Key cofactors of Photosystem II cores from four organisms identified by 1.7-K absorption, CD and MCD

Active Photosystem II (PS II) cores were prepared from spinach, pea, Synechocystis PCC 6803, and Thermosynechococcus vulcanus, the latter of which has been structurally determined Kamiya and Shen (2003) Proc Natl Acad Sci USA 100: 98–103]. Electrochromic shifts resulting from Q_A reduction by 1.7-K illumination were recorded, and the Q_x and Q_y absorption bands of the redox-active pheophytin a thus identified in the different organisms. The Q_x transition is ∼3 nm (100 cm⁻¹) to higher energy in cyanobacteria than in the plants. The predominant Q_y shift appears in the range 683–686 nm depending on species, and does not appear to have a systematic shift. Low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of the chlorophyll Q_y region are very similar in spinach and pea, but vary in cyanobacteria. We assigned CP43 and CP47 trap-chlorophyll absorption features in all species, as well as a P680 transition. Each absorption identified has an area of one chlorophyll a. The MCD deficit, introduced previously for spinach as an indicator of P680 activity, occurs in the same spectral region and has the same area in all species, pointing to a robustness of this as a signature for P680. MCD and CD characteristics point towards a significant variance in P680 structure between cyanobacteria, thermophilic cyanobacteria, and higher plants.