Research
The
photochemical reaction that plants and photosynthetic bacteria use to
capture the energy of sunlight occurs in pigment-protein "reaction
centers." Bacterial reaction centers typically contain four molecules
of bacteriochlorophyll (BChl), two bacteriopheophytins (BPh) and two
quinones (QA and QB) bound to three polypeptides. Two of the four BChls
are very close together. When this special pair (P) is excited with
light, it passes an electron to the quinones by way of the neighboring
BChl and BPh.
Dr.
Parson's research group is using fast spectroscopic techniques and
computational approaches to study the initial electron-transfer steps
in reaction centers. A homebuilt Ti-sapphire laser is used to excite a
sample with pulses of light lasting less than 10-13 s. A second pulse
of light probes the absorption spectrum of the sample as a function of
time after the excitation. The spectrum changes as an electron moves
from one molecule to another. The excited BChl dimer (P*) appears to
transfer an electron to one of the neighboring BChls in about 3 ps,
creating a radical-pair (P+BChl-). An electron moves from BChl- to a
BPh in about 1 ps and from BPh- to QA in 200 ps. In addition to being
very fast and efficient, these reactions have the unusual property of
speeding up with decreasing temperature.
The
reaction center provides an ideal system for addressing the general
question of how enzymes work because the kinetics can be measured with
high precision under a wide range of conditions in a protein with a
well-defined structure, and because the initial reactions occur on a
time scale that is amenable to molecular-dynamics simulations.
To
explore the way in which pigment-protein interactions affect the
energies of P+BChl- and P+BPh-, and thus control the rate, temperature
dependence and specificity of electron transfer, Dr. Parson's research
group is measuring the energies and electron-transfer kinetics in
bacterial mutants with amino acid substitutions near the pigments.
Mutations of a particular tyrosine residue were found to change the
rate and temperature dependence of the initial reaction dramatically.
In a current project, the electrostatic symmetry of the protein is
being modified by mutations of ionizable residues. A variety of
computational approaches also are being used, including molecular
dynamics simulations and electrostatics and quantum calculations. By
combining accurate electrostatics and molecular dynamics calculations,
it is possible to calculate the free energy surfaces of the reactant
and product states as functions of the reaction coordinate and to
identify vibrational motions of the protein that are coupled to the
electron-transfer reactions.
Dr.
Parson's group also is using experimental and computational techniques
to study antenna complexes that absorb light and transfer energy to the
reaction center. These are integral membrane proteins that typically
contain 24 to 36 BChls. Topics currently under investigation include
the extent to which excitations are delocalized over the entire
complex, the manner in which excitations move from complex to complex,
and the structural organization of the antenna complexes and reaction
centers in the membrane.
Selected
Publications
Cotton NJ, Stoddard B, Parson WW (2004) Oxidative inhibition of human soluble catechol-O-methyltransferase. J Biol Chem 279: 23710-23718.
Johnson ET, Nagarajan V, Zazubovich V, Riley K, Small GJ, Parson WW (2003) Effects of ionizable residues on the absorption spectrum and initial electron-transfer kinetics in the photosynthetic reaction center of Rhodobacter sphaeroides. Biochemistry 42: 13673-13683.
Parson WW (2003) Electron donors and acceptors in the initial steps of photosynthesis in purple bacteria: a personal account. Photosynth Res 76: 81-92.
Graf WD, Unis AS, Yates CM, Sulzbacher S, Dinulos MB, Jack RM, Dugaw KA, Paddock MN, Parson WW (2001) Catecholamines in patients with 22q11.2 deletion syndrome and the low-activity COMT polymorphism. Neurology 57: 410-416.
"Oscillations of the Energy Gap for the Initial Electron-Transfer Step
in Bacterial Reaction Centers", Parson, W.W.; Chu, Z.T.; Warshel, A.,
Photosynth. Res. 55, 147 (1998).
