An enzyme (from Greek énsimo (ένζυμο), formed by én = at or in and simo = leaven or yeast) is a protein that catalyzes, or speeds up, a chemical reaction. Enzymes are essential to sustain life, because most chemical reactions in biological cells would occur too slowly or would lead to different products without enzymes. A malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to severe diseases. For example, phenylketonuria is caused by an enzyme malfunction in the enzyme phenylalanine hydroxylase, which catalyses the first step in the degradation of phenylalanine. If this enzyme does not function, the resulting build-up of phenylalanine leads to mental retardation.
Like all catalysts, enzymes work by lowering the activation energy of a reaction, thus allowing the reaction to proceed much faster. Enzymes may speed up reactions by a factor of many thousands. An enzyme, like any catalyst, remains unaltered by the completed reaction and can therefore continue to function. Because enzymes, like all catalysts, do not affect the relative energy between the products and reagents, they do not affect equilibrium of a reaction. However, the advantage of enzymes compared to most other catalysts is their sterio-, regio- and chemoselectivity and specificity...
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