Proteins do most of the body's work: Hemoglobin carries oxygen, insulin gets cells to process sugar, endorphins produce the runner's rush. A protein's function depends on its shape, and its shape depends on how its chain of amino acids folds up. It's a fiendishly difficult puzzle, one that drug companies would dearly love to solve because it would let them design proteins with entirely new attributes. Why is protein-folding hard? Even in a small protein, with 100 amino acids, the 20 different kinds of amino acids can line up in nearly 20-to-the-100th-power different ways. That's more combinations than the number of atoms in the universe.
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