The dynamics of the non-heme iron in bacterial reaction centers from Rhodobacter sphaeroides

Abstract

We investigate the dynamical properties of the non-heme iron (NHFe) in His-tagged photosynthetic bacterial reaction centers (RCs) isolated from Rhodobacter (Rb.) sphaeroides. Mössbauer spectroscopy and nuclear inelastic scattering of synchrotron radiation (NIS) were applied to monitor the arrangement and flexibility of the NHFe binding site. In His-tagged RCs, NHFe was stabilized only in a high spin ferrous state. Its hyperfine parameters (IS=1.06±0.01mm/s and QS=2.12±0.01mm/s), and Debye temperature (θ(D0)~167K) are comparable to those detected for the high spin state of NHFe in non-His-tagged RCs. For the first time, pure vibrational modes characteristic of NHFe in a high spin ferrous state are revealed. The vibrational density of states (DOS) shows some maxima between 22 and 33meV, 33 and 42meV, and 53 and 60meV and a very sharp one at 44.5meV. In addition, we observe a large contribution of vibrational modes at low energies. This iron atom is directly connected to the protein matrix via all its ligands, and it is therefore extremely sensitive to the collective motions of the RC protein core. A comparison of the DOS spectra of His-tagged and non-His-tagged RCs from Rb. sphaeroides shows that in the latter case the spectrum was overlapped by the vibrations of the heme iron of residual cytochrome c(2), and a low spin state of NHFe in addition to its high spin one. This enabled us to pin-point vibrations characteristic for the low spin state of NHFe.

Mutational control of bioenergetics of bacterial reaction center probed by delayed fluorescence

“…The observed increase of the entropy contribution in the acceptor side mutants is in agreement with softening of the H-bond structure. It is consistent with the results from neutron scattering [69], X-ray absorption fine structure spectroscopy [70] and Mössbauer spectroscopy [71] measurements which suggested the rigidity of the native protein core around the Fe ligand that could be specifically softened by point mutations. In addition to conformational flexibility, polar side chains and proton distribution at the cytoplasmic side of the protein might play some role in entropy increase in these mutants if they are different in the ground and charge separated states.…”

A single residue controls electron transfer gating in photosynthetic reaction centers

“…Similar remote effects on protein flexibility and functionality were previously reported for numerous protein systems2425. The known sensitivity of the non-heme iron coordination and spin state to the collective motions of the RC protein core are in line with such a “remote” effect on protein flexibility5264849.…”

Discovery of a single molecule transistor in photosystem II

“…As a result of this small splitting energy, the electrons avoid pairing and distribute themselves in a high spin (S = 2) configuration. This is in full agreement with Mössbauer spectroscopy, which has clearly established that the non-heme iron complex functions in a high spin (paramagnetic) ferrous state [17]. Further evidence for the high spin (S = 2) configuration is provided by EPR spectroscopy [18][19][20] and computer simulation [21,22].…”