"This enzyme stopped working."
Toma showed experimental data.
Rei confirmed. "Because you added an inhibitor."
"Inhibitor?" Kana asked.
"A substance that stops enzyme activity. Catalyst poisoning."
Toma was confused. "Poisoning?"
"The enzyme is trapped by the inhibitor and can't do its original job."
Kana opened her notebook. "How does it stop it?"
Rei drew a diagram. "Competitive inhibition, noncompetitive inhibition, irreversible inhibition. Three methods."
"What's competitive inhibition?"
"A molecule similar to substrate enters the active site."
Toma understood. "Fighting for seats?"
"Exactly. When the inhibitor sits, substrate can't enter."
Kana asked for an example. "What kind of inhibitor?"
"Malonate and succinate dehydrogenase," Rei answered.
"Malonate resembles succinate. So it fools the enzyme."
Toma laughed. "An impostor?"
"But the enzyme can't react. So activity decreases."
Kana asked. "What if we increase substrate?"
"Sharp. Competitive inhibition can be overcome by substrate concentration."
"Why?"
"Inhibitor and substrate compete for seats. If substrate is abundant, substrate wins."
Rei continued. "Noncompetitive inhibition is different. Binds outside the active site."
"Not the active site?"
"Allosteric site. A different location."
Toma asked. "And that stops the enzyme?"
"When inhibitor binds, enzyme shape changes. Active site distorts."
Kana understood. "Even if substrate enters, it can't react?"
"Exactly. So increasing substrate is useless."
Rei redrew the diagram. "Irreversible inhibition is more serious."
"Irreversible?"
"Forms covalent bonds with enzyme. Permanently loses activity."
Toma was surprised. "Can't be cured?"
"That enzyme molecule is unusable. Must synthesize new ones."
Kana asked for examples. "What substances?"
"Nerve gas, sarin. Irreversibly inhibit acetylcholinesterase."
"Dangerous..."
"That's why it was used as poison gas," Rei said quietly.
Toma gave another example. "Are medicines also inhibitors?"
"Yes. Aspirin inhibits cyclooxygenase."
"Stops pain?"
"COX produces inflammatory substances. Stopping it reduces pain."
Kana understood. "Inhibition isn't necessarily bad?"
"Depends on use," Rei acknowledged.
"Antibiotics also inhibit bacterial enzymes."
Toma added. "Like penicillin."
"Stops cell wall synthesis enzyme. Bacteria die, but humans are safe."
"Selective inhibition," Kana said.
"Exactly. The basis of drug design."
Rei showed another example. "HIV protease inhibitors. Stop viral replication."
"AIDS treatment?"
"Yes. Medical application of inhibitors."
Kana thought deeply. "Catalyst poisoning is a sad phrase."
"Why?" Toma asked.
"The enzyme can't fulfill its original role. Imprisoned."
Rei nodded. "But that understanding gave birth to medicines."
"Sad past becomes hope?"
"By knowing inhibition mechanisms, we can fight disease."
Kana stared at the test tube. "The enzyme in here is also fighting."
"Against inhibitors and substrates."
Toma murmured. "A quiet battle in the molecular world."
"Understanding that battle is biochemistry," Rei concluded.
The three quietly watched the invisible battlefield.