"Why are there 20 amino acids?"
Kana stared at the chart.
Toma answered. "Can't be coincidence. Result of evolution."
Rei supplemented. "Could be more or fewer. But 20 was chosen."
"What's common?"
"They have amino and carboxyl groups," Rei drew a diagram. "This is basic structure."
"So the difference?"
"Side chains. Called R groups."
Toma pointed at the list. "Glycine has just one hydrogen. Smallest."
"Simple," Kana said.
"But tryptophan has complex ring structures."
Rei continued. "This diversity creates protein function."
Kana questioned. "What diversity?"
"First, hydrophobic and hydrophilic," Rei explained.
"Oil and water?"
"Yes. Leucine, isoleucine, valine... hydrophobic side chains. Avoid water."
Toma supplemented. "These hide inside proteins."
"Why?"
"In aqueous solution, hydrophobic parts gather inward. That determines protein folding."
Rei gave another example. "Meanwhile, serine and threonine are hydrophilic. Have hydroxyl groups."
"Friends with water?"
"They appear on surface. Interact with water."
Kana wrote in her notebook. "Reason positions are determined."
"Yes," Toma nodded. "Hydrophobic effect is main factor in structure."
Rei continued. "Next, charge. Aspartate and glutamate are acidic."
"Negative charge?"
"At physiological pH, carboxyl groups dissociate."
"Lysine and arginine are basic," Toma added. "Positive charge."
"When charged?"
"Can form ionic bonds. Called salt bridges," Rei explained.
"Stabilize proteins?"
"Yes. And important at enzyme active sites too."
Kana questioned. "Cysteine?"
"Special," Toma got excited. "Makes disulfide bonds!"
"Disulfide?"
Rei drew a diagram. "Sulfur atoms of two cysteines form covalent bond."
"And?"
"Firmly fixes proteins. Hair perms use this too."
Kana was surprised. "Perms are chemistry?"
"Break disulfide bonds, reform them."
Toma continued. "Proline is also unusual. Cyclic structure."
"Why unusual?"
"Restricts protein flexibility. Creates bends."
Rei supplemented. "Abundant in collagen. Supports triple helix structure."
Kana questioned. "Aromatic amino acids?"
"Phenylalanine, tyrosine, tryptophan," Rei answered. "Have benzene rings."
"Used for what?"
"Absorb light. Used for protein concentration measurement."
Toma added. "And form hydrophobic cores."
"Methionine?"
"Start codon," Rei said. "First amino acid in protein synthesis."
"Special role?"
"Yes. In genetic code, AUG specifies methionine."
Kana looked at the chart again. "All 20 have meaning."
"Each has unique role," Toma nodded.
Rei said quietly. "But mysterious why it's 20."
"Won't other numbers work?"
"Chemically possible. Actually, rarely a 21st amino acid is used."
"21st?"
"Selenocysteine. Incorporated with special genetic code."
Toma continued. "But basic is 20. This was optimal."
"Evolution's answer?"
"Result of hundreds of millions of years of experimentation," Rei said.
Kana was moved. "With 20 letters of alphabet, can write infinite sentences."
"Yes," Toma laughed. "Sentences called proteins."
Rei nodded. "Balance of diversity and economy. That's the number 20."
The three fell silent.
Twenty amino acids.
Each has history, each has story.
The chosen 20, as letters of life.