"Reorganization
Energy of the Initial Electron-Transfer Step in Photosynthetic
Bacterial Reaction Centers", Parson, W.W.; Chu, Z.T.; Warshel, A.,
Biophys. J. 74, 182 (1998).
"Excitation
Energy Transfer Between the B850 and B875 Antenna Complexes of
Rhodobacter sphaeroides", Nagarajan, V.; Parson, W.W., Biochemistry 36,
2300 (1997).
"Calculations
of Spectroscopic Properties of the LH2 Bacteriochlorophyll-Protein
Antenna Complex from Rhodopseudomonas acidophila", Alden, R.G.;
Johnson, E.; Nagarajan, V.; Parson, W.W.; Law, C.J.; Cogdell, R.J., J.
Phys. Chem. 101, 4667 (1997).
"Ultrafast
Exciton Relaxation in the B850 Antenna Complex of Rhodobacter
sphaeroides", Nagarajan, V.; Alden, R.G.; Williams, J.C.; Parson, W.W.,
Proc. Natl. Acad. Sci. USA 93, 13774 (1996).
"Comment
on two-dimensional free energy surfaces for primary electron transfer
in a photosynthetic reaction center", Warshel, A.; Chu, Z.T.; Parson,
W.W., Chem. Phys. Letts., 265, 293 (1996).
"Orientation
of the OH Dipole of Tyrosine (M)210 and its Effect on Electrostatic
Energies in Photosynthetic Bacterial Reaction Centers", Alden, R. G.,
Parson, W. W., Chu, Z. T., and Warshel, A., J. Phys. Chem. 100, 16761
(1996).
"Photosynthetic
Reaction Centers", Parson, W. W. In Protein Electron Transfer (D. S.
Bendall, ed.) Bios Scientific Publishers, Oxford, pp. 125-160 (1996).
"Macroscopic
and Microscopic Estimates of the Energetics of Charge Separation in
Bacterial Reaction Centers", Alden, R. G., Parson, W. W., Chu, Z. T.,
and Warshel, A., in Reaction Centers of Photosynthetic Bacteria:
Structure and Dynamics (M. E. Michel-Byerle, ed.) Springer-Verlag,
Berlin (1996).
"Calculations
of Electrostatic Energies in Photosynthetic Reaction Centers", Alden,
R. G., Parson, W. W., Chu, Z. T., and Warshel, A., J. Am. Chem. Soc.
117, 12284 (1995).
"Theoretical
Analyses of Electron-Transfer Reactions", Parson, W. W., and Warshel,
A., In Anoxygenic Photosynthetic Bacteria (R. E. Blankenship, M. T.
Madigan and C. E. Bauer, eds.) Kluyer Academic Publishers, Dordrecht,
pp. 559-575 (1995).
"Specific
Alteration of the Oxidation Potential of the Electron Donor in Reaction
Centers from Rhodobacter sphaeroides", Lin, X., Murchison, H. A.,
Nagarajan, V., Parson, W. W., Allen, J. P., and Williams, J. C., Proc.
Natl. Acad. Sci. USA 91, 10265 (1994).
"On
the Energetics of the Primary Electron-Transfer Process in Bacterial
Reaction Centers", Warshel, A., Chu, Z. T., and Parson, W. W.,
Photochem. Photobiol. 82, 123 (1994).
"Kinetics
and Free Energy Gaps of Electron-transfer Reactions in Rhodobacter
sphaeroides Reaction Centers", Nagarajan, V., Parson, W.W., Davis, D.,
and Schenck, C. Biochemistry 32, 12324 (1993).
"Simulations of Electron Transfer in Bacterial Reaction Centers",
Parson, W. W., and Warshel, A., In The Photosynthetic Reaction Center
(J. Deisenhofer and J. R. Norris, eds.) Academic Press, New York, pp.
23-47 (1993).
For
more information see the U.W. Biomolecular Structure
and Design Program home page.
Box 357350 