Crystal structure and molecular mechanism of an E/F type bilin lyase-isomerase

Chromophore attachment of the light-harvesting apparatus represents one of the most important post -translational modifications in photosynthetic cyanobacteria. Extensive pigment diversity of cyanobacteria critically depends on bilin lyases that covalently attach chemically distinct chromophores to phycobiliproteins. However, how bilin lyases catalyze bilin ligation reactions and how some lyases acquire additional isomerase abilities remain elusive at the molecular level. Here, we report the crystal structure of a representative bilin lyase-isomerase MpeQ. This structure has revealed a question-markprotein architecture that unambiguously establishes the active site conserved among the E/F-type bilin lyases. Based on structural, mutational, and modeling data, we demonstrate that stereoselectivity of the active site plays a critical role in conferring the isomerase activity of MpeQ. We further advance a tyrosine-mediated reaction scheme unifying different types of bilin lyases. These results suggest that lyases and isomerase actions of bilin lyases arise from two coupled molecular events of distinct origin.

Automated summary

Chromophore attachment is a crucial post-translational modification in photosynthetic cyanobacteria. This study reveals the crystal structure of a representative bilin lyase-isomerase MpeQ, providing insights into the catalytic mechanism and isomerase activity of bilin lyases.