P680 and pheophytin. The second pair of chlorophylls is coordinated by symmetry-related histidine
residues at the periphery of the RC complex. These peripheral accessory chlorophylls are excitonically
coupled to the antennae chlorophylls and to the P680 [8–10]. In addition to their light-harvesting func-
tion, it was proposed that the peripheral accessory chlorophyll(s) (ChlZ) function in an electron transfer
cycle around PSII (including QBand cytochrome b 559 ), which protects PSII from accumulating long-lived
P680states and consequently from D1 protein degradation [10].
B. Subunits of PSII Reaction Center Complex
- PsbA-D1 Protein
D1 protein is a highly conserved RC protein with a molecular mass of about 38 kDa depending on the
species [6]. From hydropathy plots and comparison with the L subunit of the reaction center of purple
bacteria, it is assumed that D1 contains five transmembrane helices (I to V) and two surface helices be-
tween II and IV (luminal) and between IV and V (stromal). The N-terminal threonine can be reversibly
phosphorylated [11]. D1 protein binds the majority of cofactors involved in PSII-mediated electron
282 DENEV AND MINKOVFigure 1 Transmission electron micrograph of a Haberlea rhodopensis(Friv.) chloroplast (magnification
20,000).
Figure 2 Scheme for PSII of higher plants emphasizing subunit composition and primary and secondary
electron transfer steps that occur in the reaction center. (From Ref. 6.)