∞ generated and posted on 2016.08.22 ∞
The substance that is acted upon by an enzyme as the enzyme effects catalysis.
In a non-enzymatically catalyzed reaction the substrate (or substrates) instead is described as a reactant (or reactants). Typically, in an enzymatically catalyzed reaction, what substrate is acted upon by an enzyme is very specifically defined so that only one or very few substrates are acted upon by a given type of enzyme. This specificity is the basis of control of metabolism by cells, that is, of what chemical reactions will tend to occur.
Figure legend: Comparing the concept of substrate with that of reactant as well as the properties of enzymatically catalyzed reactions with reactions that proceed without the help of an enzyme (or catalyst).
By analogy, your home or car have specificity for very few individuals, as determined in those cases by characteristics of the associated key. The analogy breaks down, however, because while the car has specificity for a particular key, they key itself is not acted upon by the car.