Title says it all.
I think it is the chemical name of Titin:
(Put it in a spoiler because it is 189,819 letters long)
Methionylthreonylthreonylglutaminylala…
ylglutaminylprolylleucylglutaminylsery…
lserylthreonylalanylthreonylphenylalan…
rylglycylphenylalanylprolylvalylprolyl…
lanylarginylaspartylglycylglutaminylva…
nylleucylprolylglycylvalylglutaminylis…
ylglycylarginylalanyllysylleucylthreon…
llysylalanylasparaginylserylglycylargi…
eonylasparaginylglycylserylglycylgluta…
nylglutamylleucylleucylvalyllysylalany…
asparaginylphenylalanylvalylglutaminyl…
ylthreonylvalylarginylglutaminylglycyl…
utaminylvalylarginylvalylthreonylglycy…
lylvalyllysylphenylalanyltyrosylarginy…
cylglutaminylserylserylleucylaspartylp…
glutaminylglutamylglycylaspartylleucyl…
anylglutamylalanyltyrosylprolylglutamy…
serylvalylasparaginylalanylthreonylasp…
ylthreonylserylthreonylalanylglutamyll…
tamylglutamylglutamylvalylprolylalanyl…
eucylvalylserylthreonylalanylglutaminy…
glutaminylthreonylarginylisoleucylglut…
ylhistidylphenylalanylaspartylalanylar…
lylglutamylmethionylvalylisoleucylaspa…
nylglutaminylleucylprolylhistidyllysyl…
lprolylprolyllysylprolyllysylserylargi…
erylisoleucylalanylalanyllysylalanylgl…
ylglutaminylserylprolylserylprolylisol…
lprolylvalylarginylhistidylvalylarginy…
lylvalylarginylserylvalylserylprolylal…
onylserylprolylisoleucylarginylserylva…
thionylarginyllysylthreonylglutaminyla…
ylglycylprolylglutamylvalylprolylproly…
tamylglycyltyrosylvalylalanylserylsery…
ylarginylglutamylthreonylthreonylleucy…
minylisoleucylarginylthreonylglutamylg…
cylarginyltyrosylglycylvalylglutaminyl…
eucylserylglycylalanylalanylglycylalan…
serylalanylseryltyrosylalanylalanylglu…
lalanyllysylglutamylvalyllysylglutamin…
lanylalanylvalylalanylthreonylvalylval…
lalanylarginylvalylarginylglutamylprol…
tamylglutaminylthreonylalanylglutaminy…
eonylalanylvalylhistidylisoleucylgluta…
glutaminylvalylarginyllysylglutamylala…
threonyllysylvalylvalylvalylalanylalan…
lutaminylglutamylleucyllysylserylargin…
leucylthreonylthreonyllysylglutaminylg…
alylthreonylhistidylglutamylglutaminyl…
ylglutamyllysylthreonylphenylalanylval…
ylalanylalanyllysylalanyllysylglutamyl…
oleucylserylglutamylglutamylisoleucylt…
taminylvalylthreonylglutaminylglutamyl…
ylthreonylarginyllysylthreonylvalylval…
nylthreonylprolyllysylvalyllysylglutam…
larginylglycylarginylglutamylglycyliso…
lutamylglutaminylvalylglutaminylisoleu…
ethionylarginyllysylglutamylalanylglut…
hreonylisoleucylalanylva