"Don't enzymes rest?"
Kana asked while looking at the textbook.
"They don't rest," Rei answered. "They work constantly."
"Don't they get tired?"
"They're molecules. But they're regulated."
Toma showed his experiment notebook. "This reaction starts fast but gradually slows."
"Because substrate decreases," Rei explained.
"Substrate?" Kana asked.
"What enzyme acts on. The reactant."
Rei drew a graph. "Relationship between reaction rate and substrate concentration. Michaelis-Menten curve."
"Linear at first, but flattens midway," Toma pointed out.
"When substrate is sufficient, proportional to concentration. But when enzyme saturates..."
"Saturates?"
"State where all enzymes are always bound to substrate. Can't go faster."
Kana wrote in her notebook. "Enzyme amount is the limit?"
"Yes. That's why cells regulate enzyme amount."
Toma took out a new test tube. "What happens if I add inhibitor?"
He added a few drops. The reaction slowed.
"Competitive inhibition," Rei said. "Inhibitor competes with substrate."
"Binds to same place?"
"Active site. Because shape is similar, enzyme mistakes it."
Kana showed interest. "Are drugs also inhibitors?"
"Many drugs are enzyme inhibitors. Stop specific enzymes to cure disease."
Rei drew another diagram. "There's also non-competitive inhibition. Binds to site other than active site."
"Still stops?"
"Shape changes, loses function. Allosteric inhibition."
Toma took notes. "How are enzymes regulated in vivo?"
"Feedback control," Rei answered. "When product increases, stops enzyme."
"Auto-regulation?"
"Yes. Final product of metabolic pathway inhibits first enzyme."
Kana drew a diagram. "A → B → C → D. When D increases, stops A→B enzyme?"
"Correct. Prevents wasteful production."
Milia entered the club room. "Enzyme talk?"
"Sleepless enzymes," Toma laughed.
Milia continued. "But enzymes are destroyed. Protein degradation."
"Degradation?" Kana was surprised.
"New enzymes constantly made, old ones broken down. Dynamic equilibrium."
Rei supplemented. "Enzyme lifespan is minutes to days. Varies by type."
"Short..."
"So must constantly make them. Energy cost is high."
Milia showed experimental data. "Enzyme activity also depends on temperature."
"Optimal temperature?"
"Most human enzymes optimal around 37 degrees."
Toma asked. "What about tropical organisms?"
"Higher optimal temperature. Evolutionarily adapted."
Rei organized. "Enzyme activity control. Concentration, temperature, pH, inhibitors, feedback..."
"Complex..." Kana murmured.
"But this complexity enables flexible regulation," Milia said.
Kana pondered. "Enzymes can't sleep because life can't sleep?"
"Yes. Metabolism doesn't stop," Rei answered.
"Until death?"
"Until death. Because enzymes keep working, we can live."
Toma held up the test tube. "Even in here, enzymes are working now."
"Hundreds of millions of molecules, simultaneously."
Milia looked at the window. "Day and night, enzymes move in cells."
Kana closed her notebook. "Tireless workers."
Rei smiled. "But precisely controlled. So they don't run wild."
"Balance," Toma said.
"Life stands on delicate balance," Milia said quietly.
The four fell silent. Invisible enzymes keep working without rest.
"The night enzymes can't sleep," Kana murmured. "That's proof of being alive."
Rei nodded. "Metabolism is proof of life."