Enzymes are proteins, composed of amino-acids, which are produced by all living organisms. Enzymes are responsible for many reactions and biological activities in plants, animals, human beings and microorganisms. Enzymes are essential for all metabolic processes, but are not themselves living materials. They are distinguishable from other proteins because enzymes are involved in a reaction but not used up. Enzymes have catalytic activity – increasing the rate of a reaction.  Enzymes have an active site of a specific size and form that will fit only one substrate (e.g. arabinoxylan) in a specific reaction. The enzyme and substrate form an enzyme-substrate complex, where a reaction occurs breaking down the substrate and releasing different products. This allows the enzyme to work again and form a new substrate complex.