Nano-mechanical mapping of the interactions between surface-bound RC-LH1-PufX core complexes and cytochrome c 2 attached to an AFM probe

Single-Molecule Detection of the Encounter and Productive Electron Transfer Complexes of a Photosynthetic Reaction Center

Small, diffusible redox proteins play an essential role in electron transfer (ET) in respiration and photosynthesis, sustaining life on Earth by shuttling electrons between membrane-bound complexes via finely tuned and reversible interactions. Ensemble kinetic studies show transient ET complexes form in two distinct stages: an "encounter" complex largely mediated by electrostatic interactions, which subsequently, through subtle reorganization of the binding interface, forms a "productive" ET complex stabilized by additional hydrophobic interactions around the redox-active cofactors. Here, using single-molecule force spectroscopy (SMFS) we dissected the transient ET complexes formed between the photosynthetic reaction center-light harvesting complex 1 (RC-LH1) of Rhodobacter sphaeroides and its native electron donor cytochrome c2 (cyt c2). Importantly, SMFS resolves the distribution of interaction forces into low (∼150 pN) and high (∼330 pN) components, with the former more susceptible to salt concentration and to alteration of key charged residues on the RC. Thus, the low force component is suggested to reflect the contribution of electrostatic interactions in forming the initial encounter complex, whereas the high force component reflects the additional stabilization provided by hydrophobic interactions to the productive ET complex. Employing molecular dynamics simulations, we resolve five intermediate states that comprise the encounter, productive ET and leaving complexes, predicting a weak interaction between cyt c2 and the LH1 ring near the RC-L subunit that could lie along the exit path for oxidized cyt c2. The multimodal nature of the interactions of ET complexes captured here may have wider implications for ET in all domains of life.

FRET measurement of cytochrome bc1 and reaction centre complex proximity in live Rhodobacter sphaeroides cells

In the model purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides, solar energy is converted via coupled electron and proton transfer reactions within the intracytoplasmic membranes (ICMs), infoldings of the cytoplasmic membrane that form spherical 'chromatophore' vesicles. These bacterial 'organelles' are ideal model systems for studying how the organisation of the photosynthetic complexes therein shape membrane architecture. In Rba. sphaeroides, light-harvesting 2 (LH2) complexes transfer absorbed excitation energy to dimeric reaction centre (RC)-LH1-PufX complexes. The PufX polypeptide creates a channel that allows the lipid soluble electron carrier quinol, produced by RC photochemistry, to diffuse to the cytochrome bc1 complex, where quinols are oxidised to quinones, with the liberated protons used to generate a transmembrane proton gradient and the electrons returned to the RC via cytochrome c2. Proximity between cytochrome bc1 and RC-LH1-PufX minimises quinone/quinol/cytochrome c2 diffusion distances within this protein-crowded membrane, however this distance has not yet been measured. Here, we tag the RC and cytochrome bc1 with yellow or cyan fluorescent proteins (YFP/CFP) and record the lifetimes of YFP/CFP Förster resonance energy transfer (FRET) pairs in whole cells. FRET analysis shows that that these complexes lie on average within 6 nm of each other. Complementary high-resolution atomic force microscopy (AFM) of intact, purified chromatophores verifies the close association of cytochrome bc1 complexes with RC-LH1-PufX dimers. Our results provide a structural basis for the close kinetic coupling between RC-LH1-PufX and cytochrome bc1 observed by spectroscopy, and explain how quinols/quinones and cytochrome c2 shuttle on a millisecond timescale between these complexes, sustaining efficient photosynthetic electron flow.

Unfolding pathway and intermolecular interactions of the cytochrome subunit in the bacterial photosynthetic reaction center

