TYPE-I AND TYPE-II REVERSIBLE PHOTOCHEMISTRY OF PHYCOERYTHROCYANIN ALPHA-SUBUNIT FROM MASTIGOCLADUS-LAMINOSUS BOTH INVOLVE Z-ISOMERIZATION,E-ISOMERIZATION OF PHYCOVIOLOBILIN CHROMOPHORE AND ARE CONTROLLED BY SULFHYDRYLS IN APOPROTEIN

The alpha-subunit of phycoerythrocyanin (alpha-PEC) can exist in four states (Z-alpha(I), Z-alpha(II), E-alpha(I), E-alpha(II)). They are co nnected pairwise by photoreversible photochromism. The type I photoche mistry connecting Z-alpha(I) and E-alpha(I), involves a I5Z/E phototra nsformation, alpha-PEC showing this type of photochemistry is obtained when the subunits of PEC are separated by gel permeation chromatograp hy in the presence of 63 mM formic acid, or by reduction of the alpha- subunit of phycoerythrocyanin of type II reversible photochemistry wit h mercaptoethanol. alpha-PEC showing the recently characterized [Hong et al. (1993) Photochem. Photobiol. 58, 745-747] type II photochemistr y connecting Z-alpha(II) and E-alpha(II) can be obtained when the alph a-subunit of phycoerythrocyanin of type I photochemistry is allowed to oxidize, or when it is treated with p-chloromercuribenzenesulfonate. The two types of reversible photochemistry of alpha-subunit of phycoco erythrocyanin are therefore controlled by the state of the two sulfhyd ryl group(s), viz. Cys-98,99 of the apoprotein. A quantitative analysi s of the PCMS titration showed that modification of either one of thes e two cysteine residues is sufficient to inhibit type I photochemistry and induces type II. By treatment with mercaptoethanol or PCMS, the e nd products of type I and type II photochemistry, respectively, could be pairwise transformed into each other, showing that type II also inv olves I5Z/E isomerization. The difference between them must be due to different interactions between phycoviolobilin and apoprotein, which c an be modulated by the two sulfhydryls.