* The longest word in the English language is technically "pneumonoultramicroscopicsilicovolcanoconiosis." This word refers to a lung disease caused by inhaling very fine silica dust.
* "Methionyl" is a prefix used in biochemistry to indicate the presence of the amino acid methionine at the beginning of a protein.
So, while "methionyl" is a prefix, it doesn't start the longest word in the English language.
However, you could technically create a very long word by adding "methionyl" to the beginning of "pneumonoultramicroscopicsilicovolcanoconiosis." But this wouldn't be considered a legitimate word in standard usage.