protein folding

A freshly built protein comes off the ribosome as a floppy chain of amino acids, like a long beaded string with no particular shape. But a string cannot do a job; an enzyme that grabs a specific molecule, or a channel that lets a specific ion through, only works because it has a precise three-dimensional shape. Protein folding is the process by which that limp chain collapses and twists into its correct working form.

Folding is driven mostly by the chain seeking its lowest-energy, most stable arrangement in water. Amino acids that dislike water (hydrophobic) tuck themselves into the protein's interior, away from the surrounding fluid, while water-friendly (hydrophilic) ones face outward; hydrogen bonds and other interactions then lock helices, sheets, and loops into place. Remarkably, the instructions for the final shape are largely written into the amino acid sequence itself — the order of the beads tends to determine the fold. Folding often begins even while the chain is still being made (co-translationally), and many proteins need help to fold correctly, which is where chaperones come in.

Protein folding matters because shape is function: a protein folded wrongly usually does its job poorly or not at all, and clumps of misfolded protein can be toxic. Several serious diseases — including Alzheimer's, Parkinson's, and the prion diseases — are now understood as folding gone wrong, where proteins aggregate into harmful tangles. The deep difficulty of predicting a protein's fold from its sequence (the 'folding problem') was a grand challenge in biology for decades, and recent computational advances have only made the stakes of understanding it clearer.