Atomic level structure and function of a photosynthetic vesicle
Atomic Level Structure of the Photosynthetic Unit of Purple Bacteria
Photosynthesis, the main source of energy for all life, is performed by an intricate assembly of hundreds of proteins, which harvest, transfer, convert, and store solar energy. Such a light harvesting machine is best exemplified in the purple bacterial photosynthetic unit (PSU), which performs anoxygenic photosynthesis and is significantly simpler and evolutionarily more primitive than its counterpart found in plants.
Three different views of the PSU, from the supramolecular architecture
to individual chlorophylls to the energy transfer network across the vesicle.
The vesicle shown has an inner diameter of 60 nm.
Figure made with VMD.
The photosynthetic apparatus of purple bacteria consists of six different kinds of proteins: The light harvesting complexes, LH2 and LH1, absorb light and transfer the resulting excitation to the reaction center (RC), which subsequently initiates an electron transfer reducing quinone to hydroquinone. The LH1 and RC complexes for dimeric RC-LH1-PufX supercomplexes, where PufX is a small polypeptide likely involved in regulating the quinone traffic. The bc1 complex oxidizes hydroquinone to create a proton gradient across the membrane which in turn is utilized by ATP synthase for ATP production. Electrons are shuttled back to the RC by cytochrome c2.
The bc1 complexes and ATP synthase are not included in the construction depicted above as this study focuses mainly on primary excitation transfer processes. It is possible that bc1 complexes may be located within the vesicle closer to the RC-LH1-PufX complexes, even though they are not considered in this study.
A system level view of the interactions between the
proteins that constitute the PSU.
Solving a Macromolecular Puzzle:
Stitching the Vesicle Geometry from AFM, LD, X-Ray, NMR, and cryo-EM data
Scientists have determined through decades of research the
three-dimensional structure of many of the individual
proteins that constitute the PSU from multiple species.
However, the overall architecture of the PSU remains
elusive to a direct observation. Clues
from spectroscopy, electron microscopy, and
especially the recent AFM data of the vesicles
permits us finally to reconstruct a vesicle
from multiple patches computationally stitched together.
Typical AFM images used in the construction.
(Image courtesy of John Olsen.)
An area-preserving map between planar and spherical regions (depicted below) were used for the construction of small spherical patches for the protein clusters. Since both the AFM imaging procedure as well as this projection introduces deformation artifacts, resulting steric clashes and gaps were removed manually.
The inverse Mollweide transformation is used to map small planar patches
onto a spherical geometry before "stitching" them onto the vesicle
shown above.
Two different vesicle geometries were assembled with different LH1-RC:LH2 ratios, corresponding to different growth conditions.
Architecture of two PSU vesicles.
Efficient Excitation Transfer across the Vesicle
The architecture of the protein assembly thus constructed enables us to model quantum mechanical excitation transfer process across the whole vesicle. The vesicle depicted has around 4000 chlorophylls belonging two over 200 clusters. This system displays a very high efficiency of 95% with an average excitation lifetime of around 50ps, which compares well with experiments. Also, a significant level of excitation sharing has been observed between neighboring reaction centers within the same dimer cluster.
The excitation transfer networks across the vesicle, depicted
as inter-chlorophyll couplings (right) and cluster-to-cluster
transfer rates (left). The transfer between the clusters are
computed according an assumption of Boltzman equilibrium
prior to the transfer.
Figure made with VMD.
Related web pages on our site
Organization of energy transfer networks.
Quantum Biology of the Photosynthetic Unit
Light-Harvesting by Carotenoids
Related publications
Atomic-level structural and functional model of a bacterial photosynthetic membrane vesicle .Melih K. Sener, John D. Olsen, C. Neil Hunter, and Klaus Schulten. PNAS, 104:15723, 2007.