It's more like a mutual friend. There's a connection to both reactants (aka "binding affinity"), but not as strong as the bond that is formed between the two substrates (if the reaction is forming a covalent bond between the two substrates, anyway)
Edit: I'm actually saving this meme to show my coworkers that teach biochem, because it's a pretty decent analogy. You can even extend it to other reaction classes, like a phosphorylase being like a friend who connects your buddy who is selling a guitar with your other buddy who wants to buy a guitar, or a isomerase being that friend who gives you a make-over so that another friend can set you up on a date.
