<sigh> from the Wiki-link:

Methionylglutaminy...serine is a scientific name for tryptophan synthase, which is made up of 267 amino acids. It has appeared written down a number of times and has 1913 letters.

According to the Guinness Book of World Records, 18th edition, it is the longest chemical word for C1289H2051N343O375S8.

Lots of much larger proteins exist and their "scientific names" can be written in the same way. If nobody has actually bothered to write down such a scientific name, it is a fine metaphysical point whether the names should be said to exist, and if not, whether this particular string of letters representing a rather average, modest-sized protein is better qualified to be called a "word".