1 research outputs found
Binding and Energetics of Electron Transfer between an Artificial Four-Helix Mn-Protein and Reaction Centers from <i>Rhodobacter sphaeroides</i>
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 <i>Rhodobacter sphaeroides</i> was characterized using optical spectroscopy. Upon illumination
of reaction centers, an electron is transferred from P, the bacteriochlorophyll
dimer, to Q<sub>A</sub>, the primary electron acceptor. The P1 Mn-protein
can bind to the reaction center and reduce the oxidized bacteriochlorophyll
dimer, P<sup>+</sup>, with a dissociation constant of 1.2 μM
at pH 9.4, comparable to the binding constant of <i>c</i>-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<sup>+</sup> by the P1
Mn-protein was dependent on the P/P<sup>+</sup> 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