Precise folding of photosynthetic proteins and organization of multicomponent assemblies to form functional entities are fundamental to efficient photosynthetic electron transfer. The bacteriochlorophyll b-producing purple bacterium Blastochloris viridis possesses a simplified photosynthetic apparatus. The light-harvesting (LH) antenna complex surrounds the photosynthetic reaction center (RC) to form the RC-LH1 complex. A non-membranous tetraheme cytochrome (4Hcyt) subunit is anchored at the periplasmic surface of the RC, functioning as the electron donor to transfer electrons from mobile electron carriers to the RC. Here, we use atomic force microscopy (AFM) and single-molecule force spectroscopy (SMFS) to probe the long-range organization of the photosynthetic apparatus from Blc. viridis and the unfolding pathway of the 4Hcyt subunit in its native supramolecular assembly with its functional partners. AFM images reveal that the RC-LH1 complexes are densely organized in the photosynthetic membranes, with restricted lateral protein diffusion. Unfolding of the 4Hcyt subunit represents a multi-step process and the unfolding forces of the 4Hcyt α-helices are approximately 121 picoNewtons. Pulling of 4Hcyt could also result in the unfolding of the RC L subunit that binds with the N-terminus of 4Hcyt, suggesting strong interactions between RC subunits. This study provides new insights into the protein folding and interactions of photosynthetic multicomponent complexes, which are essential for their structural and functional integrity to conduct photosynthetic electron flow.

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AFM and single-molecule force spectroscopy reveal the membrane organization and unfolding process of Blastochloris viridis RC

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RC-light-harvesting 1 complexes are densely organized in photosynthetic membranes, with restricted lateral diffusion

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Unfolding of the non-membranous cytochrome (4Hcyt) subunit represents a multi-step process;

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The average unfolding forces of the 4Hcyt α-helices are ~121 pN;

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Pulling of 4Hcyt from the RC suggests strong interactions (> 150 pN) between 4Hcyt and L subunits in the RC structure.

Atoms to Phenotypes: Molecular Design Principles of Cellular Energy Metabolism

We report a 100-million atom-scale model of an entire cell organelle, a photosynthetic chromatophore vesicle from a purple bacterium, that reveals the cascade of energy conversion steps culminating in the generation of ATP from sunlight. Molecular dynamics simulations of this vesicle elucidate how the integral membrane complexes influence local curvature to tune photoexcitation of pigments. Brownian dynamics of small molecules within the chromatophore probe the mechanisms of directional charge transport under various pH and salinity conditions. Reproducing phenotypic properties from atomistic details, a kinetic model evinces that low-light adaptations of the bacterium emerge as a spontaneous outcome of optimizing the balance between the chromatophore's structural integrity and robust energy conversion. Parallels are drawn with the more universal mitochondrial bioenergetic machinery, from whence molecular-scale insights into the mechanism of cellular aging are inferred. Together, our integrative method and spectroscopic experiments pave the way to first-principles modeling of whole living cells.

Dissecting the cytochrome c 2-reaction centre interaction in bacterial photosynthesis using single molecule force spectroscopy

The reversible docking of small, diffusible redox proteins onto a membrane protein complex is a common feature of bacterial, mitochondrial and photosynthetic electron transfer (ET) chains. Spectroscopic studies of ensembles of such redox partners have been used to determine ET rates and dissociation constants. Here, we report a single-molecule analysis of the forces that stabilise transient ET complexes. We examined the interaction of two components of bacterial photosynthesis, cytochrome c ₂ and the reaction centre (RC) complex, using dynamic force spectroscopy and PeakForce quantitative nanomechanical imaging. RC-LH1-PufX complexes, attached to silicon nitride AFM probes and maintained in a photo-oxidised state, were lowered onto a silicon oxide substrate bearing dispersed, immobilised and reduced cytochrome c ₂ molecules. Microscale patterns of cytochrome c ₂ and the cyan fluorescent protein were used to validate the specificity of recognition between tip-attached RCs and surface-tethered cytochrome c ₂ Following the transient association of photo-oxidised RC and reduced cytochrome c ₂ molecules, retraction of the RC-functionalised probe met with resistance, and forces between 112 and 887 pN were required to disrupt the post-ET RC-c ₂ complex, depending on the retraction velocities used. If tip-attached RCs were reduced instead, the probability of interaction with reduced cytochrome c ₂ molecules decreased 5-fold. Thus, the redox states of the cytochrome c ₂ haem cofactor and RC 'special pair' bacteriochlorophyll dimer are important for establishing a productive ET complex. The millisecond persistence of the post-ET cytochrome c ₂[oxidised]-RC[reduced] 'product' state is compatible with rates of cyclic photosynthetic ET, at physiologically relevant light intensities.

