No abstract

Cytochrome c-550, a low-potential c-type cytochrome, and a 12-kDa protein were recently shown to be associated extrinsically and stoichiometrically with purified photosystem II (PSII) complex of the thermophilic cyanobacterium Synechococcus vulcanus [Shen, J.-R., Ikeuchi, M., & Inoue, Y. (1992) FEBS Lett. 301, 145-149]. The binding and functional properties of these two extrinsic components in PSII were studied by means of release-reconstitution and thermoluminescence techniques. The following results were obtained: (i) cyt c-550 rebound appreciably to cyanobacterial PSII in the absence of the 33- and 12-kDa extrinsic proteins, but the presence of these two proteins facilitated the rebinding, affording a full level of binding equal to that in native PSII. (ii) The 12-kDa protein did not rebind to PSII at all unless the 33-kDa protein or cyt c-550 was present. It rebound only partially in the presence of either of these two proteins, but it rebound maximally when reconstituted together with both of them. (iii) Reconstitution with cyt c-550 or the 12-kDa protein alone in the absence of the 33-kDa protein did not restore the O2-evolving activity of CaCl2-washed PSII. Reconstitution with cyt c-550 in combination with the 33-kDa protein appreciably enhanced the activity, but the activity restoration was much more marked and reached a level close to that of the original activity when all three extrinsic proteins were included.(ABSTRACT TRUNCATED AT 250 WORDS)

Fourier transform infrared (FTIR) signals of a histidine side chain were identified in flash-induced S(2)/S(1) difference spectra of the oxygen-evolving complex (OEC) of photosystem II (PS II) using PS II membranes from globally (15)N-labeled spinach and PS II core complexes from Synechocystis cells in which both the imidazole nitrogens of histidine were selectively labeled with (15)N. A negative band at 1113-1114 cm(-1) was downshifted by 7 cm(-1) upon both global (15)N-labeling and selective [(15)N]His labeling, and assigned to the C-N stretching mode of the imidazole ring. This band was unaffected by H-D exchange in the PS II preparations. In addition, several peaks observed at 2500-2850 cm(-1) all downshifted upon global and selective (15)N-labeling. These were ascribed to Fermi resonance peaks on a hydrogen-bonding N-H stretching band of the histidine side chain. FTIR measurements of model compounds of the histidine side chain showed that the C-N stretching band around 1100 cm(-)(1) can be a useful IR marker of the protonation form of the imidazole ring. The band appeared with frequencies in the following order: Npi-protonated (>1100 cm(-1)) > imidazolate > imidazolium > Ntau-protonated (<1095 cm(-1)). The frequency shift upon N-deuteration was occurred in the following order: imidazolium (15-20 cm(-1)) > Ntau-protonated (5-10 cm(-1)) > Npi-protonated approximately imidazolate ( approximately 0 cm(-1)). On the basis of these findings together with the Fermi resonance peaks at >2500 cm(-1) as a marker of N-H hydrogen-bonding, we concluded that the histidine residue in the S(2)/S(1) spectrum is protonated at the Npi site and that this Npi-H is hydrogen bonded. This histidine side chain probably ligated the redox-active Mn ion at the Ntau site, and thus, oxidation of the Mn cluster upon S(2) formation perturbed the histidine vibrations, causing this histidine to appear in the S(2)/S(1) difference spectrum.

X-ray absorption near-edge structure spectra of the manganese (Mn) cluster in physiologically native intermediate states of photosynthetic water oxidation induced by short laser flash were measured with a compact heat-insulated chamber equipped with an x-ray detector near the sample surface. The half-height energy of the Mn Kedge showed a period-four oscillation dependent on cycling of the Joliot-Kok's oxygen clock. The flash number-dependent shift in the Mn K-edge suggests that the Mn cluster is oxidized by one electron upon the S(0)-to-S(1), S(1)-to-S(2), and S(2)-to-S(3) transitions and then reduced upon the S(3)-to-S(0) transition that releases molecular oxygen.