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Файл:Carbohydrates. Bioenergetics. Study manual
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(γ-glutamylcylcysteinyl glycine). Glutathione sulfhydrilic group (GSH) serves as an
electron donor and, oxidizing, forms a disulfide form of glutathione, in which
2 molecules of glutathione are bound through the disulfide group.
H2O2 + 2 GSH → 2 H2O + G-S-S-G.
Oxidized glutathione is reduced by glutathione reductase:
GS-SG + NADPH + H+ → 2 GSH + NADP+.
Glutathione peroxidase, which restores lipid hydroperoxides in membranes,
uses selenium as a coenzyme (a necessary food trace element). With its lack of antioxidant protection activity is reduced.
2.7. Detoxification of xenobiotics
Xenobiotics are substances alien to the body, which are synthetic materials,
products of human economic activity, household chemicals, detergents, perfumes.
Increasing pollution of the environment leads to pollution of the body, damage to
cells, mutations leading to malignant and hereditary diseases.
Hydrophilic xenobiotics are excreted from the body in the same form as urine,
hydrophobic xenobiotics can be trapped in tissues, binding to proteins or forming a
complex of lipids in cell membranes. Over time, the accumulation of foreign matter
in the cells will lead to the disruption of their functions. In order to remove such
substances unnecessary for the organism in the process of evolution the mechanisms
of their detoxification (deactivation) and excretion from the organism were developed. As a result of successive processes of xenobiotic modification, these substances become more hydrophilic and are excreted with urine. Substances that are
more hydrophobic or have a high molecular weight (>300 Da) are more often excreted with bile into the intestine and then removed with feces.
There are three main phases of xenobiotic detoxification:
The first phase, microsomal oxidation, involves cytochrome P450, specific oxi-
dases, monoxygenases (hydroxylation), and conjugation enzymes.
Cytochrome P450 is an integral hemoprotein containing a prosthetic group of
hematoprotein and has binding sites for O2 and substrate. 150 genes encoding various cytochrome P450 isoforms were discovered. Each of P450 isoforms has many
substrates and differs from other P450 isoforms only by its protein part. Cytochrome
P450 transfers 2 electrons per 1 atom of oxygen molecule, which transforms into O
when interacting with 2 protons of O2- gives water. The second atom of the oxygen
molecule is included in the substrate RH, forming ROH.
The metabolism of xenobiotics may involve enzymes of the kidneys, lungs,
skin and gastrointestinal tract, but they are most active in the liver. The most possi-
2-
ble ways of modification are reactions of hydroxylation, sulfooxidation, oxidative
deamination, epoxidation, dealkylation.
Advantages of this phase of decontamination are:
1. Variety of metabolic pathways allowing to neutralize more than 7 000 sub-
stances.
2. High power in the main pathways of penetration into the body – gastrointes-
tinal tract, liver and lungs.
Disadvantages:
1. Low degree of protection at penetration through mucous membrane, wounds,
injections.
2. Absence or lack of activity in the vital organs – heart, brain, etc.
3. Possibility of toxicity. Thus, cytochrome P450 transforms chloroform, a means
of anesthesia, into a militant poisonous gas – phosgene. Polycyclic aromatic hydrocarbon-benzapyrene (procarcinogen) turns into carcinogenic dihydroxyepoxide.
Paracetamol – an anesthetic and antipyretic agent turns into a substance that is toxic
to the liver and kidneys.
The second phase – conjugation reactions, in which a foreign substance modified by enzyme systems binds to endogenous substrates – glucuronic acid, sulfuric
acid, glycine, glutathione. As a rule, the formation of bonds takes place in the OH- or
NH2-group of xenobiotics. The resulting conjugate is low-toxic and easily excreted
from the body with urine. Glucuronide, sulfate, thiosulphate and acetyl conjugation
are distinguished. They involve endogenous compounds formed in the body with the
use of energy: UDP-glucuronate, PAPS (3’phosphoadenosyl-5’phosphosulfate), thiosulfate, acetyl-coA.
