Spectral and photochemical properties of borohydride‐treated D1‐D2‐cytochromeb‐559 complex of photosystem II

Abstract

The D1-D2-cytochrome b-559 reaction center complex of photosystem II with an altered pigment composition was prepared from the original complex by treatment with sodium borohydride (BH4-). The absorption spectra of the modified and original complexes were compared to each other and to the spectra of purified chlorophyll a and pheophytin a (Pheo a) treated with BH4- in methanolic solution. The results of these comparisons are consistent with the presence in the modified complex of an irreversibly reduced Pheo a molecule, most likely 13(1)-deoxo-13(1)-hydroxy-Pheo a, replacing one of the two native Pheo a molecules present in the original complex. Similar to the original preparation, the modified complex was capable of a steady-state photoaccumulation of Pheo- and P680+. It is concluded that the pheophytin a molecule which undergoes borohydride reduction is not involved in the primary charge separation and seems to represent a previously postulated photochemically inactive Pheo a molecule. The Qy and Qx transitions of this molecule were determined to be located at 5 degrees C at 679.5-680 nm and 542 nm, respectively.

Reaction centers of photosystem II with a chemically‐modified pigment composition: exchange of pheophytins with 131‐deoxo‐131‐hydroxy‐pheophytin a

“…PS II RCs were isolated from sugar‐beet chloroplasts by the method of Nanba and Satoh [1], essentially as described earlier [8] except that the column chromatography step was performed using Fractogel EMD DEAE 650 (S) cellulose rather then Fractogel TSK DEAE 650 (S).…”

“…A promising approach to address this problem is modification of the pigment composition of the RCs either by selective removal of constituent chromophores or by their chemical treatment. Several preparations of PS II RCs with an altered pigment content have been recently described in the literature [7–9]. The first modified preparation of the PS II RC was obtained by isolation of the RC on a Cu affinity column [7].…”

“…The preparation contained five Chls (instead of six) per two Pheos, with the missing Chl apparently having been lost from a peripheral position and serving a role in energy transfer to P680. Another type of modification has been achieved by irreversible reduction of the Pheo molecule not involved in charge separation (hereafter referred to as Pheo inactive ) to 13 1 ‐deoxo‐13 1 ‐hydroxy‐Pheo (13 1 ‐OH‐Pheo) by reaction with NaBH 4 [8].…”

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“…In general, it is now accepted that the active Pheo absorbs near 680 nm, but there is no consensus on the assignment of the inactive Pheo. In particular, the inactive Pheo Q y absorption has been reported at 668 nm (Jankowiak et al 1999), 670 nm (Konermann and Holzwarth 1996;Konermann et al 1997b;Dedic et al 2000), 671 nm (Mimuro et al 1995), 672 to 674 nm (Yruela et al 1999), 676 nm (Germano et al 2001) and near 680 nm (Shkuropatov et al 1997).…”

Dynamic protein conformations preferentially drive energy transfer along the active chain of the photosystem II reaction centre

“…A fascinating fact concerning the PSII is that cofactors are arranged in two branches with a C2 symmetry, but half of them (one complete branch) are not involved in the primary electron transfer from water to plastoquinone789. Due to this, the underlying C2 branches are termed: active and inactive chains, according to their contributions to the primary electron transfer.…”