Structure and apoprotein linkages of phycourobilin

Area of Science:

• Biochemistry
• Spectroscopy
• Protein Chemistry

Background:

• R-phycoerythrin is a photosynthetic pigment containing bilin prosthetic groups.
• Understanding chromophore structure and linkage is crucial for pigment function.

Purpose of the Study:

• To elucidate the structure and apoprotein linkages of phycourobilin in R-phycoerythrin.
• To compare phycourobilin structure with phycoerythrobilin.

Main Methods:

• Tryptic digestion of R-phycoerythrin.
• Adsorption chromatography on Sephadex G-25 for chromophore separation.
• Structural analysis of peptide-bound bilin derivatives.

Main Results:

• Separated peptide-bound phycoerythrobilin and phycourobilin.
• Identified identical structure and linkages for bound phycoerythrobilin as previously reported.
• Determined phycourobilin is a tetrapyrrole with specific peptide linkages: an ester bond and two thioether bonds (one labile, one stable).
• Characterized the unsaturated Ring D and conjugated system in phycourobilin.

Conclusions:

• Phycourobilin possesses a unique tetrapyrrole structure with defined linkages to its apoprotein.
• The structural features of phycourobilin explain its spectral properties and potential for isomerization.