Getting back to the 1,913-letter word - if you allow the chemical representation of Tryptophane Synthetase (that's the common name of the protein so specified), you introduce another category of "words" and render moot your search for the Longest Word. I'm sure there are proteins with more than 267 amino acids, and as soon as you specify one the biochemists can make one still longer by tacking on an extra amino acid...

So there can't be a Longest Word, because there can't be a "biggest protein," any more than there is a "biggest number."

Sorry, folks.