<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> Enzymes / ˈɛnzaɪmz / are macromolecular biological catalysts . Enzymes accelerate chemical reactions . The molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products . Almost all metabolic processes in the cell need enzymes in order to occur at rates fast enough to sustain life . Metabolic pathways depend upon enzymes to catalyze individual steps . The study of enzymes is called enzymology and a new field of pseudoenzyme analysis has recently grown up, recognising that during evolution, some enzymes have lost the ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual' pseudocatalytic' properties . </P> <P> Enzymes are known to catalyze more than 5,000 biochemical reaction types . Most enzymes are proteins, although a few are catalytic RNA molecules . The latter are called ribozymes . Enzymes' specificity comes from their unique three - dimensional structures . </P> <P> Like all catalysts, enzymes increase the reaction rate by lowering its activation energy . Some enzymes can make their conversion of substrate to product occur many millions of times faster . An extreme example is orotidine 5' - phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds . Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction . Enzymes differ from most other catalysts by being much more specific . Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, and activators are molecules that increase activity . Many therapeutic drugs and poisons are enzyme inhibitors . An enzyme's activity decreases markedly outside its optimal temperature and pH . </P>

Enzymes are usually composed of which type of molecule