How enzymes catalyse biological reactions

Enzymes act on substrates by attaching to them. The part of an enzyme to which one or more substrate molecules can attach is called the active site.

An enzyme represented as an oval with two sections which look like half hexagons, removed at the right hand side. The removed sections are the active site.

The diagram below shows how an enzyme catalyses the breakdown of one substrate into two products.

Three enzymes represented as ovals with two half-hexagon shaped removed.  Enzyme 1 shows the substrate - two hexagon shapes joined together vertically - collide with the active site of the enzyme and become attached.  Enzyme 2 shows the enzyme catalyse the breakdown of the substrate. Enzyme 3 shows the products - 2 hexagons, no longer joined - released from the active site.  The enzyme molecule is unchanged and can be reused.