reduction is trapped as useful energy in the form of ATP. Some energy is used by the mitochondria themselves for doing osmotic work and for mechanical work, contraction.the remaining energy appears as heat and is used to warm the body at colder temperature. There are many substances like dinitrophenol, some antibiotics, thyroxine, arsenate, bilirubin, salicylates and v.K-antogonists, which can uncouple this process. These substances, called uncoupling agents, cause a decreased formation of ATP and relatively greater loss of energy as body heat.

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