Overview

There are two sets of similar electron transfer cofactors arranged approximately symmetrically along the non- crystallographic pseudo-C2 axis, one at the D1 side and one at the D2 side (Figure 4.3a). The active electron transfer chain is located at the D1 side, since the Mn4Ca cluster is bound to the D1 side only. The charge separation occurs at four Chls bound to the D1/D2 subunits (Figure 4.3b), which are designated RC-Chls. Among these four Chls, two Chls, PD1 and PD2, are located toward the lumenal side with their head groups arranged parallel to each other and perpendicular to the membrane plane. PD1 and PD2 are considered as the R.C. Chls of PSII, namely P680 (however, see 4.3.2). The other two Chls surround PD1 and PD2 respectively; they are designated ChlD1 and ChlD2 and are called accessory Chls because of their similar location as the bacterial accessory Chls. These two accessory Chls are tilted against the membrane plane by approximate 30°. These Chls form a cluster in a region from the middle to the lumenal side of the membrane. The electrons generated in the initial charge separation events at the RC-Chls are transferred to a pheophytin molecule (PheoD1) at the D1 side, with a center-to-center distance of 10.5 A from ChlD1 to the cytoplasmic membrane surface. The electrons from PheoD1 are then transferred to QA, the first bound plastoquinone accepter, 13.1 A away from the center of PheoD1i and then to the second bound quinone acceptor QB.

At the donor side, tyrosine of D1-161 donates electrons directly to the RC-Chls; this is designated TyrZ and is located 13.6 A from the center of PD1. A symmetrically located Tyr in the D2 side, namely, D2-Tyr-160 or TyrD, is found which although not directly involved in electron transfer, may play a role in photoassembly of the Mn4Ca cluster. The MniCa cluster is located 5.4 A away from D1-TyrZ, which sequentially extracts electrons from water and donates them to TyrZ. After four electrons are removed from two molecules of water, one molecule of oxygen is formed. This process is accomplished by the release of four protons, whose exact pattern is still under debate. During this process, the configuration of the Mn4Ca cluster is changed through the S-state cycle (where i = 0-4) as defined by the Kokmodel [33, 34].

Local c2-axis

Local c2-axis

Figure 4.3 (a) Arrangement of the electron transfer chain and other cofactors in PSII-RC from a side view. Distances between the centers of each component were shown in A. (b) An enlarged picture of the four Chls located in the center of PSII-RC. Distances between the edges of each component were shown in A.

Figure 4.3 (a) Arrangement of the electron transfer chain and other cofactors in PSII-RC from a side view. Distances between the centers of each component were shown in A. (b) An enlarged picture of the four Chls located in the center of PSII-RC. Distances between the edges of each component were shown in A.

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