Enzymes are globular proteins that catalyze biological reactions with extraordinary specificity and efficiency. They lower activation energy by 10^6 to 10^12 fold compared to uncatalyzed reactions.

Lock-and-key model: Substrate fits precisely into the enzyme's active site (complementary shape). Only the correct substrate can bind — explains specificity. Induced-fit model (more accurate): active site adjusts shape slightly upon substrate binding.

Naming: Substrate name + "-ase" suffix. Urease hydrolyzes urea, lipase hydrolyzes lipids, DNA polymerase synthesizes DNA.

Six classes: Oxidoreductases (redox), Transferases (group transfer), Hydrolases (hydrolysis), Lyases $\frac{addition}{removal of groups}$, Isomerases (isomerization), Ligases (joining two molecules using ATP).

Factors affecting activity: Temperature (optimum ~37 degC for human enzymes; above 60 degC → denaturation), pH (optimum varies: pepsin pH 2, trypsin pH 8), Inhibitors (competitive: resemble substrate, bind active site; non-competitive: bind elsewhere, change active site shape).