Protein Engineering and Molecular Dynamics Studies of Electron Transfer in Photosynthetic Bacterial Reaction Centers
Wang, Xutong
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https://hdl.handle.net/2142/72409
Description
Title
Protein Engineering and Molecular Dynamics Studies of Electron Transfer in Photosynthetic Bacterial Reaction Centers
Author(s)
Wang, Xutong
Issue Date
1993
Doctoral Committee Chair(s)
Wraight, Colin A.
Department of Study
Center for Biophysics and Computational Biology
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Molecular
Biophysics, General
Abstract
We have altered Glu$\sp{\rm M234}$ in reaction centers from Rhodobacter sphaeroides, replacing it with valine, glutamine and glycine to form mutants M234EV, M234EQ and M234EG, respectively. The absence of a formate-bicarbonate effect in these mutants suggests that Glu$\sp{\rm M234}$ is not responsible for the absence of the formate-bicarbonate effect in Wt bacterial RCs, or at least that other factors must be taken into account. It is concluded that Glu$\sp{\rm M234}$ is not essential to the normal functioning of the acceptor quinone complex in bacterial RCs and that the role of bicarbonate in PS II is distinct from the role of this residue in bacterial RCs.
Electron transfer from the primary donor P* to the intermediated acceptor H$\sb{\rm A}$ was simulated, using the method of molecular dynamics, for wild type reaction centers from the photosynthetic bacterum Rhodopseudomonas viridis and for three "mutant" structure. From the simulations it could be concluded that: (1) mutations did not significantly change the average structure of the whole protein; (2) greater mobilities in the keto group of the phytol chain of the dimer were created by the mutations; (3) the formation of hydrogen bonds to the dimer from Tyr$\sp{\rm M208}$ in Wt and from Tyr$\sp{\rm L181}$ in YF is dynamic; (4) the tyrosine hydroxyl group stabilizes the position of the aromatic ring at M208 but it destabilizes the ring position at L181; (5) protein relaxations for mutants YF and YY are faster than wild type; (6) mapping of three symmetrically placed pairs of amino acid residues around the dimer shows a strongly anisotropic distribution of electrostatic interactions.
In studies designed to investigate the energetic and structural determinants of the charge separation process of photosynthesis, the residue tyrosine M210 (M210Y) has been changed to phenylalanine (M210YF), asparagine (M210YN) and serine (M210YS) in reaction centers of Rhodobacter sphaeroides (Ga). The optical absorption spectra showed that all three mutants have red shifts of the monomeric bacteriochlorophyll Qy band and the redox midpoint potential of the primary donor also varied. These observations demonstrate that mutation at M210 changed various energy states, i.e., B*, P$\sp+$I$\sp-$, P$\sp+$Q$\sb{\rm A}\sp-$ and PQ$\sb{\rm A}$. Especially, the energy gap between P$\sp+$I$\sp-$ and P$\sp+$Q$\sb{\rm A}\sp-$ undergoes significant change in the M210YS mutant. All these changes are attributed to the free energy change due to mutation, and the data are discussed in the context of current electron transfer theories. (Abstract shortened by UMI.)
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