Biosynthesis of phycobiliproteins

Phycocyanin and phycoerythrin are phycobiliproteins that function as photosynthetic antennae pigments in various types of algae (O'Carra & O'hEocha, 1977). Phytochrome, a related phycobiliprotein, plays an important role in photomorphogenesis in higher plants (Pratt, 1978). The chromophores of these proteins are bilins, which are closely related in structure to each other and to the mammalian blue pigments biliverdin and bilirubin. The biosynthesis of the phycobiliproteins is of interest both in terms of the metabolic pathway for formation of the chromophores and in terms of the linkage of chromophore to apoprotein. In recent studies, we have focused attention on the biosynthesis of phycocyanobilin (Scheme 1), the chromophore obtained from phycocyanin, in the rhodophyte Cyanidium caldarium (Brown et al., 1981). Dark-grown cells of this organism contain no photosynthetic pigments, but they may be readily induced to synthesize phycocyanin, allophycocyanin and chlorophyll a when they are resuspended in minimal medium and exposed to light (Bogorad et al., 1963). By using this method for induction of pigment, it was shown that protohaem is a biosynthetic precursor of phycocyanin in C. caldarium (Brown et al., 1981). The formation of the phycocyanin chromophore from haem requires two reduction steps in addition to the haem-cleavage step (see Scheme 1). If haem is cleaved directly to bile pigment before reduction, then biliverdin would be an intermediate in phycocyanin synthesis. If, however, one or both of the reduction steps occurs before haem cleavage, then biliverdin would not be an intermediate.