NONENZYMATIC BILIN ADDITION TO THE GAMMA-SUBUNIT OF AN APOPHYCOERYTHRIN

C-Phycoerythrin is a light-harvesting protein whose alpha and beta sub units carry thioether-linked phycoerythrobilin (PEB) at cysteine resid ues alpha-82, alpha-139, beta-48,59 (doubly-linked), beta-80, and beta -165. The two subunits of Calothrix sp. PCC 7601 C-phycoerythrin, over expressed together as apopolypeptides in Escherichia coli, formed incl usion bodies. Purified apo-alpha was soluble in the absence of urea, w hereas the apo-beta subunit was only soluble at high urea concentratio ns. Products of nonenzymatic addition of PEB and phycocyanobilin (PCB) to apo-alpha were characterized by isolation of bilin peptides and sp ectroscopy. Reaction of PEB with the apo-alpha subunit led primarily t o 15,16-dihydrobiliverdin (Cys-82) or urobilin (Cys-139) adducts, and small amounts of the natural PEB adducts at both Cys-82 and Cys-139. P CB reacted primarily with Cys-82 to form phycocyanobilin and mesobiliv erdin adducts. Both PEB and PCB also formed relatively small amounts o f adducts with Cys-59, which is not a bilin attachment residue in natu ral phycoerythrin. Sodium azide was found to promote the addition of P EB to simple thiols but not to apo-alpha phycoerythrin.