What are enzymes and how do they catalyze reactions?

Enzymes are biological catalysts, mostly proteins, that accelerate chemical reactions without being consumed. They work by lowering the activation energy required for reactions to proceed. Enzymes bind substrates at their active site, forming an enzyme-substrate complex that facilitates the reaction. Each enzyme is specific to particular substrates due to the complementary shape of its active site (lock and key or induced fit model). Enzymes increase reaction rates by factors of 10^6 to 10^12 compared to uncatalyzed reactions while maintaining specificity and regulation.