"There's a way to stop enzymes."
Milia said, picking up a molecular model.
Kana showed interest. "Enzymes speed up reactions, right? Stopping them?"
"More like interfering," Rei explained. "Molecules called inhibitors interfere with enzyme function."
"Why do that?"
Milia answered quietly. "Many drugs are enzyme inhibitors. They selectively inhibit enzymes involved in disease."
Rei began drawing a diagram. "Enzymes have an active site. Where the substrate binds."
"Active site?"
"Like a keyhole. When the substrate key fits perfectly, the reaction proceeds."
Kana nodded. "So, the inhibitor?"
"Broadly two types. Competitive inhibition and non-competitive inhibition."
Rei drew two diagrams. In one, substrate and inhibitor compete for the same site. In the other, the inhibitor binds to a different location.
"Competitive inhibition is when the inhibitor competes with the substrate for the same active site."
"Musical chairs?" Kana compared.
"Good metaphor. Substrate and inhibitor compete. When the inhibitor binds, the substrate can't enter."
Milia added. "But if you increase substrate concentration, you can overcome the inhibition to some extent."
"Why?"
"Probability issue. With more substrate, the probability that substrate binds first increases."
Rei continued. "On the other hand, non-competitive inhibition is different. The inhibitor binds to a different place, not the active site."
"Different place?"
"Called an allosteric site. When an inhibitor binds there, the enzyme's shape changes."
Kana tried to understand. "When the shape changes?"
"The active site shape also changes. Then the substrate can't bind. Or even if it binds, the reaction doesn't proceed."
Milia moved the molecular model to demonstrate. "Proteins are flexible. When one place changes, everything changes."
"So even increasing substrate?"
"Non-competitive inhibition can't be overcome," Rei answered. "The active site isn't blocked, but it doesn't function."
Kana pondered. "Which is more effective as a drug?"
"Depends on the situation," Milia said. "Competitive inhibitors need to be structurally similar to the substrate. Non-competitive inhibitors can have more diverse structures."
Rei drew a graph. "In enzyme kinetics, the difference becomes clear."
"What's Vmax?"
"Maximum reaction rate. The rate when all enzymes are saturated with substrate."
"What's Km?"
"Michaelis constant. An indicator of affinity with substrate."
Rei continued explaining. "In competitive inhibition, the apparent Km increases. Substrate binds less easily. But Vmax doesn't change."
"Why doesn't Vmax change?"
"Because if you increase substrate concentration infinitely, you can completely push out the inhibitor."
"What about non-competitive inhibition?"
"Vmax decreases. Part of the enzyme is permanently in a non-functional form. But Km doesn't change."
Kana summarized. "Competitive inhibition reversibly interferes. Non-competitive inhibition changes the structure."
"Accurate understanding," Rei smiled.
Milia gave an example. "There's a drug called statin. A competitive inhibitor of cholesterol synthesis enzyme."
"I've heard of it," Kana said.
"It inhibits HMG-CoA reductase. Has a structure similar to the substrate HMG-CoA."
Rei added. "Meanwhile, aspirin is an irreversible inhibitor. It chemically modifies COX enzyme."
"Modifies?"
"Binds to the enzyme covalently. Permanently inactivates it."
Kana was surprised. "That means breaking the enzyme?"
"You could say that. So the effect lasts a long time. Until new enzymes are synthesized."
Milia said quietly. "The battle between enzymes and inhibitors is a molecular-level strategy game."
"An invisible battle," Kana murmured.
"But understanding it leads to developing new drugs," Rei concluded.
The three contemplated the quiet battle unfolding inside cells.