What is the difference between the lock-and-key model and the induced-fit hypothesis of enzyme action?

The lock-and-key model and the induced-fit hypothesis are two potential models for how substrates may bind in the active site of an enzyme. The lock-and-key model suggests that the substrate is completely complementary in shape to the active site, so that it fits in 'perfectly' - i.e. the way a key (the substrate) fits into a lock (the enzyme). There is no change in shape of the active site when the substrate binds. It's important to remember that the induced-fit hypothesis is similar to the lock-and-key model, but tweaked slightly. It says that the substrate and active site are not completely complementary, but there is still some complementarity. This is like a glove (the enzyme and it's active site) and a hand (the susbtrate) - they're a similar shape but not an exact match. When the hand goes into the glove, the glove changes shape slightly and moulds itself around the hand so that it fits snugly. In the same way, the active site changes shape to tightly bind the substrate. At the moment, this model is supported by a lot of evidence. For example, some enzymes can catalyse reactions with more than one substrate, but these different substrates are still similar in shape. This is much like how a single glove can fit different hands (as hands are generally similarly shaped!).