Systematic Analysis of the MIO-forming Residues of Aromatic Ammonia Lyases.

Aromatic ammonia lyases (AALs) and tyrosine/phenylalanine ammonia mutases (TAM/PAM) are 3,5-dihydro-5-methylidene-4H-imidazol-4-one (MIO)-dependent enzymes. Usually, the MIO moiety is autocatalytically formed from the tripeptide Ala-Ser-Gly (ASG) and acts as an electrophile during the enzymatic reaction. However, the MIO-forming residues (ASG) have some diversity in this enzyme class. In this work, a systematic investigation on the variety of MIO-forming residues was carried out using in-depth sequence analyses. Several protein clusters of AAL-like enzymes with unusual MIO-forming residues such as ACG, TSG, SSG, and CSG were identified, including two novel histidine ammonia lyases and one PAM with CSG and TSG residues, respectively, as well as three novel ergothioneine trimethylammonia lyases without MIO motif. The mutagenesis of common MIO-groups confirmed the function of these MIO variants, which provides good starting points for future functional prediction and mutagenesis research of AALs.