Electron transport chain inhibitors
These substances target specific enzyme complexes, interrupting electron transport and disrupting normal cellular respiration.
- Complex I (NADH dehydrogenase). The function of this complex is disrupted by barbiturates (e.g., amobarbital sodium) and rotenone. They block electron transfer within the enzyme at the site between iron-sulfur clusters and ubiquinone (coenzyme Q).
- Complex III ($QH_2$ dehydrogenase). The main inhibitor at this stage is antimycin A.
- Complex IV (cytochrome c oxidase). The terminal segment of the chain is sensitive to such potent toxins as cyanides, carbon monoxide (CO), and hydrogen sulfide ($H_2S$).
Toxic effects of barbiturates
High doses of barbiturates (particularly amobarbital sodium) cause a profound energy crisis in cells.
By blocking the first complex, these drugs completely stop the oxidation of NADH-dependent substrates, which are a vital source of energy for the entire organism. Due to the blockade, proton pumping stops, no electrochemical gradient is established at the first complex, and consequently, ATP synthesis drops sharply.
The central nervous system is the most sensitive to energy deprivation. The primary cause of death in severe barbiturate poisoning is energy deficit in the neurons of the respiratory center located in the medulla oblongata. This leads to function depression, respiratory arrest, and death.
Cyanide poisoning and tissue hypoxia
Cyanides (e.g., hydrogen cyanide $HCN$) are dangerous cellular poisons causing histotoxic (tissue) hypoxia.
Their target is cytochrome c oxidase (Complex IV). This heme-protein enzyme contains two hemes ($a$ and $a_3$) and two copper centers ($Cu_A$ and $Cu_B$). Heme $a_3$ together with copper $Cu_B$ forms a binuclear center where molecular oxygen is normally reduced.
Mechanism of toxicity:
- The cyanide anion ($CN^-$) has an extremely high affinity for ferric iron ($Fe^{3+}$).
- During normal catalysis, iron in cytochrome c oxidase transitions to the $Fe^{3+}$ state, making the enzyme an ideal target.
- By binding to $Fe^{3+}$ in heme $a_3$, cyanide tightly blocks the terminal segment of the electron transport chain.
A characteristic clinical sign is bright red venous blood. Since tissues cannot consume oxygen due to malfunctioning cytochrome c oxidase, blood passes through the capillary bed unchanged, retaining oxygenated hemoglobin ($HbO_2$). Death occurs due to CNS cell death and respiratory arrest against the background of a complete absence of ATP, despite normal or even elevated oxygen levels in the bloodstream.
Oxidative phosphorylation inhibitors
Unlike electron transport blockers, there are compounds that act on the enzyme apparatus responsible for ATP synthesis.
A classic representative of this group is the antibiotic oligomycin. It is a specific inhibitor of ATP synthase. Oligomycin does not directly interfere with electron movement along the mitochondrial respiratory chain, but it completely blocks the phosphorylation process—the generation of ATP using the energy of the proton gradient.
Briefly: answers to topical questions
Which substances are uncouplers of tissue respiration and oxidative phosphorylation?
Uncouplers of tissue respiration and oxidative phosphorylation include free fatty acids and unconjugated bilirubin.
- Free fatty acids are involved in proton transport across the mitochondrial membrane, providing non-shivering thermogenesis during cold adaptation.
- Unconjugated bilirubin—in neonatal jaundice, it crosses the blood-brain barrier into neurons, reducing ATP synthesis and causing a pyrogenic effect.
Which antidotes are used in cyanide poisoning?
In cyanide poisoning, high concentrations of methylthioninium chloride and nitrofural are administered intravenously.
- Methylthioninium chloride (Methylene blue) oxidizes hemoglobin into methemoglobin, which then binds cyanides, forming non-toxic cyanmethemoglobin.
- Nitrofural (Nitrofural) has high surface activity and is used in toxicology as an antidote for intravenous administration.
For quick memorization of toxin targets: "Barbiturates — Complex I, Antimycin — Complex III, Cyanide — Complex IV" (B-1, A-3, C-4).
Class and exam questions
Why does venous blood turn bright red in cyanide poisoning?
Due to the blockade of cytochrome c oxidase, tissues completely lose the ability to utilize oxygen from the blood. Blood passes through the capillary bed without releasing oxygen and returns to the veins in an oxygenated state, retaining a bright arterial color.
How does oligomycin affect cellular respiration?
This antibiotic inhibits the enzyme ATP synthase. It does not stop electron transport directly, but makes ATP synthesis impossible by blocking oxidative phosphorylation.
Which specific ion does cyanide bind to in mitochondria?
The cyanide anion has a high chemical affinity for ferric iron ($Fe^{3+}$), which is formed during catalysis as part of heme $a_3$ of the fourth respiratory chain complex.
What else to study on this topic
- Organization of the binuclear center (heme $a_3$ and $Cu_B$) in cytochrome c oxidase and the mechanism of oxygen reduction.
- The role of iron-sulfur clusters in the function of NADH dehydrogenase (Complex I).
- Pathogenesis of histotoxic hypoxia in carbon monoxide (CO) and hydrogen sulfide poisoning.
- Biochemical mechanisms of energy dependence in the neurons of the respiratory center.
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