This phase has an important advantage: all cells have enzymes responsible for
the addition of neutralizing molecules. Therefore, in the second phase, the entire
body's cells fight against toxins, which allows for effective implementation or completion of detoxification.
The third phase. Most xenobiotics as a result of metabolism become more hydrophilic, come into blood plasma, from where they are removed by kidneys with
urine. Substances with more hydrophobic or high molecular weight (>300 Da) are
more often excreted with bile into the intestines and then removed with feces. The
liver and kidneys play a crucial role in the elimination and excretion of most xenobiotics. However, despite the dominant role of the liver and kidneys in the metabolism of xenobiotics, other organs are also involved in this process. In the detoxification of the body, although to a lesser extent take the mucous membranes - gastrointestinal tract, lungs and upper respiratory tract. Due to the diffusion of xenobiotics
can also be excreted with the milk of nursing mothers and the secret of sweat, sebaceous and salivary glands.
Test questions on the topic of "Bioenergetics"
1. The role of mitochondria in energy metabolism, especially the structure of the
inner mitochondrial membrane and the localization of enzymes of the respiratory chain.
2. The mechanism of conjugation of respiration and phosphorylation in mitochondria.
3. Scheme of formation of the proton potential in the course of respiration and
its role in the process of phosphorylation.
4. Chemiosmotic theory of Mitchell.
5. Structure and localization of H+-ATP synthase and its role in the phosphory-
lation reaction
6. Ways of ATP synthesis in the cell (substrate and oxidative phosphorylation).
7. Characteristic points of interface of tissue respiration and oxidative phos-
phorylation.
8. Full and short ETC, their energy value.
9. Free oxidative phosphorylation.
10. The general idea of microsomal oxidation and the role of mono - and di-
oxygenase.
11. Free radical oxidation and its regulators.
13. Substances that affect energy metabolism in the cells.
14. Drugs as regulators of energy metabolism.
Situational task on the theme of "Bioenergetic"
Task 1. Explain why during intensive physical work activates the rate of reactions of citrate cycle? Write reactions whose speed increases. Explain why?
Task 2. Directly in the reactions of the Krebs cycle oxygen is not involved. However, the citrate cycle is an aerobic process. Explain why it is inhibited
in the absence of oxygen.
Task 3. In the experiment to the homogenate, containing all the enzymes of citrate cycle and respiratory chain was added acetyl CoA. What will measuring the
amount of oxaloacetate and acetyl CoA before and after incubation?
a) was there an increase in the oxaloacetate? Explain the role of oxaloacetate in
the process.
b) whether the content has changed acetyl CoA? What happens to him in the
Krebs cycle?
Task 4. After suffering a severe illness the patient have any hypoenergetics
condition. The doctor recommended to the patient vitamins group B. Justify a doctor's appointment.
Task 5. When deficiency of vitamins group B may reduce oxidative decarboxy-
The composition of pyruvate
dehydrogenase enzyme complex
Coenzymes
cofactors
Vitamins
Place
of the reaction
lation of pyruvate. Explain the cause of this decline. Write the overall reaction of
oxidative decarboxylation of pyruvate, fill in the table:
Task 6. In the experiment with isolated mitochondria as the oxidizable substrate
used malate. Can the suspension of mitochondria to oxidize malate, if
a) on Wednesday to add rotenone;
b) together with rotenone add succinic acid. Argues response.
Task 7. How does the energy potential of the cell (ADP/ATP) for the rate of the
citric acid cycle? Justify your answer. Write the reaction cycle sensitive to changes
in energy potential.
Task 8. In the study of tissue respiration of muscles in vitro the researchers used
as a substrate of oxidation of succinate. Additional add to the environment of malonic acid was stopped the absorption of oxygen and the accumulated intermediate
metabolite in the Krebs cycle. Answer the question:
a) what is the cause of respiratory arrest?
b) is it possible to remove caused by malonate inhibition?
c) if Yes, in what way?
