What are the major mechanisms enzymes use for catalysis?

Enzymes use several catalytic mechanisms, often in combination. Acid-base catalysis: amino acid side chains donate/accept protons to stabilize transition states (His, Asp, Glu as acids; Lys, Arg, His as bases). Covalent catalysis: enzyme forms transient covalent bond with substrate, creating reactive intermediate (Ser proteases, pyridoxal-dependent enzymes). Metal ion catalysis: metals stabilize negative charges, activate water, or participate in redox (carbonic anhydrase, cytochromes). Electrostatic catalysis: charged groups stabilize transition state. Proximity and orientation effects: bringing reactants together in optimal geometry. Strain/distortion: binding substrate in high-energy conformation. These mechanisms lower activation energy, providing rate enhancements up to 10^17.