In my Frontiers in Plant Science hypothesis paper [1] I suggested that the steps in the process of photoactivation of the Mn4CaO5 cluster and the assembly of Photosystem II recapitulated evolutionary transitions. Then, using that as a framework and the available biochemical and biophysical literature on the topic, I made a prediction on how the early steps on photoactivation would actually go. This scenario had not been proposed before. Now, Gisriel et al., in a paper just published August 2020 [2], provide evidence supporting my prediction. The new paper is on the CryoEM structure of Photosystem II before photoactivation. Then, only a few months later further confirmation of my prediction was provided in a preprint from the group Marc Nowaczyk and collabarotaros [3].
Gisriel et al. 2020 [2] writes based on their structural analysis:
Since both D1-Glu189 and D1-Asp170 are clearly present in a similar location in apo-PSII as they are in mature PSII, and since a Mn2+ coordinated by this pair of residues agrees with the distance to it from YD⋅ established by EPR spectroscopy, we suggest that they form the high-affinity Mn2+ binding site; however, more investigation is required to confirm this. The distance between the two carboxylate moieties of D1-Asp170 and D1-Glu189 is ∼6.0 Å. If a Mn2+ ion is coordinated between them, and assuming there would not be an accompanying structural shift, the distance between a carboxylate and the Mn2+ would be ∼3.0 Å, which is slightly longer than but similar to the carboxylate-Mn ion distances found in mature PSII. If these residues indeed comprise the high-affinity binding site, it also suggests that the first Mn2+ binds near where the Ca2+ is observed in mature PSII, not the Mn4 site. This is reasonable because Ca2+ in solution is not a prerequisite for high-affinity Mn2+ site occupation, and the binding of Ca2+ following initial photo-oxidation of Mn2+ to Mn3+ results in a structural rearrangement of the OEC-binding site74 as further discussed below. We also note that the Ca2+ site is closest to YZ of the five metal-ion sites present in the fully assembled OEC, which may be important for efficient electron transfer from Mn2+ to YZ⋅ during the initial step of photoactivation.
In my paper, back to 2016, I wrote the following:
The first ligand to the Mn4CaO5 cluster to have evolved was likely a glutamate at position 170, followed by glutamate 189. This is paralleled during the process of photoactivation, as the first Mn(II) is oxidized to Mn(III) and bound to the high-affinity Mn binding site, known to be in part provided by D170 (Nixon and Diner, 1992; Campbell et al., 2000; Asada and Mino, 2015). The oxidation of Mn(II) to Mn(III) is accompanied by a deprotonating event of one of the ligating water molecules (Dasgupta et al., 2008). This is considered to be the first intermediate, which is unstable until the binding of Ca2+ followed by an uncharacterized conformational change (Tamura and Cheniae, 1987; Tamura et al., 1989; Chen et al., 1995; Tyryshkin et al., 2006). Beside D170 being part of the high affinity binding site, Dasgupta et al. (2007) suggested that the first bound Mn may also be coordinated by a N-donor ligand and speculated to be H332 or H337. Because these two histidines are too far from D170 to bind the same Mn, doubt was cast on the role of D170 as part of the high-affinity binding site (Becker et al., 2011). However, from an evolutionary perspective one would expect the first Mn to be bound by D170 and E189. If this is true, it can be predicted that the high-affinity binding site is located in a position near to the Ca2+ in the fully assembled cluster. In this position the first bound Mn is coordinated by D170, E189, and A344; the N-donor ligand detected by electron spin echo envelope modulation spectroscopy may be due to the presence of R357 from the CP43 subunit (Figure 2C). Although, R357 might appear counterintuitive as a N-donor ligand, arginine-metal interactions are not uncommon in metalloproteins; and arginine is known to ligate Mn in arginase (Di Costanzo et al., 2006). This position is not completely inconsistent with the distance measured by Asada and Mino (2015) using pulsed electron-electron double resonance spectroscopy considering that in the absence of the cluster the ligand sphere should be somewhat shifted. It is also consistent with a six-coordinate tetragonally-elongated or a five-coordinate square-pyramidal geometry as measured using EPR (Campbell et al., 2000). Upon Ca2+ binding, the uncharacterized conformational change is due to Ca2+ shifting the position of Mn and taking its correct position. Mn(III) then moves to a position similar to that of Mn4 in the crystal structure, where it is coordinated by D170 and E333. This is also consistent with the work by Cohen et al. (2007) that showed E333 mutants were impaired in the binding of the first Mn. After the binding and oxidation of Mn(II) to Mn(III), and the subsequent binding of Ca2+, a second Mn(II) is oxidized to Mn(III), which quickly leads to a fully assembled Mn4CaO5 cluster. Unfortunately, the fast events after the binding of the second Mn remain to be characterized; however, the evidence for an intermediary photoactivation step made of two Mn and involving at least D170 and E333 seems to be strong (Dasgupta et al., 2008; Becker et al., 2011).
This is Figure 2 from my paper. My hypothesis is represented in panel C.
I should note that it was the first time this had been proposed. I was then a bit nervous and considered the idea somewhat wild. I have to admit that I was feeling a bit embarrassed and thought that no one would buy it, but there you go… :)
References
Cardona, T., Reconstructing the origin of oxygenic photosynthesis: Do assembly and photoactivation recapitulate evolution? Front. Plant Sci., 2016. 7: 257. DOI: 10.3389/fpls.2016.00257.
Gisriel, C.J., K. Zhou, H.-L. Huang, R.J. Debus, Y. Xiong, and G.W. Brudvig, Cryo-EM Structure of Monomeric Photosystem II from Synechocystis sp. PCC 6803 Lacking the Water-Oxidation Complex. Joule, 2020. 4: 1-8. DOI: 10.1016/j.joule.2020.07.016.
Zabret, J., S. Bohn, S.K. Schuller, O. Arnolds, M. Möller, J. Meier-Credo, P. Liauw, A. Chan, E. Tajkhorshid, J.D. Langer, R. Stoll, A. Krieger-Liszkay, B.D. Engel, T. Rudack, J.M. Schuller, and M.M. Nowaczyk, How to build a water-splitting machine: structural insights into photosystem II assembly. bioRxiv, 2020: 2020.09.14.294884. DOI: 10.1101/2020.09.14.294884.