Arsenic exerts its toxic effects primarily by inhibiting which enzyme complex involved in cellular respiration?

Explanation

## Arsenic Toxicity: Mechanism of Action **Key Point:** Arsenic (trivalent arsenite, As³⁺) inhibits the pyruvate dehydrogenase complex by binding to sulfhydryl (-SH) groups on the enzyme's lipoic acid cofactor, blocking the conversion of pyruvate to acetyl-CoA. ### Molecular Mechanism ```mermaid flowchart TD A[Arsenite As³⁺]:::action --> B[Binds to SH groups on lipoic acid]:::action B --> C[Inhibits Pyruvate Dehydrogenase Complex]:::urgent C --> D[Blocks Pyruvate → Acetyl-CoA]:::urgent D --> E[Decreased ATP production]:::outcome E --> F[Cellular energy depletion]:::outcome E --> G[Lactate accumulation]:::outcome G --> H[Metabolic acidosis]:::outcome ``` ### Why Pyruvate Dehydrogenase? The pyruvate dehydrogenase complex contains multiple lipoic acid cofactors with critical sulfhydryl groups. Arsenic has a strong affinity for these -SH groups, forming stable complexes that render the enzyme inactive. This is the primary mechanism of arsenic toxicity, not direct inhibition of the electron transport chain. **High-Yield:** Arsenic's affinity for sulfhydryl groups explains why it also inhibits other SH-containing enzymes (α-ketoglutarate dehydrogenase, succinate dehydrogenase), but pyruvate dehydrogenase is the most clinically significant target. ### Cellular Consequences | Effect | Mechanism | Clinical Manifestation | |--------|-----------|------------------------| | ATP depletion | Blocked acetyl-CoA entry into TCA cycle | Weakness, organ failure | | Lactate accumulation | Shift to anaerobic metabolism | Metabolic acidosis | | ROS generation | Mitochondrial dysfunction | Oxidative stress, apoptosis | | Peripheral neuropathy | Myelin damage from energy depletion | Sensory/motor deficits | **Clinical Pearl:** The metabolic acidosis seen in acute arsenic poisoning is due to lactate accumulation from impaired aerobic metabolism and shunting to anaerobic pathways. **Mnemonic:** **LIPOIC** — Lipoic acid (cofactor) is the target; Inhibition of pyruvate dehydrogenase; Pyruvate cannot enter TCA; Oxidative metabolism blocked; Intracellular ATP drops; Cellular dysfunction ensues.