Determination of the Molar Extinction Coefficients of the B800 and B850 Absorption Bands in Light-harvesting Complexes 2 Derived from Three Purple Photosynthetic Bacteria Rhodoblastus acidophilus, Rhodobacter sphaeroides, and Phaeospirillum molischianum by Extraction of Bacteriochlorophyll a

Protein Interactions in Rhodopseudomonas palustris TIE-1 Reveal the Molecular Basis for Resilient Photoferrotrophic Iron Oxidation

Rhodopseudomonas palustris is an alphaproteobacterium with impressive metabolic versatility, capable of oxidizing ferrous iron to fix carbon dioxide using light energy. Photoferrotrophic iron oxidation is one of the most ancient metabolisms, sustained by the pio operon coding for three proteins: PioB and PioA, which form an outer-membrane porin-cytochrome complex that oxidizes iron outside of the cell and transfers the electrons to the periplasmic high potential iron-sulfur protein (HIPIP) PioC, which delivers them to the light-harvesting reaction center (LH-RC). Previous studies have shown that PioA deletion is the most detrimental for iron oxidation, while, the deletion of PioC resulted in only a partial loss. The expression of another periplasmic HiPIP, designated Rpal_4085, is strongly upregulated in photoferrotrophic conditions, making it a strong candidate for a PioC substitute. However, it is unable to reduce the LH-RC. In this work we used NMR spectroscopy to map the interactions between PioC, PioA, and the LH-RC, identifying the key amino acid residues involved. We also observed that PioA directly reduces the LH-RC, and this is the most likely substitute upon PioC deletion. By contrast, Rpal_4085 demontrated significant electronic and structural differences from PioC. These differences likely explain its inability to reduce the LH-RC and highlight its distinct functional role. Overall, this work reveals the functional resilience of the pio operon pathway and further highlights the use of paramagnetic NMR for understanding key biological processes.

Effects of Detergents on the Spectral Features of B820 Bacteriochlorophyll a in Light-Harvesting Complex 3

Excitonic coupling of bacteriochlorophyll (BChl) a in light-harvesting (LH) proteins of purple photosynthetic bacteria is key for efficient photon capture and energy transfer. Environmental factors can affect the spectral features of these BChl a pigments and investigating these effects can provide insight into the molecular mechanisms underlying the photosynthetic spectral tuning. The present study analyzes the spectral alterations of the Q_y band of B820 BChl a within the LH3 protein in relation to the type and concentration of detergents in the buffer. Changing the detergent from lauryl dimethylamine N-oxide (LDAO) to n-dodecyl-β-d-maltoside (DDM) caused a red shift in the B820 Q_y band accompanied by hyperchromism; these spectral alterations were completely reversed by exchanging back from DDM to LDAO. These results reflect the different effects of harsh vs mild detergents on the perturbation of LH3. The B820 Q_y band did not change when LDAO or NaCl concentration was altered, suggesting that electrostatic effects by external components have little influence on the spectral features of B820 BChl a in LH3.

Excitation Energy Transfer from Bacteriochlorophyll b in the B800 Site to B850 Bacteriochlorophyll a in Light-Harvesting Complex 2

Control of the spectral overlap between energy donors and acceptors provides insight into excitation energy transfer (EET) mechanisms in photosynthetic light-harvesting proteins. Substitution of energy-donating B800 bacteriochlorophyll (BChl) a with other pigments in the light-harvesting complex 2 (LH2) of purple photosynthetic bacteria has been extensively performed; however, most studies on the B800 substitution have focused on the decrease in the spectral overlap integral with energy-accepting B850 BChl a by reconstitution of chlorophylls into the B800 site. Here, we reconstitute BChl b into the B800 site of the LH2 protein from Rhodoblastus acidophilus to increase the spectral overlap with B850 BChl a. BChl b in the B800 site had essentially the same hydrogen-bonding pattern as B800 BChl a, whereas it showed a red-shifted Q_y absorption band at 831 nm. The EET rate from BChl b to B850 BChl a in the reconstituted LH2 was similar to that of native LH2 despite the red shift of the Q_y band of the energy donor. These results demonstrate the importance of the contribution of the density of excitation states of the B850 circular assembly, which incorporates higher lying optically forbidden states, to intracomplex EET in LH2.

