What defies a species?

their color, their size, their life I guess

Acord to nature.com a species can be defined as a biological species is a group of organisms that can reproduce with one another in nature and produce fertile offspring. The term species can also be defined as the most basic category in the system of taxonomy.

Explanation:

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products