methionylglutaminylarginyltyrosylglutamylserylleucyl phenylalanylalanylglutaminylleucyllysylglutamylarginyl lysylglutamylglycylalanylphenylalanylvalylprolylphenyl alanylvalylthreonylleucylglycylaspartylprolylglycylisol eucylglutamylglutaminylserylleucyllysylisoleucylaspartyl threonylleucylisoleucylglutamylalanylglycylalanylaspartyl alanylleucylglutamylleucylglycylisoleucylprolylphenyl alanylserylaspartylprolylleucylalanylaspartylglycylprolyl threonylisoleucylglutaminylasparaginylalanylthreonylleucyl arginylalanylphenylalanylalanylalanylglycylvalylthreonyl prolylalanylglutaminylcysteinylphenylalanylglutamyl methionylleucylalanylleucylisoleucylarginylglutaminyllysyl histidylprolylthreonylisoleucylprolylisoleucylglycylleucyl leucylmethionyltyrosylalanylasparaginylleucylvalylphenyl alanylasparaginyllysylglycylisoleucylaspartylglutamylphenyl alanyltyrosylalanylglutaminylcysteinylglutamyllysylvalyl glycylvalylaspartylserylvalylleucylvalylalanylaspartylvalyl prolylvalylglutaminylglutamylserylalanylprolylphenylalanyl arginylglutaminylalanylalanylleucylarginylhistidylasparaginyl valylalanylprolylisoleucylphenylalanylisoleucylcysteinyl prolylprolylaspartylalanylaspartylaspartylaspartylleucyl leucylarginylglutaminylisoleucylalanylseryltyrosylglycyl arginylglycyltyrosylthreonyltyrosylleucylleucylserylarginyl alanylglycylvalylthreonylglycylalanylglutamylasparaginyl arginylalanylalanylleucylprolylleucylasparaginylhistidyl leucylvalylalanyllysylleucyllysylglutamyltyrosylasparaginyl alanylalanylprolylprolylleucylglutaminylglycylphenylalanyl glycylisoleucylserylalanylprolylaspartylglutaminylvalyllysyl alanylalanylisoleucylaspartylalanylglycylalanylalanylglycyl alanylisoleucylserylglycylserylalanylisoleucylvalyllysylisol eucylisoleucylglutamylglutaminylhistidylasparaginylisoleucyl glutamylprolylglutamyllysylmethionylleucylalanylalanylleucyl lysylvalylphenylalanylvalylglutaminylprolylmethionyllysyl alanylalanylthreonylarginylserine
Doesn't really count as a real word, but it has been used in a medical Journal
"The word with the most letters is a name of protein, as a name, it technically does not qualify as a word, but at 1913 characters, we just had to include it. Theoretically these names can contain an infinite amount of characters, but this one qualifies as it has actually been used in a medical journal."