What gives an enzyme specificity?

Enzymes are typically globular proteins that catalyse a specific reaction. Being proteins, enzymes have a well-defined overall conformation (shape), or 'tertiary structure', which allows formation of a catalytic 'active site'. This active site has a shape that is 'complementary' to its substrate, i.e. it can specifically recognise a molecule that fits into the active site, and convert this molecule into the product(s) of the reaction while excluding other molecules that do not fit the active site. This has been described as the 'lock and key' mechanism of enzyme activity, which leads to formation of the 'enzyme-substrate complex', and subsequent release of the reaction product(s).