A mechanism proposed in 1890 by Emil Fischer (1852–1919) to explain the binding between the active site of an enzyme and a substrate molecule. The active site is seen as the fixed structure (lock) and exactly matches the structure of the substrate (key). An example is the interaction of an enzyme and substrate in which the lock and key mechanism produces an enzyme–substrate complex. The substrate is then converted to products, which no longer fit the active site, releasing the product and liberating the enzyme. Recent X-ray diffraction studies have shown that the active site of an enzyme is more flexible than this simple mechanism would suggest.