"When you boil an egg, it turns white, right?"
Kana said abruptly.
Milia nodded quietly. "Protein denaturation."
"Denaturation?"
Rei began explaining. "Protein's three-dimensional structure breaks. It loses function."
"But the amino acid sequence doesn't change."
"Doesn't change, yet structure collapses?"
"Yes. Protein tertiary structure is maintained by non-covalent bonds."
Milia drew a diagram. A complexly folded protein.
"Primary structure is peptide bonds. Strong covalent bonds."
"Secondary structure is α-helices and β-sheets. Hydrogen bonds."
"Tertiary structure is overall folding. Hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bonds."
Rei explained in detail. "Non-covalent bonds are weaker than covalent bonds."
"Easily broken by temperature, pH, or solvents."
Kana asked. "When you boil eggs, what happens?"
"Heat breaks non-covalent bonds," Milia answered.
"Proteins unfold and entangle with each other."
"So it solidifies white."
Rei supplemented. "Soluble proteins become insoluble aggregates."
"Can't return to original. Irreversible denaturation."
Kana proposed an experiment. "I want to see it actually."
Milia prepared a diluted egg white solution in a test tube.
"Adding hydrochloric acid to this."
The solution became cloudy white.
"Denaturation by pH change," Rei explained.
"Amino acid side chains become protonated, charge changes."
"Ionic bonds break, electrostatic repulsion occurs."
Kana understood. "Structure changes with pH."
Milia added ethanol to another test tube.
It became cloudy white again.
"When solvent polarity changes, hydrophobic interactions collapse," Rei explained.
"Protein interior has hydrophobic amino acids gathered."
"In aqueous solution, hydrophobic amino acids hide inside."
"But in ethanol, hydrophobic interactions weaken."
Kana thought deeply. "What if body temperature rises?"
"Proteins begin to denature," Rei answered seriously.
"That's why high fever is dangerous. Above 42 degrees, many proteins denature."
Milia supplemented. "But heat shock proteins protect."
"Heat shock proteins?"
"Expressed under stress conditions. Molecular chaperones that help other proteins fold."
Rei explained in detail. "GroEL and Hsp70. They recognize partially denatured proteins and fold them correctly."
"Guardians of structure."
Milia gave another example. "Prion diseases."
"Prions?"
"Diseases where normal proteins convert to abnormal structures."
"Mad cow disease and Creutzfeldt-Jakob disease."
Rei's face became serious. "Prion protein PrP has normal and abnormal forms."
"Same amino acid sequence but different three-dimensional structures."
"Abnormal form converts normal form to abnormal."
Kana felt fear. "Structure is infectious?"
"Yes. This is the most dramatic example showing protein structure's importance."
Milia said quietly. "Structure is function. Function is life."
"When structure collapses, function is lost and life is threatened."
Rei said finally. "That's why cells use enormous energy to maintain protein structure."
"Chaperones, ubiquitin-proteasome system, autophagy."
"All are protein quality control."
Kana closed her notebook. "When tertiary structure collapses, it's a crisis for life."
"But life has also evolved mechanisms to protect structure."
Milia wrote finally. "Stability is not a given. It is maintained."
"Stability is not given. It is maintained."
Night deepened. In the body, countless proteins continue fighting to maintain correct structure at this very moment.