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
Kh. Zhao et H. Scheer, 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, Biochimica et biophysica acta. Bioenergetics, 1228(2-3), 1995, pp. 244-253
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.