Influence of the 33 kDa Manganese-Stabilizing Protein on the Structure and Substrate Accessibility of the Oxygen-Evolving Complex of Photosystem II

Abstract

The 33 kDa manganese-stabilizing extrinsic protein binds to the lumenal side of photosystem II (PS II) close to the Mn 4 Ca cluster of the oxygen-evolving complex, where it limits access of small molecules to the metal site. Our previous finding that the removal of this protein did not alter the magnetic coupling regime within the manganese cluster, measured by electron spin-echo envelope modulation [Gregor, W., and Britt, R. D. (2000) Photosynth. Res. 65,[175][176][177][178][179][180][181][182][183][184][185], prompted us to examine whether this accessibility control is also true for substrate water, using the same pulsed EPR technique. Comparing the deuteron modulation of the S 2 -state multiline signal of PS II membranes, equilibrated with deuterated water (D 2 O) after removal or retention of the 33 kDa protein, we observed no change in the number and the distance of deuterons magnetically coupled to manganese, indicating that the number and distance of water molecules bound to the manganese cluster are independent of bound 33 kDa protein in the S 1 state, in which the sample was poised prior to cryogenic illumination. A simple modulation depth analysis revealed a distance of 2.5-2.6 Å between the closest deuteron and manganese. These results are in agreement with our refined Xray absorption analysis. The manganese K-edge positions, reflecting their oxidation states, and the extended X-ray absorption fine structure amplitudes and distances between the manganese ions and their oxygen and nitrogen ligands (1.8, 2.7, and 3.3-3.4 Å) were independent of bound 33 kDa protein.Oxidation of ubiquitous water to dioxygen is the basis for plant-type (oxygenic) photosynthesis, carried out by cyanobacteria, algae, and green plants. It produces the majority of the earth's biomass, and the resultant dioxygen delivered into the atmosphere † This work was funded by National Institutes of Health Grants GM48242 (R.D.B.) and GM55302 (V.K.Y.), the

18O-Water exchange in photosystem II: Substrate binding and intermediates of the water splitting cycle

“…However, 18 O exchange measurements of PSII samples in which the PsbO protein was biochemically removed did not show any significant increase in the exchange rates in the S 3 state, implying this was not a limiting kinetic property [40]. Similarly, ESEEM measurements of the proton couplings with the catalytic Mn in the S 1 state did not reveal any significant differences in PsbO-depleted samples [50]. However, it may be possible that excess Ca 2+ added for activity results in subtle conformational changes in PSII that recreates a kinetic protein barrier previously formed by the PsbO protein.…”

The Possible Role of Proton‐Coupled Electron Transfer (PCET) in Water Oxidation by Photosystem II

“…[15,19,21,32,35,129,213] K-edge difference spectra from XANES measurements show that structural changes in the S 1 !S 2 transition, which involve oxidation of Mn III to Mn IV , are significantly different from changes in the S 2 !S 3 transition. [140,214] EXAFS data also point to a structural change between the S 2 and S 3 states, [20,42,49,203,[215][216][217][218] but different results have been reported from different studies. An equilibrium between at least two states in S 3 has been proposed by Renger.…”

Proton-Coupled Electron Transfer

“…647 EXAFS data point to a significant structural change between S 2 and S 3 , but different interpretations have been offered in different studies. 657,670,701,726,728,758,[770][771][772][773][774][775] It has also been proposed that there is an equilibrium between at least two states in S 3 . 677,770 From XANES measurements, K edge difference spectra for the S 1 f S 2 and S 2 f S 3 transitions are significantly different, with oxidation of Mn(III) to Mn(IV) occurring in the former.…”