TYPE-I REVERSIBLE PHOTOCHEMISTRY OF PHYCOERYTHROCYANIN INVOLVES Z/E-ISOMERIZATION OF ALPHA-84 PHYCOVIOLOBILIN CHROMOPHORE

Type I reversible photochemistry of the alpha-subunit of phycoerythrocyanin (alpha-PEC) from the cyanobacterium, Mastigocladus laminosus, and of chromopeptides derived from both states, was studied by UV-Vis absorption and H-1-NMR spectroscopy. (1) Peptic peptides bearing the two isomeric chromophores were separated by chromatography in acid medium on Bio-Gel P10. This allowed a quantitation of the isomer composition in denatured as well as in native alpha-PEC, based on the extinction coefficient of the Z-peptide (Bishop et al. (1987) J. Am. Chem. Soc. 109, 875-881). (2) H-1-NMR spectra of peptic chromopeptides in both states, were assigned by comparison with spectra of a tryptic peptide (Bishop et al., see above) and from the difference spectra. They differed mainly by the positions of the 10-H and 15-H signals of the chromophore, and of signals related to the substituents of ring D. (3) These signals are (partially) interconvertible photochemically, without effect on most of the signals derived from the amino-acid residues. (4) Phototransformation of the native chromoprotein is accompanied by several reversible changes in the H-1-NMR spectrum, in particular by a 0.11 ppm shift of an isolated sinlet at delta = 5.7 ppm assigned to the 15H-proton of the chromophore, and in the region of aromatic amino-acid residues. The data suggest that the reversible phototransformation involves the 15,16-Z/E-isomerization of the phycoviolobilin chromophore between rings C and D, similar to the reaction of the phytochromobilin chromophore in the plant photoreceptor, phytochrome. They also indicate changed interactions between chromophore and apoprotein, in particular with nearby aromatic residues, in the two isomeric states.