Enzymes

Life cannot exist without enzymes. Thousands of chemical reactions occur in living cells: old cells are being replaced by new ones; simple molecules link to form complex ones; food is digested and converted to energy; waste materials are disposed of; and cells reproduce. These reactions, involving buildup and breakdown, are collectively referred to as metabolism. For each of these reactions to occur, a certain degree of energy is required (activation energy) and in the absence of such energy, these reactions would not occur spontaneously. The presence of these enzymes—which are usually proteins or RNA enzymes—reduces the amount of activation energy required for these reactions to occur and increases the rate of these reactions by millions. In the process, enzymes are neither consumed nor chemically changed.

Each of the chemical reactions in the body is a component of a pathway or cycle, and most enzymes are highly specific and act on only a single substrate (reactant) in the pathway to produce a product in the metabolic sequence. Most of the more than 4,000 enzymes in living cells are proteins, with a unique three-dimensional configuration, the shape of which accounts for their specificity. An enzyme is commonly named by adding the suffix ase to the root name of the substrate on which it acts, although more specific (and descriptive) names are used in chemically oriented literature.

Certain anticancer and immunosuppressive drugs target purine nucleoside phosphorylase (PNP), an enzyme that carries out housekeeping functions by clearing away certain waste molecules that are formed when DNA is broken down. The image depicts a computer-generated model of PNP .

It was known in the late seventeenth and early eighteenth centuries that meat was digested by secretions in the stomach and starch could be broken down to simple sugars by saliva and plant extracts. Wilhelm Kühne, a German physiologist, was the first to coin the name enzyme in 1878 to refer to trypsin, a protein-digesting enzyme he had discovered, and, in 1897, Eduard Buchner at the University of Berlin first demonstrated that enzymes could function outside living cells. In 1926, working with the jack bean, James Summer at Cornell University isolated and crystallized the first enzyme, urease, and provided conclusive proof that it was a protein. Summer was the co-recipient of the 1946 Nobel Prize in Chemistry.