CP47 binds 16 Chls and CP43 binds 13 Chls . Most of these Chls are arranged in two layers, one toward the stromal surface and the other toward the lumenal surface (Figure 4.5a), which is similar to the N -ierminal domains of PsaA and PsaB in PSI [ 18]. There are eightChls each in CP47 and CP43 in the layer close to the cytoplasmic surface, seven Chls in CP47, and four Chls in CP43 in the layer close to the lumenal surface. The arrangement of Chls in a layer close to the stromal surface may facilitate the absorption of light-energy, as well as energy transfer from phycobilisome antenna attached on the cytoplasmic surface of PSII. The Chl layer close to the lumenal side, however, should be less involved in excitation energy transfers from phycobilisomes and energy absorption (as the light intensity reaching to the lumenal layer would become weaker); this may account for why the numbers of Chls present in the lumenal layer are fewer than those in the cytoplasmic layer, especially in the case of CP43. Nevertheless, the Chls in the lumenal side layer may be necessary to fulfill the spaces between TMHs in both CP47 and CP43, in order to maintain their stable structures. In fact, mutagenesis of a single His residue that is coordinated to a Chl has been found to produce a significant effect on PSII oxygen-evolving activity, in most cases in both CP47 and CP43 (for review, see ).
The Chls within the two layers of both CP47 and CP43 are arranged closely with each other, with their Mg-Mg distances ranging between 8.1-13.6A, enabling a fast equilibrium of excitation energy within each layer [76, 77]. The distances between the two layers, however, are above 16 A, rendering a direct energy transfer between the Chls in two layers less efficient. This is overcome by a single Chl each in CP47 and CP43 located in the middle of the two pigment layers (Figure 4.5a);
Figure 4.5 (a) Arrangement of Chls and other cofactors in a PSII monomer of T. elongatus from a side view. The protein backbone structures were shown in light colors. The arrows in CP47 and CP43 indicate two Chls located in the middle between the two Chl layers in the cytoplasmic and lumenal side. Two Pheo molecules and QA, QB are shown in blue, and heam rings of Cyt c550, Cyt
Figure 4.5 (a) Arrangement of Chls and other cofactors in a PSII monomer of T. elongatus from a side view. The protein backbone structures were shown in light colors. The arrows in CP47 and CP43 indicate two Chls located in the middle between the two Chl layers in the cytoplasmic and lumenal side. Two Pheo molecules and QA, QB are shown in blue, and heam rings of Cyt c550, Cyt b559 in red, TyrZ, TyrD in orange, Mn in red, Ca in yellow. (b) Same as (a) with a top view on to the membrane plane from the lumenal side. Arrows indicate four Chls located between CP47 and RC, and between CP43 and RC that are suggested as mediating energy transfer from bulk Chls of CP47 and CP43 to PSII-RC.
the distances between this Chl and each of the two layers are 9- 10 A . This Chl is thus important for mediating energy transfer between the two layers, enabling the fast equilibrium of excitation energy also possible between the two layers.
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