The chemiosmotic theory can account for respiratory control and the action of uncouplers The action of uncouplers
chemiosmotic theory can account for respiratory control and the action of uncouplersThe electrochemical potential difference across the membrane, once established as a result of proton translocation, inhibits further transport of reducing equivalents through the respiratory chain unless discharged by back translocation of protons across the membrane through the vectorial ATP synthase. This in turn depends on availability of ADP and Pi. Uncouplers (eg, dinitrophenol) are amphipathic and increase the permeability of the lipoid inner mitochondrial membrane to protons, thus reducing the electrochemical potential and shortcircuiting the ATP synthase. In this way, oxidation can proceed withoutphosphorylation.
Impermeability of the inner mitochondrial membraneThe relative impermeability of the inner
mitochondrial membrane necessitates exchange transporters. Exchange diffusion systems are present in the membrane for exchange of anions against OH- ions and cations against H+ ions. Suchsystems are necessary for uptake and output of ionized metabolites while preserving electrical and osmotic equilibrium. The inner bilipoid mitochondrial membrane is freely permeable to uncharged small molecules, such as oxygen, water, CO2, and NH3, and to monocarboxylic acids, such as 3-hydroxybutyric, acetoacetic, and acetic. Long-chain fatty acids are transported into mitochondria via the carnitine system, and there is also a special carrier for pyruvate involving a symport that utilizes the H+ gradient from outside to inside the mitochondrion. However, dicarboxylate and tri- carboxylate anions and amino acids require specific transporter or carrier systems to facilitate their passage across the membrane. Monocarboxylic acids penetrate more readily in their undissociated and more lipid-soluble form. The transport of di- and tricarboxylate anions isclosely linked to that of inorganic phosphate, which penetrates readily as the H2PO4 – ion in exchange for OH–.
Ionophores permit specific cations to penetrate membranesIonophores are lipophilic molecules that
complex specific cations and facilitate their transport through biologic membranes, eg, valinomycin (K+). The classic uncouplers such as dinitrophenol are, in fact, proton ionophores.
A proton-translocating transhydrogenase is a source of intramitochon-drial NADPH
Energy-linked transhydrogenase, a protein in the inner mitochondrial membrane, couples the passage of protons down the electrochemical gradient from outsid to inside the mitochondrion with the transfer of H from intramitochondrial NADH to NADPH for intramitochondrial enzymes such as glutamate dehydrogenase and hydroxylases involved in steroid synthesis.
Oxidation of extramitochondrial NADH is mediated by substrate shuttles
NADH cannot penetrate the mitochondrial membrane, but it is produced continuously in the cytosol by 3-phosphoglyceraldehyde dehydrogenase, an enzyme in the glycolysis sequence. However, under aerobic conditions,extramitochondrial NADH does not accumulate and is presumed to be oxidized by the respiratory chain in mitochondria. The transfer of reducing equivalents through the mitochondrial membrane requires substrate pairs, linked by suitabledehydrogenases on each side of the mitochondrial membrane. The mechanism of transfer uses glycerophosphate shuttle. Since the mitochondrial enzyme is linked to the respiratory chain via a flavoprotein rather than NAD, only 2 mol rather than 3 mol of ATP are formed per atom of oxygen consumed. Although this shuttle is present in some tissues (eg, brain, white muscle), in others (eg, heart muscle) it is deficient. It is therefore believed that the malate shuttle system is of more universal utility. The complexity of this system is due to the impermeability of the mitochondrial membrane to oxaloacetate, which mustreact with glutamate and transaminate to aspartate and á-ketoglutarate before transport through the mitochondrial membrane and reconstitution to oxaloacetate in the cytosol.
Ion transport in mitochondria is energy-linked
Mitochondria maintain or accumulate cations such as K+, Na+, Ca2+, and Mg2+, and Pi. It is assumed that a primary proton pump drives cation exchange.
The creatine phosphate shuttle facilitates transport of high-energy phosphate from mitochondria
The creatine phosphate shuttle augments the functions of creatine phosphate as an energy buffer by acting as a dynamic system for transfer of high-energy phosphate from mitochondria in active tissues such as heart and skeletal muscle. An isoenzyme of creatine kinase is found in the mitochondrial intermembrane space, catalyzing the transfer of high-energy phosphate to creatine from ATP emerging from the adenine nucleotide transporter. In turn, the creatine phosphate is transported into the cytosol via protein pores in the outer mitochondrial membrane, becoming available for generation of extramitochondrial ATP.
