What is an enzyme? Explain the 'Lock and Key' Hypothesis

An enzyme is a biological catalyst. This means that (like all catalysts) it increases the rate of the reactions it takes part in without being used up. Each enzmye has an active site which is specifc for a certain substrate (reactant particle). The idea of the substrate fitting 'just right' to the active site of the right enzyme is called the 'Lock and Key' hypothesis.If the active site is altered by exptremes of teperature or changes in pH then the substrate won't fit into it as well and will lead to a decrease in the activity of the enzyme. Hope this was helpful! :)