It's no synonym, it's the proper name for the longest protein we've discovered, and its etymology consists of a biologist painstakingly recording each and every amino acid in the chain that makes up the protein. The name tells you exactly what the protein is made of. It's also not so much a word as it is a name (proper noun) so I don't think you can count that. The longest word (45 letters) that is not a proper noun in the English language is the name of a type of lung disease resulting from inhaling fine silica particles (specifically from a volcano I guess?). I'd imagine it'd feel like you constantly just smoked an entire cuban cigar, but with more stinging. I'm not sure how moist the lining of our lungs is, but if it is moist and that's important to its function, you'd have a lot more problems as well.
Fag or faggot, though I'm entirely too uncomfortable typing those words out. I don't approve of their use in a derogatory manner and it really hurts me to see it used in the skype group chat from time to time.