Hello there, thanks for the question -- and it's a very good question, too. We are taught that enzymes have exquisite specificity for their substrates: like a hand fitting in a glove -- and we have hundreds of examples of this in all phyla of life. However, it's also true that there are many "promiscuous" enzymes that have a large, and diverse, set of substrates. Common features of such enzymes is that they bind to structural motifs common of diverse substrates. An immediate example of these are cyotochrome P oxidases (cyp enzymes) found in many places, but are most active in your liver. Their main job is to metabolize xenobiotics (things your body didn't make, such as drugs -- prescription or otherwise) into less reactive metabolites; this usually involves attaching -OH groups to the compound to make it more hydrophilic, to prepare it for excretion via urine, or further modification in the liver. Kinases are another example: they add phosphate groups to their substrates (usually proteins, but can include lipids or carbohydrates) based on sequence homology. In the case of protein substrates, a protein that can be phosphorylated will have a particular amino acid sequence that is specific to a particular kinase -- the rest of the structure is irrelevant to the kinase. When a kinase is active, it will phosphorylate any protein it can find that has the sequence to which it can bind. So, again, it's binding to a common structural motif (the amino acid sequence) that may be found on a diverse set of proteins.
