A protein (or protein-based molecule) that speeds up a chemical reaction in a living organism. An enzyme acts as catalyst for specific chemical reactions, converting a specific set of reactants (called substrates) into specific products. Without enzymes, life as we know it would not exist.
Enzymes are nonetheless subject to error. In 1902 Sir Archibald Garrod was the first to attribute a disease to an enzyme defect, to what Garrod called an -inborn error of metabolism.- Today, newborns are routinely screened for certain enzyme defects such as PKU (phenylketonuria) and galactosemia, an error in the handling (metabolism) of the sugar galactose.