Electron Transport Chain Inhibitors
These substances specifically target particular enzyme complexes, interrupting electron transport and disrupting normal cellular respiration.
- Complex I (NADH dehydrogenase). The function of this complex is impaired by barbiturates (e.g., amobarbital) and rotenone. They block electron transfer within the enzyme 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 powerful poisons such as cyanides, carbon monoxide (CO), and hydrogen sulfide ($H_2S$).
Toxic Action of Barbiturates
High doses of barbiturates (specifically amobarbital) cause a profound cellular energy crisis.
By blocking Complex I, these drugs completely halt the oxidation of NADH-dependent substrates, which are critical energy sources for the entire organism. Due to this blockade, proton pumping ceases, no electrochemical gradient is generated at Complex I, and as a result, 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 depletion 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 (such as hydrogen cyanide, $HCN$) are dangerous cellular poisons that cause histotoxic (tissue) hypoxia.
Their target is cytochrome c oxidase (Complex IV). This hemoprotein enzyme contains two heme groups ($a$ 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 cycles to the $Fe^{3+}$ state, making the enzyme an ideal target.
- By binding to $Fe^{3+}$ in heme $a_3$, cyanide irreversibly blocks the terminal segment of the electron transport chain.
A characteristic clinical sign is bright red venous blood. Because tissues cannot consume oxygen due to non-functional cytochrome c oxidase, blood passes through the capillary bed unchanged, retaining oxygenated hemoglobin ($HbO_2$). Death results from central nervous system cell death and respiratory arrest amidst a complete lack of ATP, despite normal or even elevated oxygen levels in the bloodstream.
Inhibitors of Oxidative Phosphorylation
Unlike electron transfer blockers, there are compounds that act directly on the enzymatic machinery 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 phosphorylation—the generation of ATP using the energy of the proton gradient.