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Flow-chart diagrams are good models to use in biology to show reactions like how enzymes work. At first a lock-and key model was proposed: it was thought that the active site is rigid and the small substrate molecule is reciprocally shaped and fits into the active site, like a lock fits a key. In this model, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme). Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock. Once this enzyme –substrate complex has formed, the close proximity of the molecules allows the reaction to be rapidly catalysed and the products of the reaction are released. Not all experimental evidence could be adequately explained by using the lock and key enzyme model theory. For this reason, a modification called the induced-fit theory has been proposed. The amended induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. Evidence suggests that the binding of a substrate to the active site of an enzyme induces the enzyme to alter its shape slightly, to fit more tightly around the substrate. This explains why certain compounds can bind to the enzyme but do not react because the enzyme has been distorted too much. Other molecules may be too small to induce the proper alignment and therefore cannot react. Only the proper substrate is capable of inducing the proper alignment of the active site. You will need to know both of these models for future assessment tasks. You will need to know both of these models for future assessment tasks. Just a simple representation will do.
