Enzymes bring about high rates of chemical conversions through three key mechanisms: 1) By lowering the activation energy needed for reactions to occur, making transitions between substrate and product states more facile. 2) By forming transient complexes with substrates at their active sites, stabilizing higher-energy transition states that allow bonds to break and form. 3) Enzyme activity is optimized at particular temperatures, pH levels, and substrate concentrations, and can be inhibited by chemicals that bind competitively to the active site and prevent substrate binding.