Define enzyme and describe their characteristics and specifications.

An enzyme is a protein molecule that acts as a catalyst to speed up biochemical reactions without being consumed in the process. Enzymes are highly specific for their substrates, meaning each enzyme catalyzes only a particular reaction. They have an active site where the substrate binds, and the enzyme-substrate complex forms. Enzymes lower the activation energy required for a reaction to occur, making reactions happen faster. Characteristics of enzymes include high specificity, sensitivity to temperature and pH, and the ability to work at low concentrations. Most enzymes are proteins, though some RNA molecules also function as enzymes (ribozymes).