Evaluation of β1-integrin expression on chondrogenically differentiating human adipose-derived stem cells using atomic force microscopy

The expression of β1-integrin on human adipose-derived stem cells, differentiating toward a chondrogenic lineage, is hypothesized to decrease when cells are grown under in vivo-like environments due to sufficient extracellular matrix (ECM) buildup in the engineered tissues. The opposite is true when cells are grown in static cultures such as in pellet or micromass. To probe β1-integrin distribution on cellular surfaces, atomic force microscopy cantilevers modified with anti-β1-integrin antibodies were used. Specific antibody-antigen adhesion forces were identified and indicated the locations of β1-integrins on cells. ECM properties were assessed by estimating the Young's modulus of the matrix. Specific single antibody-antigen interactions averaged 78 ± 10 pN with multiple bindings occurring at approximate multiples of 78 pN. The author's results show that upregulated β1-integrin expression coincided with a less robust ECM as assessed by mechanical properties of tissues. In micromass and pellet cultures, transforming growth factor β3(TGF-β3) elicited a decrease in Young's modulus by 3.7- and 4.4-fold while eliciting an increase in β1-integrin count by 1.1- and 1.3-fold, respectively. β1-integrin counts on cells grown in the presence of TGF-β3 with oscillating hydrostatic pressure decreased by a 1.1-fold while the Young's modulus increased by a 1.9-fold. Collectively, our results suggest that cells in insufficiently robust ECM express more integrin perhaps to facilitate cell-ECM adhesion and compensate for a looser less robust ECM.

Visualizing the dynamic structure of the plant photosynthetic membrane

The chloroplast thylakoid membrane is the site for the initial steps of photosynthesis that convert solar energy into chemical energy, ultimately powering almost all life on earth. The heterogeneous distribution of protein complexes within the membrane gives rise to an intricate three-dimensional structure that is nonetheless extremely dynamic on a timescale of seconds to minutes. These dynamics form the basis for the regulation of photosynthesis, and therefore the adaptability of plants to different environments. High-resolution microscopy has in recent years begun to provide new insights into the structural dynamics underlying a number of regulatory processes such as membrane stacking, photosystem II repair, photoprotective energy dissipation, state transitions and alternative electron transfer. Here we provide an overview of the essentials of thylakoid membrane structure in plants, and consider how recent advances, using a range of microscopies, have substantially increased our knowledge of the thylakoid dynamic structure. We discuss both the successes and limitations of the currently available techniques and highlight newly emerging microscopic methods that promise to move the field beyond the current 'static' view of membrane organization based on frozen snapshots to a 'live' view of functional membranes imaged under native aqueous conditions at ambient temperature and responding dynamically to external stimuli.

Large photocurrent response and external quantum efficiency in biophotoelectrochemical cells incorporating reaction center plus light harvesting complexes

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 μA cm(-2), respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 μA cm(-2), respectively, with illumination power of ∼6 mW cm(-2) at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material.

Nanodomains of cytochrome b6f and photosystem II complexes in spinach grana thylakoid membranes

The cytochrome b6f (cytb6f) complex plays a central role in photosynthesis, coupling electron transport between photosystem II (PSII) and photosystem I to the generation of a transmembrane proton gradient used for the biosynthesis of ATP. Photosynthesis relies on rapid shuttling of electrons by plastoquinone (PQ) molecules between PSII and cytb6f complexes in the lipid phase of the thylakoid membrane. Thus, the relative membrane location of these complexes is crucial, yet remains unknown. Here, we exploit the selective binding of the electron transfer protein plastocyanin (Pc) to the lumenal membrane surface of the cytb6f complex using a Pc-functionalized atomic force microscope (AFM) probe to identify the position of cytb6f complexes in grana thylakoid membranes from spinach (Spinacia oleracea). This affinity-mapping AFM method directly correlates membrane surface topography with Pc-cytb6f interactions, allowing us to construct a map of the grana thylakoid membrane that reveals nanodomains of colocalized PSII and cytb6f complexes. We suggest that the close proximity between PSII and cytb6f complexes integrates solar energy conversion and electron transfer by fostering short-range diffusion of PQ in the protein-crowded thylakoid membrane, thereby optimizing photosynthetic efficiency.