Energy Capture in Respiratory Chain| Biologic Oxidation

In this article we will discuss about the Energy Capture in Respiratory Chain in Mitochondria.

1. When substrates are oxidized through an NAD-linked dehydrogenase, 3 mol inor­ganic phosphate are incorporated into 3 mol of ADP to form 3 mol of ATP per half mol of O₂ (1/2 O₂) consumed, i.e. the P; O ratio = 3.

2. When a substrate is oxidized through a flavoprotein-linked dehydrogenase, only 2 mol of ATP are formed, i.e. P: O ratio = 2. These reactions are known as oxidative phosphorylation at the respiratory chain level.

3. There must be a redox potential of about 0.2 volts or free energy change of about 9 K Cal between components of the respira­tory chain if that particular site is to sup­port the coupled formation of 1 mol of ATP.

Four sites fulfil these requirements in the respiratory chain:

(a) Between NAD and flavoprotein.

(b) Between flavoprotein and cytochrome b.

(c) Between cytochrome b and cyto­chrome c.

(d) Between cytochrome a and oxygen.

4. When ADP is deficient in the presence of excess substrate, 3 crossover points can be identified. These crossover points co­incide with 3 of the possible sites on ther­modynamic grounds. The 3 sites of phosphorylation have been designated as sites I, II and III, respectively. So P : O ratio of succinate is only 2 as site I is bypassed by the flavoprotein-linked succinate dehy­drogenase.

Inhibitors:

(a) Inhibitors act at three sites of the respira­tory chain to arrest respiration by block­ing this chain.

(b) The first is inhibited by barbiturates such as amobarbital, antibiotic piericidin A, fish poison rotenone etc. These inhibitors pre­vent the oxidation of substrates that com­municate directly with the respiratory chain via an NAD-linked dehydrogenase by blocking the transfer from FeS to Q.

(c) Antimycin A and dimercaprol inhibit the respiratory chain between cyt b and cyt c.

(d) H₂S, CO, cyanide totally block respiration because they inhibit cytochrome oxidase.

(e) Carboxin and TTFA inhibit transfer of re­ducing equivalents from succinate dehy­drogenase to Q, whereas malonate is a competitive inhibitor of succinate dehy­drogenase.

Free Energy Change:

ΔG = -nF DE’₀, coulomb joules.

where, n = number of electrons transferred.

F = Faraday constant.

ΔE’₀ = Difference in redox potential between two systems.

ΔG = Free energy.

This can be converted into calories by divid­ing by the factor 4.18.

Oxidative Phosphorylation:

The process by which ADP is phosphorylated by P_i to ATP in the respiratory chain is called oxidative phosphorylation.

This process is limited to the mitochondria. Various oxidations occur in other parts of the cell and liberate heat instead of energy.

Various mechanisms have been proposed for this process. Chance and Williams have proposed the following mechanism in which I and X are in­termediates of unknown nature. B is the oxidized form of a carrier in the chain. AH₂ is the reduced carrier which reduces B.

Uncoupling Agent:

A large number of substances uncouple oxidative phosphorylation in the respiratory chain; i.e. they prevent the formation of ATP but permit oxidation to proceed with the generation of heat. One of the uncoupling agents is 2, 4-dinitrophenol (DNP). DNP uncouples phosphorylation by the hy­drolysis of X~I or X~Pi. It has been considered to increase the ATPase activity of mitochondria:

Other uncoupling agents are:

(i) Methylene blue,

(ii) Arsenite,

(iii) Dicoumarol,

(iv) Aureomycin,

(v) Gramicidin.

Thyroxin is an uncoupling agent and causes swelling of the mitochondria.

Inhibitors:

1. Oligomycin completely blocks oxidation and phosphorylation in intact mitochon­dria.

2. Atractyloside inhibits oxidative phos­phorylation. It inhibits the transporter of ADP into the mitochondrion and of ATP out of the mitochondrion.

Coenzyme Q (Ubiquinone, CoQ):

1. It exists in mitochondria in the oxidized quinone form under aerobic conditions and in the reduced quinol form under anaerobic conditions.

2. It has a structure very similar to vitamin K and vitamin E. It is also similar to plastoquinone found in chloroplast. All of these possess polyisoprenoid side chain.

3. It is the additional carrier present in the respiratory chain linking the flavoprotein to cytochrome b.

4. It is a constituent of the mitochondrial lipids.

5. In mitochondria, there is a large stoichometric excess of CoQ compared to other members of the respiratory chain.

6. It is possible that there is more than one pool of CoQ and that some of it is not in the direct pathway of oxidation.

Role of High Energy Phosphates:

Low-energy Phosphates:

These are the ester-phosphates i.e. AMP found in the intermedi­ates of glycolysis.

High-energy Phosphates:

The value is higher than that of ATP, e.g., ATP, ADP, phosphoenolpyruvate, creatine phosphate, arginine phosphate.

High-energy Compounds:

Acetyl-CoA, active methionine (S-adenosylmethionine) and UDPG (Uridine diphosphate glucose).

Lipmann introduced the symbol ~ p indicating high-energy phosphate bond. ATP contains 2 high- energy phosphate groups and ADP contains one.