The Accumulation of the Light-Harvesting 2 Complex during Remodeling of the Rhodobacter sphaeroides Intracytoplasmic Membrane Results in a Slowing of the Electron Transfer Turnover Rate of Photochemical Reaction Centers

Abstract

A functional proteomic analysis of the intracytoplasmic membrane (ICM) development process was performed in Rhodobacter sphaeroides during adaptation from high-intensity illumination to indirect diffuse light. This initiated an accelerated synthesis of the peripheral light-harvesting 2 (LH2) complex relative to that of LH1-reaction center (RC) core particles. After 11 days, ICM vesicles (chromatophores) and membrane invagination sites were isolated by rate-zone sedimentation and subjected to clear native gel electrophoresis. Proteomic analysis of gel bands containing the RC-LH1 and -LH2 complexes from digitonin-solubilized chromatophores revealed high levels of comigrating electron transfer enzymes, transport proteins, and membrane assembly factors relative to their equivalent gel bands from cells undergoing adaptation to direct low-level illumination. The GroEL chaperonin accounted for >65% of the spectral counts in the RC-LH1 band from membrane invagination sites, which together with the appearance of a universal stress protein suggested that the viability of these cells was challenged by light limitation. Functional aspects of the photosynthetic unit assembly process were monitored by near-IR fast repetition rate analysis of variable fluorescence arising from LH-bacteriochlorophyll a components. The quantum yield of the primary charge separation during the early stages of adaptation showed a gradual increase (variable/maximal fluorescence = 0.78-0.83 between 0 and 4 h), while the initial value of ~70 for the functional absorption cross section (σ) gradually increased to 130 over 4 days. These dramatic σ increases showed a direct relation to gradual slowing of the RC electron transport turnover rate (τ(QA)) from ~1.6 to 6.4 ms and an ~3-fold slowing of the rate of reoxidation of the ubiquinone pool. These slowed rates are not due to changes in UQ pool size, suggesting that the relation between increasing σ and τ(QA) reflects the imposition of constraints upon free diffusion of ubiquinone redox species between the RC and cytochrome bc(1) complex as the membrane bilayer becomes densely packed with LH2 rings.

Overall energy conversion efficiency of a photosynthetic vesicle

“…With increasing light intensity, the cycling time of quinones at the RC, t RC ðIÞ as given by Equation 19, increases; fewer RCs are found in a state available to receive photoexcitation, described by the probability, p RC ðIÞ, given by Equation 13, and resulting in a corresponding loss of electronic excitation. The time-scale with which the quinone/quinol pool redox state adapts to a change in light conditions is reported to be about 0.5 s (Woronowicz et al, 2011a).…”

“…The ðn B ; n L Þ value also has an effect on the size of the quinone/quinol pool relevant for intermittent energy storage under fluctuating light conditions, since the number of quinones in the system correlates with the number of RCs (Comayras et al, 2005;Woronowicz et al, 2011a;Cartron et al, 2014). Energy conversion through the quinone/quinol pool also involves electron exchange processes from outside the chromatophore as furnished, for example, through the enzymes NADH dehydrogenase and succinate dehydrogenase (Klamt et al, 2008).…”

“…The absorbed light power I is given in units of photons absorbed per second for the entire vesicle, i.e., it holds approximately, I ¼ F s total where F is the flux of useable photons and s total is the total absorption cross-section of the chromatophore determined via the functional absorption crosssection reported in (Woronowicz et al, 2011a).…”

“…The architecture of the chromatophore, reported in (Ş ener et al, 2007, 2010Cartron et al, 2014), has been determined by combining atomic force microscopy (AFM) (Bahatyrova et al, 2004;Olsen et al, 2008), cryo-electron microscopy (cryo-EM) (Qian et al, 2005;Cartron et al, 2014), crystallography (Koepke et al, 1996;McDermott et al, 1995;Papiz et al, 2003;Jamieson et al, 2002), optical spectroscopy Sener et al, 2010), mass spectroscopy (Cartron et al, 2014), and proteomics (Jackson et al, 2012;Woronowicz and Niederman, 2010;Woronowicz et al, 2013) data. The composition of the chromatophore depends on growth conditions such as light intensity (Adams and Hunter, 2012;Woronowicz et al, 2011bWoronowicz et al, , 2011a) and can also be influenced by mutations Hsin et al, 2010b).…”

“…and Novoderezhkin, 2006b;Strümpfer andSchulten, 2011, 2012a) methods. The chromatophore exhibits also the features of modularity, repair, and assembly of components (Hsin et al, 2010a), high quantum yield of organelle-scale pigment networks (Ş ener et al, 2007, 2010Cartron et al, 2014), isolation of the electron transfer chains (Ş ener and Schulten, 2008), co-accommodation of competing functions such as efficient energy transfer and diffusion in the quinone/quinol pool Sener et al, 2010), as well as adaptation to changing external conditions (Adams and Hunter, 2012;Woronowicz et al, 2011a;Niederman, 2013;Woronowicz et al, 2013).…”