An enzyme is a type of protein that acts as a catalyst in biological reactions, speeding up processes without being consumed in the reaction. Enzymes are essential for various functions in living organisms, including digestion, metabolism, and DNA replication. Each enzyme is specific to a particular reaction or type of substrate, which is the substance it acts upon. Enzymes work by lowering the activation energy required for a reaction to occur, making it easier for the reaction to take place. They often require co-factors, such as vitamins or minerals, to function effectively. Examples of enzymes include amylase, which helps break down carbohydrates, and lipase, which aids in fat digestion.