Enzymes are proteins that catalyse metabolic reactions. There are also a number of enzymes that are not proteins but are catalytic molecules of RNA (section 9.2.2) — these are sometimes referred to as ribozymes.

As discussed in section 4.4.2, proteins are linear polymers of amino acids. Any protein adopts a characteristic pattern of folding, determined largely by the order of the different amino acids in its sequence. This folding of the protein chain results in reactive groups from a variety of amino acids, which might be widely separated in the primary sequence, coming together at the surface and creating a site that has a defined shape and array of chemically reactive groups. This is the active site of the enzyme. It is the site that both binds the compounds which are to undergo reaction (the substrates) and catalyses the reaction.

Many enzymes also have a non-protein component of the catalytic site; this may be a metal ion, an organic compound that contains a metal ion (e.g. haem; section, or an organic compound, which may be derived from a vitamin (see Chapter 11) or readily synthesized in the body. This non-protein part of the active site may be covalently bound, in which case it is generally referred to as a prosthetic group, or may be tightly, but not covalently, bound, in which case it is usually referred to as a coenzyme (section 2.4).

Amino acid side-chains at the active site provide chemically reactive groups which can facilitate the making or breaking of specific chemical bonds in the substrate by donating or withdrawing electrons. In this way, the enzyme can lower the activation energy of a chemical reaction (Figure 2.2) and so increase the speed at which the reaction attains equilibrium under much milder conditions than are required for a simple chemical catalyst. In order to hydrolyse a protein into its constituent amino acids in the laboratory, it is necessary to use concentrated acid as a catalyst and to heat the sample at 105 °C overnight to provide the activation energy of the hydrolysis. As discussed in section 4.4.3, this is the process of digestion of proteins, which occurs in the human gut under relatively mild acid or alkaline conditions, at 37 °C, and is complete within a few hours of eating a meal.

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