Task 9. In experimental animals we investigated the effect of antimycin A and
rotenone. It is shown that both these substances are toxic to the body. Knowing the
point of application of the action antimycin A and rotenone on ETC enzymes, explain:
a) what accounts for their toxicity?
b) decide which of these 2 compounds are more toxic.
Give the explanation.
Task 10. Under physiological conditions, the human body temperature above
the ambient temperature (36,6oC vs 20oC). Explain what caused this difference?
What role mitochondria play? Justify your answer.
Task 11. Prolonged use of an uncoupling agent 2,4-dinitrophenol as a drug
against obesity has had negative consequences: developed malaise, increased body
temperature, in some cases there is a lethal outcome. Explain:
a) what was the basis of the use of 2,4-dinitrophenol as a drug to reduce body
weight;
b) explain the reasons for developing complications.
Task 12. In the experiment with isolated mitochondria as studied substrate oxidation used α-ketoglutarate. Measured absorption O2and inorganic phosphorus,
which was measured by the ratio P/O. Imagine the scheme of the transport of electrons
and protons along the chain of electron transport from α - ketoglutarate to oxygen.
a) What is the coefficient of P/O with the full mate?
b) How will it affect the breathing rate and the ratio of the pairing, the addition
of rotenone?
q) How to change these settings if together with rotenone add succinate?
g) How will the ratio P/O if you add protonophore 2,4-dinitrophenol.
Problem 13. When the concentration of ADP in the cell, the speed of the tricarboxylic acid cycle is increasing rapidly. Increase activity of what enzyme (enzymes)
leads to the acceleration of reactions of the whole cycle? What is the mechanism of
activating effect of excess ADP?
Problem 14. The increase of the ATP concentration and NADH, H+leads to a
reduction in the rate of the tricarboxylic acid cycle. The activity of some enzymes
decreases with increasing concentration of ATP and NADH, H+in a cage? What is
the mechanism of the inhibitory effect of an excess of ATP and NADH, H+?
Task 15. The allocation of mitochondria and use them to study the rate of oxidative phosphorylation to the buffer system containing oxidizable substrate, ADP
and H3PO4usually add a small amount of cytochrome C extracted from any
source. Why is it necessary to add cytochrome C and why it is not necessary to allocate from the same source as the mitochondria?
Problem 16. Can the suspension of mitochondria in the presence of rotenone
oxidize succinate? Why?
Problem 17. Newborns in the neck and upper back has a special fatty tissue the so-called "brown fat", which in adults is missing. Brown color depends on the
high content of mitochondria. Brown fat has also animals in hibernation. In the mitochondria of brown fat per atom of absorbed oxygen, produces less than one ATP
molecule. What physiological function is determined by the lowest value of P/O in
brown fat tissue in newborns?
Problem 18. The patient with increased function of the thyroid gland a decrease
in weight, low-grade fever, irritability. Explain the reasons for the observed symptoms.
Task 19. Most of the substrates in the Krebs cycle is oxidized under the action
of NAD-dependent dehydrogenases. The exception is succinatedehydrogenase that
as a hydrogen acceptor uses FAD. Why FAD is a more appropriate acceptor hydrogen than NAD. Response using the known data: the normal redox potential
FAD/NADH2= –0.32 V, and FAD/FADH2= +0.05 V and the fumarate/succinate =
+0.03 V.
Task 20. People suffering from insomnia. Turning to the doctor, he got the recommendations. In particular, the doctor made an appointment for the night in small
doses of amylobarbital. Sick for a long time taken this drug, with the result that he
developed fatigue, sudden muscle weakness. Explain the causes and mechanism
complications.
Task 21. During the fire from the burning houses made the victim, who had
burns, but was in an unconscious state. With great difficulty managed to bring him
back to life. What is the cause of the serious condition of the victim and what steps
need to be taken to rescue the patient?
Task 21. The preparation of rat liver mitochondria was added NAD+. The activ-
ity of some enzymes of the Krebs cycle would increase?
