Enzymes are highly selective catalysts, greatly accelerating both the rate and specificity of metabolic chemical reactions, from the digestion of food to the synthesis of DNA. Enzymes act by converting starting molecules – substrates – into different molecules – products. Most enzymes are proteins, although some catalytic RNA molecules have been identified. An enzyme inhibitor is a substrate-like molecule that binds to enzymes and decreases their activity. Since blocking an enzyme’s activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors.