It's the former.
Titin, also known as connectin, is a protein that is
important in the contraction of striated muscle tissues.
As the largest known protein,
titin also has the longest IUPAC name.
The full chemical name, containing 189,819 letters,
is sometimes stated to be the longest word in the English language.
However, some professional dictionary writers regard generic names
of chemical compounds as verbal formulae rather than as English words