TY - JOUR
T1 - Substitution of a Chlorophyll into the Inactive Branch Pheophytin-Binding Site Impairs Charge Separation in Photosystem II
AU - Xiong, Ling
AU - Seibert, Michael
AU - Gosev, Alexey V.
AU - Wasielewski, Michael R.
AU - Hemann, Craig
AU - Hille, C. Russ
AU - Sayre, Richard T.
PY - 2004
Y1 - 2004
N2 - All photosynthetic reaction centers (RCs) have two parallel sets of electron transfer cofactors that cross the membrane. In quinone-type RCs (including photosystem II (PSII)), however, only one pathway (the active branch) is used for electron transfer. Since the electron transfer cofactors of each pathway have nearly identical distance and orientation relationships, it is assumed that local differences in protein environment determine the directionality of electron transfer. To understand further the factors that affect energy distribution among the PSII RC cofactors, we altered the PSII RC cofactor symmetry by replacing the inactive-branch pheophytin (Pheo) with a chlorophyll (Chl). We mutated the D1-L210 residue to a histidine (D1-L210H) to provide a Mg ligand for Chl. Analyses of the pigment composition of D1-L210H RCs indicated that the inactive-branch Pheo had been replaced by a Chl. Comparisons of wild-type and D1-L210 transient absorption spectra confirmed that the red-shifted Pheo Qx absorption band (543.5 um) belonged to the active-branch Pheo. Surprisingly, intact D1-L210H PSII complexes were unable to evolve oxygen, lacked Chl variable fluorescence, (following a flash), and were unable to photoaccumulate reduced Qa, indicating that electron transfer in D1-L210H PSII complexes was severely perturbed. The kinetics of primary charge separation, however, were not substantially altered in D1-L210H RCs, indicating that the Chl substitution had not perturbed the energetics of the primary electron donor/acceptor pair. Significantly, intact D1-L210H PSII core complexes had a substantially increased and red-shifted Chl fluorescence emission band attributed to fluorescence from Chl's of the distal antenna complex as well as a blue-shifted fluorescence emission peak attributed to Chl's of the proximal antenna complex (77 K). These results are interpreted in terms of a redistribution of the excited-state energy among the pigments of the RC multimer, leading to loss of the excited state via fluorescence in the D1-L210H mutant.
AB - All photosynthetic reaction centers (RCs) have two parallel sets of electron transfer cofactors that cross the membrane. In quinone-type RCs (including photosystem II (PSII)), however, only one pathway (the active branch) is used for electron transfer. Since the electron transfer cofactors of each pathway have nearly identical distance and orientation relationships, it is assumed that local differences in protein environment determine the directionality of electron transfer. To understand further the factors that affect energy distribution among the PSII RC cofactors, we altered the PSII RC cofactor symmetry by replacing the inactive-branch pheophytin (Pheo) with a chlorophyll (Chl). We mutated the D1-L210 residue to a histidine (D1-L210H) to provide a Mg ligand for Chl. Analyses of the pigment composition of D1-L210H RCs indicated that the inactive-branch Pheo had been replaced by a Chl. Comparisons of wild-type and D1-L210 transient absorption spectra confirmed that the red-shifted Pheo Qx absorption band (543.5 um) belonged to the active-branch Pheo. Surprisingly, intact D1-L210H PSII complexes were unable to evolve oxygen, lacked Chl variable fluorescence, (following a flash), and were unable to photoaccumulate reduced Qa, indicating that electron transfer in D1-L210H PSII complexes was severely perturbed. The kinetics of primary charge separation, however, were not substantially altered in D1-L210H RCs, indicating that the Chl substitution had not perturbed the energetics of the primary electron donor/acceptor pair. Significantly, intact D1-L210H PSII core complexes had a substantially increased and red-shifted Chl fluorescence emission band attributed to fluorescence from Chl's of the distal antenna complex as well as a blue-shifted fluorescence emission peak attributed to Chl's of the proximal antenna complex (77 K). These results are interpreted in terms of a redistribution of the excited-state energy among the pigments of the RC multimer, leading to loss of the excited state via fluorescence in the D1-L210H mutant.
UR - http://www.scopus.com/inward/record.url?scp=8344274263&partnerID=8YFLogxK
U2 - 10.1021/jp040262d
DO - 10.1021/jp040262d
M3 - Article
AN - SCOPUS:8344274263
SN - 1520-6106
VL - 108
SP - 16904
EP - 16911
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 43
ER -