Intex Questions
I. Choose the best answer
1. Chemically, the removal and the gain of electrons is defined respectively as
(a) Oxidation and reduction
(b) Reduction and oxidation
(c) Oxidation and dehydrogenase
(d) Reduction and dehydrogenase
2. Chemical carcinogens (xenobiotics) are metabolized by the enzymes system known as
(a) Cytochrome P450 (b) Xanthine oxidase
(c) Succinate dehydrogenase (d) Hydroperoxides
3. Oxidative phosphorylation is inhibited usually with fatal consequences by
(a) Quinalones (b) Cyanide
(c) Anacin (d) Amoxycillin
4. The action of uncouplers is to dissociate oxidation in the respiratory chain from
(a) Gluconeogenesis (b) Glycolysis
(c) TCA cycle (d) Phosphorylation
5. This antibiotic completely blocks oxidation and phosphorylation by acting on a step in phosphorylation.
(a) Dinitrophenol (b) Benzpyrene
(c) Oligomycin (d) Morphine
II. Fill in the blanks
6. Flavoprotein enzymes contain ……………… or ……………… as prosthetic
groups.
7. Generally, NAD-linked dehydrogenases catalyze ……………… reactionsm in the oxidative pathways of metabolism.
8. ……………… protect the body against harmful peroxides.
9. The function of ……………… is assumed to be the destruction of hydrogen peroxide formed by the action of oxidases.
10. Cytochromes P450 are an important superfamily of heme containing ………………
III. Match the following
11. Mitochondria (a) Energy currency of the cell
12. Cardiolipin (b) Uncouplers
13. ATP (c) Powerhouses of the cell
14. Valinomycin (d) Phospholipid
15. Dinitrophenol (e) Ionophores
1. Oxidation is defined as the removal of electrons and reduction as the gain of electrons.
2. Many drugs, pollutants, and chemical carcinogens (xenobiotics) are metabolized oxygenases known as cytochrome P450 system.
3. Enzymes involved in oxidation and reduction are called oxidoreductases and are classified into four groups: oxidases, dehydrogenases, hydroperoxidases, and oxygenases.
4. Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups. FMN and FAD are formed in the body from the vitamin riboflavin.
5. Oxidation-reduction reactions carried out by dehydrogenases are specific for their substrates but often utilize common coenzymes or hydrogen carriers, eg, NAD+.
6. The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe3+ and Fe2+ during oxidation and reduction.
7. Cytochromes are also found in the endoplasmic reticulum (cytochromes P450 and b5), and in plant cells, bacteria, and yeasts.
8. Two type of enzymes found both in animals and plants are peroxidases and catalase. Hydroperoxidases protect the body against harmful peroxides.
9. Accumulation of peroxides can lead to generation of free radicals, which in turn can disrupt membranes and perhaps cause cancer and atherosclerosis.
10. Mitochondrial cytochrome P450 systems are found in steroidogenic tissues such as adrenal cortex, testis, ovary, and placenta and are concerned withthe biosynthesis of steroid hormones from cholesterol.
11. The potential toxicity of oxygen is due to its conversion to superoxide in tissues and the enzyme superoxide dismutase is responsible for its removal.
12. Mitochondria are termed as the “powerhouses” of the cell. Respiration is coupled to the generation of the high-energy intermediate, ATP, by oxidative phosphorylation, and the chemiosmotic theory offers insight into how this is accomplished.
13. A number of drugs (eg, amobarbital) and poisons (eg, cyanide, carbon monoxide) inhibit oxidative phosphorylation, usually with fatal consequences.
14. Mitochondria contain the respiratory chain, which collects and transports reducing equivalents directing them to their final reaction with oxygen to form water, the machinery for trapping the liberated free energy as highenergy phosphate, and the enzymes of ß-oxidation and of the citric acidcycle that produce most of the reducing equivalents.
15. Mitchell’s chemiosmotic theory postulates that the energy from oxidation of components in the respiratory chain is coupled to the translocation of hydrogen ions (protons, H+) from the inside to the outside of the inner mitochondrial membrane. The electrochemical potential difference resulting from the asymmetric distribution of the hydrogen ions is used to drive the
mechanism responsible for the formation of ATP.
16. Uncouplers (eg, dinitrophenol) are amphipathic and increase the permeability of the lipoid inner mitochondrial membrane to protons, thus reducing the electrochemical potential and short-circuiting the ATP synthase. In this way,
oxidation can proceed without phosphorylation.
17. Ionophores are lipophilic molecules that complex specific cations and facilitate their transport through biologic membranes, eg, valinomycin (K+).
Terminal Question
1. Write short note on electron transfer chain.
2. Write short note on oxidative phosphorylation.
Answers to Indext questions
I. 1. (a) 2. (a) 3. (b) 4. (d) 5. (c)
II. 6.Flavin mononucleotide (FMN) or Flavin adenine dinucleotide (FAD)
7. Oxidoreduction8. Hydroperoxidases9. Catalase10. Monooxgenases
III. 11. (c) 12. (d) 13. (a) 14. (e) 15. (b)
