Enzymes are proteins that speed up specific reactions, and are a type of catalyst. The enzyme provides an alternative route for a reaction to occur, with a much lower activation energy. The reactant, in an enzyme-catalysed reaction, is called the substrate.
Enzymes are incredibly specific about what substrate they can act upon; even small changes in the reactant molecules can stop the enzyme from catalysing the reaction. The substrate reacts at the active site of the enzyme, which is often a cleft or hole in the protein, within which the substrate can fit, caused by the way the protein folds itself up into its tertiary structure; molecules of just the right shape, and with just the right arrangement of attractive groups can fit into these active sites. Other molecules won’t fit or won’t have the right groups to bind to the surface of the active site.
Most enzymes reactions require additional non-protein molecules to make them work. These are known as cofactors; in the absence of the right cofactor, the enzyme doesn’t work. Cofactors can be thought of as ‘helper molecules’ that assist in biochemical reactions, many of which are derived from vitamins.
An example of an enzyme-catalysed reaction is used by bacteria to protect itself against the invasion of foreign DNA from viruses, by a DNA methyltransferase enzyme; these enzymes attach a methyl (CH3-) group to DNA molecules. When foreign DNA (which are not methylated in this manner) is introduced into the cell, by a virus for instance, they are quickly degraded by another type of enzyme, called a restriction enzyme. The bacteria’s own DNA, however, remains untouched because it has been methylated, and therefore is not attacked by the restriction enzyme.
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