In the BRC, the SP of pigments is formed by BChl a molecules, which are denoted as D A and D B (green). Similar BChl a molecules compose the pair of accessory co-factors B A and B B (blue). It is interesting to note that the central magnesium atoms of these four pigments are all coordinated by axial His molecules. In contrast to the case of PSII, the BRC features a pair of BPhe a molecules instead of Phe a, which are called H A and H B and depicted in red in Fig. . The pair of quinone molecules is represented by menaquinone Q A and ubiquinone Q B . Additionally, the BRC contains, just like in PSII, a Fe(II) ion (brown) located between the Q A and Q B co-factors. PDB entries 3WU2 , 1JB0 , and 1M3X . For the color code used, see the main text.