Chromopeptides from phycoerythrocyanin. Structure and linkage of the three bilin groups

Phycoerythrocyanin carries two covalently attached phycocyanobilin (PCB) groups on the β subunit and a phycobiliviolinoid (PXB) group on the α subunit. Three distinct bilipeptides were obtained by proteolytic digestion of this protein: Asn-Gln-Ala-Ala-Cys(PCB)-Ile-Arg, Gly-Asp-Cys(PCB)-Ser-Gln, and Cys(PXB)-Val-Arg. Correlation 500-MHz 1H NMR analyses showed that the heptapeptide and pentapeptide were attached by cysteinyl thioether linkage to the A ring of the PCB moiety. 1H NMR and mass spectrometry determinations led to structural assignment for the hitherto uncharacterized PXB moiety, with peptide-thioether bonding possible to either ring A or D. Amino acid sequence homologies strongly favor A-ring linkage