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Structure of Phycoerythrobilin Synthase PebS from the Cyanophage P-SSM2 in Complex with the bound Substrate Biliverdin IXa
Structural highlights
FunctionPEBS_BPPRM Plays a role in phycoerythrobilin biosynthesis, the red pigment chromophore photosynthetically active biliproteins of the host cyanobacteria. Uses a four-electron reduction to carry out the reactions catalyzed by two enzymes (EC 1.3.7.2 and EC 1.3.7.3) in host. Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedThe reddish purple open chain tetrapyrrole pigment phycoerythrobilin (PEB; A(lambdamax) approximately 550 nm) is an essential chromophore of the light-harvesting phycobiliproteins of most cyanobacteria, red algae, and cryptomonads. The enzyme phycoerythrobilin synthase (PebS), recently discovered in a marine virus infecting oceanic cyanobacteria of the genus Prochlorococcus (cyanophage PSSM-2), is a new member of the ferredoxin-dependent bilin reductase (FDBR) family. In a formal four-electron reduction, the substrate biliverdin IXalpha is reduced to yield 3Z-PEB, a reaction that commonly requires the action of two individual FDBRs. The first reaction catalyzed by PebS is the reduction of the 15,16-methine bridge of the biliverdin IXalpha tetrapyrrole system. This reaction is exclusive to PEB biosynthetic enzymes. The second reduction site is the A-ring 2,3,3(1),3(2)-diene system, the most common target of FDBRs. Here, we present the first crystal structures of a PEB biosynthetic enzyme. Structures of the substrate complex were solved at 1.8- and 2.1-A resolution and of the substrate-free form at 1.55-A resolution. The overall folding revealed an alpha/beta/alpha-sandwich with similarity to the structure of phycocyanobilin:ferredoxin oxidoreductase (PcyA). The substrate-binding site is located between the central beta-sheet and C-terminal alpha-helices. Eight refined molecules with bound substrate, from two different crystal forms, revealed a high flexibility of the substrate-binding pocket. The substrate was found to be either in a planar porphyrin-like conformation or in a helical conformation and is coordinated by a conserved aspartate/asparagine pair from the beta-sheet side. From the alpha-helix side, a conserved highly flexible aspartate/proline pair is involved in substrate binding and presumably catalysis. Phycoerythrobilin synthase (PebS) of a marine virus. Crystal structures of the biliverdin complex and the substrate-free form.,Dammeyer T, Hofmann E, Frankenberg-Dinkel N J Biol Chem. 2008 Oct 10;283(41):27547-54. Epub 2008 Jul 28. PMID:18662988[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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