the longest word is actually a term for the tryptophan synthetase A protein formula (1913 letters) namely :
methionylglutaminylarginyltyro sylglutamylse-
rylleucylphenylalanylalanylglu taminylleucyl-
lysylglutamylarginyllysylgluta mylglycylalan-
ylphenylalanylvalylprolylpheny lalanylvalylt-
hreonylleucylglycylaspartylpro lylglycylisol-
eucylglutamylglutaminylserylle ucyllysylisol-
eucylaspartylthreonylleucyliso leucylglutamy-
lalanylglycylalanylaspartylala nylleucylglut-
amylleucylglycylisoleucylproly lphenylalanyl-
serylaspartylprolylleucylalany laspartylglyc-
ylprolylthreonylisoleucylgluta minylasparagi-
nylalanylthreonylleucylarginyl alanylphenyla-
lanylalanylalanylglycylvalylth reonylprolyla-
lanylglutaminylcysteinylphenyl alanylglutamy-
lmethionylleucylalanylleucylis oleucylarginy-
lglutaminyllysylhistidylprolyl threonylisole-
ucylprolylisoleucylglycylleucy lleucylmethio-
nyltyrosylalanylasparaginylleu cylvalylpheny-
lalanylasparaginyllysylglycyli soleucylaspar-
tylglutamylphenylalanyltyrosyl alanylglutami-
nylcysteinylglutamyllysylvalyl glycylvalylas-
partylserylvalylleucylvalylala nylaspartylva-
lylprolylvalylglutaminylglutam ylserylalanyl-
prolylphenylalanylarginylgluta minylalanylal-
anylleucylarginylhistidylaspar aginylvalylal-
anylprolylisoleucylphenylalany lisoleucylcys-
teinylprolylprolylaspartylalan ylaspartylasp-
artylaspartylleucylleucylargin ylglutaminyli-
soleucylalanylseryltyrosylglyc ylarginylglyc-
yltyrosylthreonyltyrosylleucyl leucylserylar-
ginylalanylglycylvalylthreonyl glycylalanylg-
lutamylasparaginylarginylalany lalanylleucyl-
prolylleucylasparaginylhistidy lleucylvalyla-
lanyllysylleucyllysylglutamylt yrosylasparag-
inylalanylalanylprolylprolylle ucylglutaminy-
lglycylphenylalanylglycylisole ucylserylalan-
ylprolylaspartylglutaminylvaly llysylalanyla-
lanylisoleucylaspartylalanylgl ycylalanylala-
nylglycylalanylisoleucylserylg lycylserylala-
nylisoleucylvalyllysylisoleucy lisoleucylglu-
tamylglutaminylhistidylasparag inylisoleucyl-
glutamylprolylglutamyllysylmet hionylleucyla-
lanylalanylleucyllysylvalylphe nylalanylvaly-
lglutaminylprolylmethionyllysy lalanylalanyl-
threonylarginylserine.
or a more simplified word that you can use on a daily basis :
Pneumonoultramicroscopicsilicovolcanoconeosis : Its a certain lung condition caused by breathing in certain harmfull particles like silica dust. (45 letters)
so there ya have it answer mystery solved :P