An enzyme is a protein molecule which acts as a catalyst in biochemical reactions. Enzymesn are normally named after the reactions they catalyze because their molecular structure is very complex and in many cases unknown. For example, superoxide dismutase is the enzyme responsible for the destruction of the superoxide ion in living organisms.
Enzymes act either in solution or attached to membranes.
The material whose reaction is catalyzed by an enzyme is called a substrate . In some reactions catalyzed by enzymes, the reaction appears to take place on the surface of the catalyst enzyme molecules themselves, which may or may not be in homogeneous solution. Most reactions in biological systems do not occur at any perceptible rate in the absence of an enzyme to catalyze them. Enzymes are very selective catalysts. A given enzyme will catalyze only specific reactions of very specific substrate molecules.