Enzymes are catalysts that increase the rate of chemical reactions in cells. Proteins are the most common enzymes in biological reactions. Without enzymatic catalysis, many biochemical reactions would be much slower or would not occur under normal conditions. Enzymes can reduce reaction time to a fraction of a second. Within the thousands of enzymes in a cell, different ones determine particular reactions. Enzymes have two fundamental properties: they increase the rate of chemical reactions (but they are not consumed or permanently altered in doing so), and they do not alter the reactant-to-product chemical equilibrium.
Enzymes function by binding substrates, forming an enzyme-substrate complex. The active site is the specific region of the enzyme to which the substrate binds. While it is so bound, the substrate is converted into the reaction’s product, which is then released from the enzyme. Active sites often bind other small molecules involved in catalysis. Some small molecules are bound to specific proteins; these are called prosthetic groups. In blood, heme is a prosthetic group that carries oxygen. Metal ions bound to enzymes and organic molecules, called co-enzymes, also play specific, significant roles in catalysis.