Bound manganese oxides capable of reducing the bacteriochlorophyll dimer of modified reaction centers from Rhodobacter sphaeroides

Eduardo Espiritu, Kori D. Chamberlain, Jo Ann C. Williams, James P. Allen

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Abstract

A biohybrid model system is described that interfaces synthetic Mn-oxides with bacterial reaction centers to gain knowledge concerning redox reactions by metal clusters in proteins, in particular the Mn4CaO5 cluster of photosystem II. The ability of Mn-oxides to bind to modified bacterial reaction centers and transfer an electron to the light-induced oxidized bacteriochlorophyll dimer, P+, was characterized using optical spectroscopy. The environment of P was altered to obtain a high P/P+ midpoint potential. In addition, different metal-binding sites were introduced by substitution of amino acid residues as well as extension of the C-terminus of the M subunit with the C-terminal region of the D1 subunit of photosystem II. The Mn-compounds MnO2, αMn2O3, Mn3O4, CaMn2O4, and Mn3(PO4)2 were tested and compared to MnCl2. In general, addition of the Mn-compounds resulted in a decrease in the amount of P+ while the reduced quinone was still present, demonstrating that the Mn-compounds can serve as secondary electron donors. The extent of P+ reduction for the Mn-oxides was largest for αMn2O3 and CaMn2O4 and smallest for Mn3O4 and MnO2. The addition of Mn3(PO4)2 resulted in nearly complete P+ reduction, similar to MnCl2. Overall, the activity was correlated with the initial oxidation state of the Mn-compound. Transient optical measurements showed a fast kinetic component, assigned to reduction of P+ by the Mn-oxide, in addition to a slow component due to charge recombination. The results support the conjecture that the incorporation of Mn-oxides by ancient anoxygenic phototrophs was a step in the evolution of oxygenic photosynthesis.

Original languageEnglish (US)
JournalPhotosynthesis research
DOIs
StateAccepted/In press - Jan 1 2019

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Keywords

  • Bacterial reaction centers
  • Electron transfer
  • Mn-cofactors
  • Optical spectroscopy
  • Secondary electron donors

ASJC Scopus subject areas

  • Biochemistry
  • Plant Science
  • Cell Biology

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