Journal Article

Five-coordinate Mn IV intermediate in the activation of nature’s water splitting cofactor

Nilsson, Håkan, Cox, Nicholas, Kutin, Yury, Neese, Frank, Chrysina, Maria, Reus, Michael, Messinger, Johannes, Lubitz, Wolfgang, DeBeer, Serena, Heyno, Eiri, Nowaczyk, Marc M.

2019

Overview

Recent results have shown that nature’s water splitting catalyst inserts an additional water molecule into what appears to be a solvent inaccessible site late in its reaction cycle. The emerging consensus of the field is that this water molecule is one of the substrates of the reaction. Here, we show that this water molecule does not come directly from solvent. It instead represents an earlier bound water, which is inserted into this site via facile structural tautomerism. The trigger for this process is cofactor oxidation. This then allows an additional water to bind from solvent to a more open site of the cofactor. In this way the cofactor carefully regulates water uptake, preventing water insertion earlier in the reaction cycle. Nature’s water splitting cofactor passes through a series of catalytic intermediates (S 0 -S 4 ) before O-O bond formation and O 2 release. In the second last transition (S 2 to S 3 ) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all Mn IV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S 4 . How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S 3 ') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (>50%) in ( i ) chemical-modified cofactors in which Ca 2+ is exchanged with Sr 2+ ; the Mn 4 O 5 Sr cofactor remains active, but the S 2 -S 3 and S 3 -S 0 transitions are slower than for the Mn 4 O 5 Ca cofactor; and ( ii ) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S 3 ' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S 3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S 2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S 3 ' cofactor is similar to the high spin form of the preceding S 2 state, which contains a cuboidal Mn 3 O 4 Ca unit tethered to an external, 5-coordinate Mn ion (Mn4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn4. The interaction of ammonia with the cofactor is also discussed.

• Subject: EDNMR / EPR / methanol / Photosystem II / WOC/OEC