Photosynthetic apparatus of purple bacteria

Abstract

1. Introduction 22. Structure of the bacterial PSU 52.1 Organization of the bacterial PSU 52.2 The crystal structure of the RC 92.3 The crystal structures of LH-II 112.4 Bacteriochlorophyll pairs in LH-II and the RC 132.5 Models of LH-I and the LH-I-RC complex 152.6 Model for the PSU 173. Excitation transfer in the PSU 183.1 Electronic excitations of BChls 22 3.1.1 Individual BChls 22 3.1.2 Rings of BChls 22 3.1.2.1 Exciton states 22 3.1.3 Effective Hamiltonian 24 3.1.4 Optical properties 25 3.1.5 The effect of disorder 263.2 Theory of excitation transfer 29 3.2.1 General theory 29 3.2.2 Mechanisms of excitation transfer 32 3.2.3 Approximation for long-range transfer 34 3.2.4 Transfer to exciton states 353.3 Rates for transfer processes in the PSU 37 3.3.1 Car→BChl transfer 37 3.3.1.1 Mechanism of Car→BChl transfer 39 3.3.1.2 Pathways of Car→BChl transfer 40 3.3.2 Efficiency of Car→BChl transfer 40 3.3.3 B800-B850 transfer 44 3.3.4 LH-II→LH-II transfer 44 3.3.5 LH-II→LH-I transfer 45 3.3.6 LH-I→RC transfer 45 3.3.7 Excitation migration in the PSU 46 3.3.8 Genetic basis of PSU assembly 494. Concluding remarks 535. Acknowledgments 556. References 55Life as we know it today exists largely because of photosynthesis, the process through which light energy is converted into chemical energy by plants, algae, and photosynthetic bacteria (Priestley, 1772; Barnes, 1893; Wurmser, 1925; Van Niel, 1941; Clayton & Sistrom, 1978; Blankenship et al. 1995; Ort & Yocum, 1996). Historically, photosynthetic organisms are grouped into two classes. When photosynthesis is carried out in the presence of air it is called oxygenic photosynthesis (Ort & Yocum, 1996). Otherwise, it is anoxygenic (Blankenship et al. 1995). Higher plants, algae and cyanobacteria perform oxygenic photosynthesis, which involves reduction of carbon dioxide to carbohydrate and oxidation of water to produce molecular oxygen. Some photosynthetic bacteria, such as purple bacteria, carry out anoxygenic photosynthesis that involves oxidation of molecules other than water. In spite of these differences, the general principles of energy transduction are the same in anoxygenic and oxygenic photosynthesis (Van Niel, 1931, 1941; Stanier, 1961; Wraight, 1982; Gest, 1993). The primary processes of photosynthesis involve absorption of photons by light-harvesting complexes (LHs), transfer of excitation energy from LHs to the photosynthetic reaction centers (RCs), and the primary charge separation across the photosynthetic membrane (Sauer, 1975; Knox, 1977; Fleming & van Grondelle, 1994; van Grondelle et al. 1994). In this article, we will focus on the anoxygenic photosynthetic process in purple bacteria, since its photosynthetic system is the most studied and best characterized during the past 50 years.

Anticrossings in Förster coupled quantum dots

“…2 However, excitons within adjacent QD's are also able to interact through their resonant (Förster) energy transfer, 3 some evidence for which has been obtained experimentally in a range of systems. 4,5,6,7 As is shown below, this resonant transfer of energy gives rise to offdiagonal Hamiltonian matrix elements and therefore to a naturally entangling quantum evolution. It is proposed here that this interaction may be observed in a straightforward way through the observation of anticrossings induced in the coupled dot energy spectra by the application of an external static electric field.…”

Overall energy conversion efficiency of a photosynthetic vesicle

“…Such adaptation has been reported for the individual protein level; it is not as well understood at the system integration level. For instance, pigment networks of individual light harvesting proteins were reported to display optimality and robustness in their quantum yield with respect to the spatial organization of pigments and the site energy distribution Noy et al, 2006;Damjanovic et al, 2002); a similar robustness was reported with respect to size-scaling deformations of an entire vesicle (Sener et al, 2010). Prior studies did not take into account optimization of the complete energy conversion process, including ATP synthesis, the effects of vesicle composition influenced by growth conditions such as light intensity (Niederman, 2013;Woronowicz et al, 2013), the regulation of the redox state of the quinone/ quinol pool (Klamt et al, 2008), or the effects of cell-scale concentration and connectivity of chromatophores also influenced by light intensity at growth (Tucker et al, 2010).…”

“…The functional principles displayed by the chromatophore and prevalent also in other photosynthetic systems include efficient excitonic coupling between components (Hu et al, 1997van Grondelle and Novoderezhkin, 2006b;Olaya-Castro et al, 2008;Sener et al, 2011), the utilization of quantum coherence (Ishizaki and Fleming, 2009a;Strümpfer et al, 2012), photoprotection by carotenoids (Damjanovic´et al, 1999), accommodation of thermal fluctuations, studied through experimental (Visscher et al, 1989;van Grondelle et al, 1994;Pullerits et al, 1994;Gobets et al, 2001;Janusonis et al, 2008;Freiberg et al, 2009) as well as theoretical (Damjanovic´et al, 2002;Ishizaki and Fleming, 2009b; van Grondelle eLife digest Photosynthesis, or the conversion of light energy into chemical energy, is a process that powers almost all life on Earth. Plants and certain bacteria share similar processes to perform photosynthesis, though the purple bacterium Rhodobacter sphaeroides uses a photosynthetic system that is much less complex than that in plants.…”

“…Though chromatophore vesicles share structural motifs (Bahatyrova et al, 2004;Cartron et al, 2014) that vary gradually with growth conditions, inevitable irregularities in the distribution of their constituent proteins and their quinone/quinol pools render chromatophores heterogeneous, requiring energy conversion processes to be insensitive to structural inhomogeneity. Robustness in photosynthetic systems had been demonstrated computationally for the excitation transfer step of light harvesting at the single protein level with respect to loss or rearrangement of pigments (Ş ener et al, 2002) as well as against fluctuations of pigment site-energies (Damjanovic et al, 2002) and at the vesicle level against deformations of the pigment network (Sener et al, 2010).…”

Light harvesting complex II B850 excitation dynamics

“…13 Energy that is absorbed by the B800 BChls or Cars is quickly transferred to the B850 exciton states which then transfer to exciton states of nearby LH2 and LH1 complexes. 14 With an accurate description of the excitation dynamics within the B850 ring and of the excitation transfer between B850 rings, insight into the function of an important component of the purple bacteria light harvesting system is gained.…”