Radical mechanism of cyanophage phycoerythrobilin synthase (PebS)

Andrea W. U. Busch , Edward J Reijerse , Wolfgang Lubitz , Eckhard Hofmann , Nicole Frankenberg-Dinkel (1)

1 Biology and Biotechnology

Phycoerythrobilin (PEB) is a pink-colored open-chain tetrapyrrole molecule found in the cyanobacterial light-harvesting phycobilisome. Within the phycobilisome, PEB is covalently bound via thioether bonds to conserved cysteine residues of the phycobiliprotein subunits. In cyanobacteria, biosynthesis of PEB proceeds via two subsequent two-electron reductions catalyzed by the ferredoxin-dependent bilin reductases (FDBR) PebA and PebB starting from the open-chain tetrapyrrole biliverdin IXalpha. Recently, a new member of the FDBR family was identified in the genome of a marine cyanophage. In contrast to the cyanobacterial enzymes, PEB-synthase (PebS) from cyanophage combines both two-electron reductions for PEB synthesis. Here we show that PebS acts via a substrate radical mechanism and that two conserved aspartate residues at position 105 and 206 are critical for stereospecific substrate protonation and conversion. Based on the crystal structures of both PebS variants and presented biochemical and biophysical data a mechanism for BV conversion to PEB is postulated and discussed with respect to other FDBR family members.

Mots clés: Life Sciences

Andrea W. U. Busch, Edward J Reijerse, Wolfgang Lubitz, Eckhard Hofmann, Nicole Frankenberg-Dinkel. Radical mechanism of cyanophage phycoerythrobilin synthase (PebS). Biochemical Journal, 2011, 433 (3), pp.469-476. ⟨10.1042/BJ20101642⟩. ⟨hal-00558101⟩