This has been debated before, I don't remember if it was in this forum or elsewhere, so I will reply once again.
If you consider any protein's name in this - I'm sorry for the word - ridiculously expanded form to be an actual word, then there deifinitely exist bigger proteins (and thus words) than the one mentioned above.
However, nobody ever uses this kind of name for proteins, and as I've never seen this name form in any kind of literature (and as I am studying the subject I can speak with a fair amount of certainty), I'm seriously doubting how valid it is to call such contraptions 'words'.