Toward a Mechanism for Biliprotein Lyases: Revisiting Nucleophilic Addition to Phycocyanobilin

摘要

Biliproteins are a remarkable example of molecular adaptation. The photophysics and photochemistry of the linear-tetrapyrrolic bilin chromophores are profoundly changed by interactions with the apoprotein, thereby optimizing them for their light-sensing and light-harvesting functions. The chromophores are covalently bound to the apoproteins via thioether bonds to conserved cysteine sites of the apoproteins. This binding is autocatalytic in the phyto-chromes but enzymatic in the phycobiliproteins of cyanobacteria. Up to four chromophores are attached in these light-harvesting pigments by a set of lyases, of which three phylogenetically unrelated types (E/F, S/U, and T) have recently been characterized; they are distinguished by their specificities for the acceptor proteins, the binding sites, and the chromophores. All three lyase types directly attach phycocyanobilin (PCB, 1) (or phycoerythrobilin, which differs by the length of the conjugated π system) by nucleophilic addition to the common Δ3,3~1 double bond of the two chromophores.