Oral Presentation 18th International Congress on Photobiology 2024

Activation of the Mn4CaO5 cofactor of Photosystem II as studied by High Field EPR and MCD spectroscopy (#76)

Nick Cox 1 , Julien Langley 1 , Jennifer Morton 1 , Robin Purchase 1 , Jian-Ren Shen 2 , Elmars Krausz 1
  1. Australian National University, Acton, ACT, Australia
  2. Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan

The structure of the S3 state of the Mn4CaO5 of Photosystem II (PSII) was recently reported using high field EPR spectroscopy [1] and XFEL crystallography [2-3]. It is this ‘final’ meta-stable S3 state that proceeds to O2 formation step following a further photo-oxidation event. These data are consistent with an all octahedral MnIV complex, requiring an additional water molecule to bind to cofactor to during the S2 to S3 transition, but the precise mechanism of water molecule insertion remains unclear. Historically, two approaches have been used to investigate intermediates of the S-state cycle that cannot be readily trapped and characterized: i) chemical modification of the cofactor; and ii) low temperature photochemistry. Here we describe new high field EPR and MCD data targeting intermediates of the S2 to S3 transition.

  1. High Field EPR data of chemical modified forms of the S3 state are consistent with the cofactor adopting two, structural distinct forms. These data include Ca2+/Sr2+ ion exchange, the binding of substrate analogs and the pH dependence of the S2 to S3 transition [4].
  2. MCD identifies the chromophore(s) responsible for the low temperature photochemistry of the cofactor - a series of sharp bands assigned to MnIV (4A2→ 2E) spin-flip transitions [5]. It is shown that these data are fully consistent with spin coupling models developed from earlier EPR/ENDOR studies and with the redox isomerism model, which explains the two S2 state forms of the cofactor.

Together, these data suggest that the S2 to S3 state transition proceeds in a step-wise fashion, with cofactor deprotonation and oxidation occurring before water molecule insertion [6]. Furthermore, they support substrate water insertion being coupled to spin state conversion of the cofactor [7]. The possible extension of these same methods towards the study of the O-O bond formation step is briefly discussed.

 

  1. Cox N, et al. (2014) Electronic structure of the oxygen-evolving complex in photosystem II prior to O-O bond formation. Science 345:804-808.
  2. Suga M, et al. (2015) Native structure of photosystem II at 1.95 A resolution viewed by femtosecond X-ray pulses. Nature 517:99-103.
  3. Kern J, et al. (2018) Structures of the intermediates of Kok’s photosynthetic water oxidation clock Nature 563:412-425.
  4. Chrysina et al. (2019) New intermediate in the activation of nature’s water splitting cofactor. Proc. Natl. Acad. Sci. U.S.A 116 (34), 16841-16846
  5. Morton J et al. (2018) Structured near-infrared Magnetic Circular Dichroism spectra of the Mn₄CaO₅ cluster of PSII in T. vulcanus are dominated by Mn (IV) d-d 'spin-flip' transitions. Biochim. Biophys. Acta 1859:88-98
  6. Retegan M, et al. (2016) A five-coordinate Mn(IV) intermediate in biological water oxidation: spectroscopic signature and a pivot mechanism for water binding. Chemical Science 7(1):72-84.
  7. Krewald V et al. (2016) Spin state as a marker for the structural evolution of nature’s water-splitting catalyst. Inorg. Chem. 55:488–501