Pigment Stoichiometry of the Photosystem II Reaction Center from Higher Plants

A preparation of Photosystem II reaction center complex containing 4 chlorophylls per 2 pheophytins was obtained from three different species of higher plants. A 'standard' reaction center preparation (6 Chl/2 Pheo) was used as starting material. This material was loaded onto a sieve chromatography Superose-6 column run on a FPLC system. Three different sets of fractions were eluted from the column. The first few fractions showed a distinct absorption band at 435 nm and pigment content of 10 Chl per 2 Pheo. The peak fractions did not show a distinct band at 435 nm, but rather a shoulder, and contained 4 Chl per 2 Pheo. The few last fractions exhibited relatively higher absorption at 416 nm, most probably due to higher cytochrome content, but they also contained 4 Chl per 2 Pheo. Similar results were observed when a reaction center preparation of about 5 Chl was loaded onto the Superose-6 column. This indicates that the 'standard' Photosystem II reaction center preparation is heterogenous, composed of 4 Chl reaction centers and contaminants containing higher pigment content. To further validate this conclusion, we obtained a 4 Chl preparation by modification of the 'standard' reaction center purification procedure. After loading this preparation onto the Superose-6 column, one single chromatographic band was obtained, and all fractions contained 4 Chl per 2 Pheo. This strongly suggests that a 4 Chl per 2 Pheo preparation corresponds to that of a native core Photosystem II reaction center preparation.