The 1,913 letter long word is the full name for the Protein; tryptophan synthetase A, an enzyme with 267 amino acids.  The 1,913 letters are simply achieved by latching together the names of the amino acids.  I would say that this is the NAME of a protein rather than a WORD as such.  However by the same logic Pneumonoultramicroscopicsilicovolcanoconiosis is also the NAME of a disease, which would leave Hippopotomonstrosesquipidelian as the longest English word.  Ironically the meaning of the word is "long".
Pete.