from my searching, Titin is the largest protein yet discovered. It's full chemical name is 189,819 letters long and, depending on how you define a "word" is the longest word in the English language. It is not included in the dictionary - few lexicographers accept chemical formulae as words. But when people go round Googling "What's the longest word in the English language?" , they don't really care if it's a "real" word or not.
So, I can't present to you the full chemical name of the protein Titin (it's a long one)