Lock and Key Model

The lock and key model explains why an enzyme acts on one specific molecule, or one specific stereoisomer of a molecule, and not on close chemical relatives. Emil Fischer proposed it in 1894 after finding that yeast and plant enzymes discriminated sharply between mirror-image and near-identical forms of the same sugar, some hydrolyzing one stereoisomer and leaving the other untouched; he concluded that an enzyme's active site and its substrate must fit one another with the geometric precision of a lock and its key, so that only the correctly shaped molecule can bind and react. The model rested on the same principle of molecular shape and stereochemistry that Louis Pasteur's 1848 discovery of molecular chirality had opened and that Jacobus van't Hoff and Joseph Le Bel had formalized in 1874 as the tetrahedral carbon atom, applied here for the first time to explain a specifically biological process rather than a crystal or a simple organic reaction. It remained the standard account of enzyme specificity for roughly sixty years, until Daniel Koshland's 1958 induced fit model proposed that an enzyme's shape adjusts around its substrate rather than pre-existing as a rigid fit; induced fit is now the more complete picture, but Fischer's lock and key remains the concept most descriptions of enzymes still begin from.