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Publication Abstract from PubMed

Pigmentation of light-harvesting phycobiliproteins of cyanobacteria requires covalent attachment of open-chain tetrapyrroles, bilins, to the apoproteins. Thioether formation via addition of a cysteine residue to the 3-ethylidene substituent of bilins is mediated by lyases. T-type lyases are responsible for attachment to Cys155 of phycobiliprotein beta-subunits. We present crystal structures of CpcT (All5339) from Nostoc sp. PCC7120 and its complex with phycocyanobilin, at 1.95 and 2.50 Angstrom resolution, respectively. CpcT forms a dimer and adopts a calyx-shaped beta-barrel fold. While the overall structure of CpcT is largely retained upon chromophore binding, arginine residues at the opening of the binding pocket undergo major rotameric rearrangements anchoring the propionate groups of phycocyanobilin. Based on the structure and mutational analysis, a reaction mechanism is proposed that accounts for chromophore stabilization, and regio- and stereospecificity of the addition reaction. At the dimer interface, a loop extending from one subunit partially shields the opening of the phycocyanobilin binding pocket in the other subunit. Deletion of the loop or disruptions of the dimer interface significantly reduce CpcT lyase activity, suggesting functional relevance of the dimer. Dimerization is further enhanced by chromophore binding. The chromophore is largely buried in the dimer but in the monomer the 3-ethylidene group is accessible for the apo-phycobiliprotein, preferentially from the chromophore alpha-side. Asp163 and Tyr65 at the beta- and alpha-face near the E-configured ethylidene group, respectively, support the acid-catalyzed nucleophilic Michael addition of cysteine-155 of the apoprotein to an N-acylimmonium intermediate proposed by K. Grubmayr and U.G. Wagner (Monatsh. Chem. 119, 965, 1988).

Structure and Mechanism of the Phycobiliprotein Lyase CpcT.,Zhou W, Ding WL, Zeng XL, Dong LL, Zhao B, Zhou M, Scheer H, Zhao KH, Yang X J Biol Chem. 2014 Jul 29. pii: jbc.M114.586743. PMID:25074932^[2]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.