Phycocyanobilins

The algal extract of P. aerugineum is blue, with maximum absorbance at a wavelength of 620 nm and a red fluorescence with maximum emission at 642 nm. The main phycobiliprotein, C-phycocyanin, is the same type of phycocyanin found in most Cyanobacteria. The chromophores are composed of phycocyanobilins, conjugated to an apoprotein via thioether bonds. 

Although there is very little data on the mechanistic aspects of the biosynthetic enzymes, it is thought that their reaction mechanisms are very similar to those of the mammalian heme oxygenases. Indirect evidence that the mechanism of action of the biosynthetic enzymes is similar to that of the mammalian heme oxygenases was provided by 0-labeling studies showing that the biosynthesis of phycocyanobilin occurred via a two-molecule mechanism (141, 142) comparable to that of the mammalian heme oxygenases (8). 

Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19-40b) as their light-harvesting pigments. These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or... 

FIGURE 19-43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text. 

Figure 5. The red-light absorbing chromophore of Pr phytochrome. A stretched conformation with Z,Z,Z configuration of the exocyclic double bonds was chosen in analogy to that of the phycocyanobilin chromophores a-84 and jS-84 in C-phyco-cyanin of cyanobacteria [52], For other proposals of conformation based on spectroscopic comparisons, see Scheer [47] and Rudiger [48], and for the protonated form of the pyrrolenine nitrogen in Pr, see text, and Lagarias and Rapoport [41] and Fodor et al. [53].

Photomorphogenesis, 231 Phycocyanobilin, 237 Phytochrome, 230 molecular weight of, 232 native undegraded, 232 P(r (far-red absorbing form), 236 bilatrene chromophore structure of, 236 photophysical properties of, 236 Pfr — Pr transformation, 234 Pr — Pfr transformation of, 234 chemical nature of individual reaction steps of, 263... 

Cysteinylheme S-Cysteinyl phycocyanobilin Cysteine-S-phosphate Threonine O -Phosphothreonine Glycosylation... 

Four major phycobilins occur in photosynthetic organisms, the main ones being phycocyanobilin and phycoerythrobilin (structures in Fig. 5-9 note the great structural similarity between the phycobilins and the chro-mophore for phytochrome in Fig. 4-15). Phycoerythrobilin plus the protein to which it is covalently attached is called phycoerythrin.1 Phycoery thrin is... 

Figure 5-9. Structure of two phycobilins that act as important accessory pigments. Phycoerythrobilin has fewer double bonds in conjugation than phycocyanobilin, so its Xmax occurs at shorter wavelengths (Fig. 5-8). Phycobilins occur covalently bound to proteins that is, they are the chromo-phores for phycobiliproteins.

Peptide - linked PHYCOCYANOBILIN Peptide-linked PHYCOBILIVIOLIN... 

Phycobiliproteins, such as phycocyanin and phycoerythrin, are members of a family of fluorescent accessory, nonchlorophyll-based pigments found in cyanobacteria and eukaryotic algae. The phycobiliproteins have characteristic broad absorption profiles spanning 450-600 nm, emissions ranging 570-660 nm, and small stokes shifts see Fig. 3. The major structural subunits, phycoerythrobilin (PEB) or phycocyanobilin... 

The investigation of the spectral properties of unfolded phytochrome65) revealed that the Pr chromophore has a A-dihydrobiliverdin structure (like phycocyanobilin in situ). The red shift of the phytochrome chromophore compared with the phyco-cyanin chromophore (Table 4) can be explained by the presence of a vinyl group in the former and a ethyl group in the latter. The A-dihydrohiliverdin structure has been confirmed by comparison with the synthetic model compound /7158), (cf. Table 5). Structure 18 has been derived from these data — together with the results of the degradation experiments — for the Pr chromophore in situ. 

Fig. 8. Phycobilin pigments (phycocyanobilin and phycoerythrobilin) linked to polypeptide.

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