"Without catalysts, do chemical reactions not occur?"
Kana asked.
Rei shook his head. "They occur. Just very slowly."
"How slow?"
"Hydrolyzing peptide bonds without enzymes can take hundreds of years for some reactions."
Toma was surprised. "With enzymes?"
"Milliseconds."
"Billions of times faster!"
Milia said quietly. "Enzymes create life's time."
Rei began explaining. "Catalysts lower activation energy."
"Activation energy?"
"The energy barrier needed for a reaction to occur."
Toma drew a diagram. A mountain-like curve.
"For reactants to become products, they must cross this mountain."
"Catalysts lower the mountain. That is, they reduce the energy barrier."
Kana understood. "So reactions become faster."
"But," Rei emphasized, "they don't change the equilibrium position. Catalysts don't change directionality."
"They only change speed."
Milia supplemented. "Forward and reverse reactions are equally accelerated."
"So the final amount of product doesn't change."
Kana asked. "How do enzymes lower activation energy?"
Rei began detailed explanation. "They bind substrates to the active site."
"The active site has a shape complementary to the substrate."
"Induced fit model. When substrate binds, enzyme shape changes slightly."
Toma assembled a molecular model. "Lock and key, but a flexible keyhole."
"Upon binding, the substrate is activated."
Milia gave specific mechanisms. "Charge stabilization, temporary covalent bond formation, substrate strain."
"All stabilize the transition state."
"Transition state?"
Rei answered. "The intermediate state of the reaction with the highest energy."
"Enzymes specifically stabilize this transition state."
"So activation energy decreases."
Kana drew a diagram in her notebook. "So what determines enzyme speed?"
"Substrate concentration, enzyme concentration, temperature, pH," Toma enumerated.
Rei supplemented. "Can be described by the Michaelis-Menten equation."
"v = Vmax[S] / (Km + [S])."
"Vmax is maximum velocity, Km is the Michaelis constant."
Kana was confused. "What's Km?"
"An indicator of enzyme-substrate affinity," Milia answered.
"Smaller Km means higher affinity."
"Km is the substrate concentration at half-maximal velocity."
Toma drew a graph. A hyperbola.
"At low substrate concentration, velocity is proportional to substrate concentration."
"At high substrate concentration, velocity becomes constant. The enzyme is saturated."
Rei explained. "All enzymes are bound to substrate."
"Even adding more substrate, velocity doesn't increase."
Kana understood. "Enzyme amount becomes rate-limiting."
"Yes. So Vmax is proportional to enzyme concentration."
Milia brought up another factor. "Temperature."
"As temperature rises, reaction rate increases."
"But too high causes enzyme denaturation."
Toma supplemented. "Optimal temperature. Human enzymes are around 37 degrees."
"pH is also important," Rei said.
"The charge state of active site amino acids changes with pH."
"Activity decreases away from optimal pH."
Kana asked seriously. "What are inhibitors?"
"Substances that lower enzyme activity," Rei answered.
"There's competitive and non-competitive inhibition."
Milia drew a diagram. "Competitive inhibition involves substrate-like structures competing for the active site."
"Non-competitive inhibition binds to a different site, changing enzyme shape."
Toma gave examples. "Many drugs are enzyme inhibitors."
"Penicillin inhibits bacterial cell wall synthesis enzymes."
"Aspirin inhibits cyclooxygenase."
Rei said finally. "Enzymes are the conductors of biochemical reactions."
"They control reaction rate, specificity, regulation. Everything."
Milia said quietly. "The ideal for catalysts is perfect selectivity and efficiency."
"Through billions of years of evolution, enzymes have approached that ideal."
Kana closed her notebook. "Ideal conditions catalysts speak of. That is life itself."
Outside the window, stars were shining. In the body, countless enzymes are precisely controlling life's chemical reactions at this very moment.