The molecule or molecules on which an enzyme exerts its catalytic action. An enzyme may be specific for only one substrate, e.g. aspartase, which catalyses the interconversion of aspartic and fumaric acids, and is strictly specific for the L-isomer of aspartic acid. Other enzymes can act on a range of substrates; alkaline phosphatase can hydrolyse many esters of phosphoric acid. The substrate reflects the structure of the enzyme active site in that the substrate must have a binding group, by which it can bind to the *active site
, and a susceptible bond at which the enzyme can attack the substrate. These groups must have complementary structures in the active site.