"Can you tell the shape from this sequence?"
Kana stared at the amino acid sequence.
Rei answered. "Theoretically possible. But practically extremely difficult."
"Why?"
"Because the number of possible conformations is astronomical."
Milia wrote numbers in her notebook. "For a 100 amino acid protein, over 10^300 possibilities."
Kana was surprised. "That many?"
"Yet in reality, it folds into the correct structure in seconds," Rei explained.
"How?"
"Levinthal's paradox. Random search would take longer than the age of the universe."
Kana was confused. "Then how does it fold?"
"Because the pathway is determined," Milia answered. "Follows the energy landscape."
Rei drew a diagram. "Funnel-shaped energy landscape. From many starting points to one minimum."
"Naturally rolls down?"
"Yes. Hydrophobic effect is the main driving force."
Kana took notes. "Water-avoiding amino acids go inside?"
"First the hydrophobic core forms. Then the structure solidifies from there."
Milia added. "Secondary structure comes first. α-helices and β-sheets."
"These combine to form tertiary structure?"
"Exactly. Hierarchical folding."
Rei warned. "But sometimes it fails."
"Fails?" Kana worried.
"Misfolding. Folds into the wrong structure."
Milia had a serious expression. "Alzheimer's disease, Parkinson's disease..."
"Related to disease?"
"Misfolded proteins aggregate. Amyloid fibrils."
Rei continued. "Prion disease too. Normal protein converts to abnormal form."
"Infectious?"
"Structurally. Abnormal form converts normal form to abnormal."
Kana was frightened. "Chain reaction?"
"Yes. That's why it's scary."
Milia showed her tablet. "Anfinsen's experiment. Denatured ribonuclease spontaneously refolds."
"Anfinsen's dogma," Rei explained. "Protein structure is determined solely by amino acid sequence."
"So if we know the sequence, we know the structure?"
"Theoretically. But computation can't keep up."
Kana thought. "What about AI?"
"AlphaFold. Recently made dramatic progress," Rei acknowledged.
Milia supplemented. "But the dynamic process of folding isn't fully understood yet."
"Dynamic?"
"Time evolution. Which pathway it takes."
Rei summarized. "Sequence determines structure. But the process is complex."
Kana drew in her notebook. "Peptide chains tangle while searching for the right shape."
"Complex like a nightmare," Milia murmured.
"But most succeed," Rei added.
"With chaperone help too?"
"Yes. Especially for large proteins or complex structures."
Kana asked. "So without chaperones?"
"Many proteins fold spontaneously. But chaperones increase efficiency and reduce errors."
Milia looked out the window. "Folding is life's miracle."
"From a single chain to a functional machine," Rei continued.
Kana murmured. "The moment information becomes shape."
"And shape becomes function."
The three fell silent. The nightmare of peptide chains speaks to life's mystery.