To justify the answer:
1. Write the scheme of reactions of the Krebs cycle.
2. What is the function NAD+?
3. What enzymes of the Krebs cycle it works?
Problem 22. To the preparation of mitochondria added pyruvate labeled
14
Ccarbon atom by methyl group. What position is14Cin oxaloacetate after one turn
of the Krebs cycle?
To answer:
1. Write the reactions of the Krebs cycle.
2. Make sure the position of the label in each metabolite.
Problem 23. In the experiment with isolated mitochondria was determined by
the intensity of the Krebs cycle to NADH accumulation. Will the work of the Krebs
cycle, if stop the outflow of it restored equivalents?
To justify the answer remember:
1. In any reactions of the Krebs cycle NADH formed?
2. What enzymes catalyze these reactions?
3. What determines the speed of the Krebs cycle?
Task 24. How much ATP is formed during the full oxidation of 5 moles of pyruvate in the presence of rotenone and 2,4-dinitrophenol?
To perform the calculations:
1. Provide diagram of the oxidation of pyruvate to СО2и Н2О
2. Specify how to change the energy yield from the oxidation of pyruvate in the
presence of rotenone and 2,4-dinitrophenol (full separation).
Problem 25. How many ATP molecules synthesized during the oxidation of one
molecule of pyruvate to 2-oxoglutarate; one molecule of isocitrate to succinate; one
molecule of succinate to oxaloacetate, provided that dehydrogenase reactions involve the respiratory chain?
For calculations:
1. Write the reactions at these sites of the Krebs cycle.
2. Specify reactions associated with the respiratory chain.
3. Remember how much ATP is formed during the oxidation of NADH and
FADH2.
Problem 26. Rotenone (a toxic substance produced by one species) sharply inhibits the activity of mitochondrial NADH dehydrogenase. Toxic antibiotic antimycin strongly inhibits the oxidation of ubiquinol. Let's say that both these substances
block the sites of the respiratory chain with equal effectiveness. Which one will be
more powerful poison? Give a reasoned answer.
To justify the answer remember:
1. What are the blockers of the respiratory chain?
2. What areas of the respiratory chain receives hydrogen from NADH and
FADH2?
Problem 27. To the preparation of isolated mitochondria added to amytal. Will
this affect oxygen consumption?
To justify the answer remember:
1. How does amytal in the respiratory chain?
2. On what part of the respiratory chain, it manifests its effect?
Task 28. Some bacteria, yeast, parasitic worms do not need oxygen. Which of the
two ways of making ATP is used by these organisms for the accumulation of energy?
To answer remember:
1. What is phosphorylation?
2. What is a substrate and oxidative phosphorylation?
3. What these types of phosphorylation are different from each other?
Task 29. Adding to the mitochondria oligomycin causes a reduction as transfer
of electrons from NADH to O2and the rate of formation of ATP. Subsequent addition of 2,4-DNP increases the rate of electron transfer without the accompanying
change in the velocity of ATP synthesis. What kind of reaction inhibits oligomycin?
To answer remember:
1. Than is 2,4-DNP for the respiratory chain?
2. What is the effect of different blockers of the action of uncouplers?
Problem 30. The immediate introduction of methylene blue has a very effective
therapeutic effect in cases of poisoning by cyanides. What is the heart of its detoxification action, given that methylene blue is able to oxidize part of hemoglobin
(theFe2+) the blood methemoglobin (Fe3+)?
To answer remember:
1. What are the similarities of the prosthetic groups of cytochromes and hemo-
globin?
2. What is the valency of iron which react with cyanide?
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УЧЕБНОЕ ИЗДАНИЕ
УГЛЕВОДЫ. БИОЭНЕРГЕТИКА
Учебное пособие
На английском языке
Beeva Dzhuletta Anatol’evna
Mukozheva Radina Aslanovna
Grineva Larisa Georgievna et. al.
CARBOHYDRATES. BIOENERGETICS
Manual of biological chemistry for independent work
of students specialties "General Medicine", "Dentistry"
Нальчик, «Каб.-Балк. ун-т», 2022
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