"Enzymes are like magic."
Kana said while assembling a protein model.
Milia smiled. "Not magic, but precision machinery."
"But they speed up reactions, right?"
Rei supplemented. "Million to trillion times. Catalytic ability is phenomenal."
Kana asked. "How?"
"The active site," Milia pointed to a cavity in the model. "The substrate enters here."
"Substrate?"
"The molecule to be reacted. Like a lock and key relationship."
Rei drew a diagram. "It used to be called the lock-and-key model. The idea that shapes fit perfectly."
"Perfectly?"
"But actually different," Milia corrected. "The induced fit model is correct."
Kana tilted her head. "Induced fit?"
"The enzyme changes shape to fit the substrate. Like putting on a glove."
Rei moved the molecular model. "When substrate approaches, the active site closes."
"Closes?"
"Deforms to envelop the substrate. This makes the reaction easier."
Kana wrote in her notebook. "Enzymes are flexible."
"Because they're proteins," Milia explained. "Amino acid chains fold to create three-dimensional structure."
"That structure is important?"
"Absolutely. When shape changes, function is lost."
Rei gave an example. "Heat denaturation destroys the active site. That's why enzymes don't work at high temperature."
Kana stared at the model. "How big is the active site?"
"A few percent of the whole protein," Milia answered. "But important amino acids gather there."
"Important amino acids?"
Rei explained. "Catalytic residues. Amino acids with roles to help the reaction."
"For example?"
"Serine, cysteine, histidine... each helps specific chemical reactions."
Milia continued. "Inside the active site, the substrate reaches ideal arrangement."
"Ideal?"
"Angles and distances favorable for reaction. Stabilizing the transition state."
Kana was confused. "Transition state?"
Rei drew a diagram. "The intermediate state from reactant to product. Highest energy."
"Enzymes stabilize that transition state. So activation energy decreases."
Kana began to understand. "Lowering the energy barrier?"
"Correct. If the barrier is low, the reaction proceeds quickly."
Milia manipulated the model. "Look. When substrate enters, the active site closes."
Kana observed. Indeed, the molecule was being enveloped.
"At this moment, the substrate is isolated from the external environment."
"Isolated?"
"Inside the active site is a special environment. pH and hydrophobicity are adjusted."
Rei added. "There are hydrophobic pockets where water can't enter. Specific reactions occur there."
Kana was surprised. "There's a small reaction chamber inside the enzyme?"
"You could say that," Milia admitted. "Perfectly controlled space."
"Substrate specificity also comes from there," Rei explained.
"Substrate specificity?"
"The ability to recognize only specific substrates. If both shape and chemical properties don't match, it can't enter."
Kana was convinced. "That's why each enzyme catalyzes different reactions."
"Right. Digestive enzymes break down food, DNA polymerase replicates DNA."
Milia asked. "Kana, what do you think happens when the active site closes?"
Kana thought. "Molecules are... fixed in optimal positions?"
"Wonderful," Milia smiled. "Position-fixing effect. That's also part of catalysis."
Rei summarized. "Enzymes capture the substrate, arrange it optimally, help the reaction, and release the product."
"And then wait for the next substrate again."
Kana murmured. "The active site opens, closes, opens..."
"The rhythm of life," Milia said quietly.
"Repeated thousands, tens of thousands of times. That's metabolism."
Kana imagined the invisible opening and closing of active sites. The invisible door that supports life.