Binding and Energetics of Electron Transfer between an Artificial Four-Helix Mn-Protein and Reaction Centers from Rhodobacter sphaeroides

Eduardo Espiritu, Tien L. Olson, Joann Williams, James Allen

Research output: Contribution to journalArticle

Abstract

The ability of an artificial four-helix bundle Mn-protein, P1, to bind and transfer an electron to photosynthetic reaction centers from the purple bacterium Rhodobacter sphaeroides was characterized using optical spectroscopy. Upon illumination of reaction centers, an electron is transferred from P, the bacteriochlorophyll dimer, to QA, the primary electron acceptor. The P1 Mn-protein can bind to the reaction center and reduce the oxidized bacteriochlorophyll dimer, P+, with a dissociation constant of 1.2 μM at pH 9.4, comparable to the binding constant of c-type cytochromes. Amino acid substitutions of surface residues on the Mn-protein resulted in increases in the dissociation constant to 8.3 μM. The extent of reduction of P+ by the P1 Mn-protein was dependent on the P/P+ midpoint potential and the pH. Analysis of the free energy difference yielded a midpoint potential of approximately 635 mV at pH 9.4 for the Mn cofactor of the P1 Mn-protein, a value similar to those found for other Mn cofactors in proteins. The linear dependence of -56 mV/pH is consistent with one proton being released upon Mn oxidation, allowing the complex to maintain overall charge neutrality. These outcomes demonstrate the feasibility of designing four-helix bundles and other artificial metalloproteins to bind and transfer electrons to bacterial reaction centers and establish the usefulness of this system as a platform for designing sites to bind novel metal cofactors capable of performing complex oxidation-reduction reactions.

Original languageEnglish (US)
Pages (from-to)6460-6469
Number of pages10
JournalBiochemistry
Volume56
Issue number49
DOIs
StatePublished - Dec 12 2017

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Rhodobacter sphaeroides
Electrons
Bacteriochlorophylls
Proteins
Dimers
Cytochrome c Group
Metalloproteins
Photosynthetic Reaction Center Complex Proteins
Proteobacteria
Redox reactions
Amino Acid Substitution
Lighting
Free energy
Oxidation-Reduction
Protons
Spectrum Analysis
Substitution reactions
Metals
Amino Acids
Oxidation

ASJC Scopus subject areas

  • Biochemistry

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Binding and Energetics of Electron Transfer between an Artificial Four-Helix Mn-Protein and Reaction Centers from Rhodobacter sphaeroides. / Espiritu, Eduardo; Olson, Tien L.; Williams, Joann; Allen, James.

In: Biochemistry, Vol. 56, No. 49, 12.12.2017, p. 6460-6469.

Research output: Contribution to journalArticle

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