label: N-terminal L-glutamyl-[protein] + N(alpha)-acetyl-L-methionine => N-terminal N(alpha)-acetyl-L-methionyl-L-glutamyl-[protein] + H2O
label_lc: n-terminal l-glutamyl-[protein] + n(alpha)-acetyl-l-methionine => n-terminal n(alpha)-acetyl-l-methionyl-l-glutamyl-[protein] + h2o