In situ formation of photoactive B-ring reduced chlorophyll isomer in photosynthetic protein LH2

Natural chlorophylls have a D-ring reduced chlorin π-system; however, no naturally occurring photosynthetically active B-ring reduced chlorins have been reported. Here we report a B-ring reduced chlorin, 17,18-didehydro-bacteriochlorophyll (BChl) a, produced by in situ oxidation of B800 bacteriochlorophyll (BChl) a in a light-harvesting protein LH2 from a purple photosynthetic bacterium Phaeospirillum molischianum. The regioselective oxidation of the B-ring of B800 BChl a is rationalized by its molecular orientation in the protein matrix. The formation of 17,18-didehydro-BChl a produced no change in the local structures and circular arrangement of the LH2 protein. The B-ring reduced 17,18-didehydro-BChl a functions as an energy donor in the LH2 protein. The photoactive B-ring reduced Chl isomer in LH2 will be helpful for understanding the photofunction and evolution of photosynthetic cyclic tetrapyrrole pigments.

Reconstitution of 3-Acetyl Chlorophyll a into Light-Harvesting Complex 2 from the Purple Photosynthetic Bacterium Phaeospirillum molischianum

The manipulation of B800 bacteriochlorophyll (BChl) a in light-harvesting complex 2 (LH2) from the purple photosynthetic bacterium Phaeospirillum molischianum (molischianum-LH2) provides insight for understanding the energy transfer mechanism and the binding of cyclic tetrapyrroles in LH2 proteins since molischianum-LH2 is one of the two LH2 proteins whose atomic-resolution structures have been determined and is a representative of type-2 LH2 proteins. However, there is no report on the substitution of B800 BChl a in molischianum-LH2. We report the reconstitution of 3-acetyl chlorophyll (AcChl) a, which has a 17,18-dihydroporphyrin skeleton, to the B800 site in molischianum-LH2. The 3-acetyl group in AcChl a formed a hydrogen bond with β'-Thr23 in essentially the same manner as native B800 BChl a, but this hydrogen bond was weaker than that of B800 BChl a. This change can be rationalized by invoking a small distortion in the orientation of the 3-acetyl group in the B800 cavity by dehydrogenation in the B-ring from BChl a. The energy transfer from AcChl a in the B800 site to B850 BChl a was about 5-fold slower than that from native B800 BChl a by a decrease of the spectral overlap between energy-donating AcChl a and energy-accepting B850 BChl a.

Selective oxidation of B800 bacteriochlorophyll a in photosynthetic light-harvesting protein LH2

Engineering chlorophyll (Chl) pigments that are bound to photosynthetic light-harvesting proteins is one promising strategy to regulate spectral coverage for photon capture and to improve the photosynthetic efficiency of these proteins. Conversion from the bacteriochlorophyll (BChl) skeleton (7,8,17,18-tetrahydroporphyrin) to the Chl skeleton (17,18-dihydroporphyrin) produces the most drastic change of the spectral range of absorption by light-harvesting proteins. We demonstrated in situ selective oxidation of B800 BChl a in light-harvesting protein LH2 from a purple bacterium Rhodoblastus acidophilus by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The newly formed pigment, 3-acetyl Chl a, interacted with the LH2 polypeptides in the same manner as native B800. B850 BChl a was not oxidized in this reaction. CD spectroscopy indicated that the B850 orientation and the content of the α-helices were unchanged by the B800 oxidation. The nonameric circular arrangement of the oxidized LH2 protein was visualized by AFM; its diameter was almost the same as that of native LH2. The in situ oxidation of B800 BChl a in LH2 protein with no structural change will be useful not only for manipulation of the photofunctional properties of photosynthetic pigment-protein complexes but also for understanding the substitution of BChl to Chl pigments in the evolution from bacterial to oxygenic photosynthesis.

Selective Removal of B800 Bacteriochlorophyll a from Light-Harvesting Complex 2 of the Purple Photosynthetic Bacterium Phaeospirillum molischianum

The selective removal of B800 bacteriochlorophyll (BChl) a from light-harvesting complex 2 (LH2) in purple photosynthetic bacteria is a clue about elucidation of the mechanism for the transfer of energy from these pigments to B850 BChl a and their roles in the LH2 protein structure. We demonstrated that the kinetics of the removal of B800 BChl a from two representative LH2 proteins derived from Phaeospirillum molischianum and Rhodoblastus acidophilus differed significantly, in contrast to the calculated binding enthalpy. These results may be interpreted as changes in the local structure near B800 BChl a with respect to the geometries of the original crystal structures upon removal of B800 BChl a. Despite the difficulty of removing B800 BChl a from molischianum-LH2, we prepared the molischianum-LH2 protein lacking B800 BChl a by combination of two detergents, n-dodecyl β-d-maltoside and n-octyl β-d-glucoside, under acidic conditions. Spectral and atomic force microscopy analyses indicated that the absence of B800 BChl a had little effect on the local structure in the vicinity of B850 BChl a and the circular arrangement in this protein. These results suggest that the hydrophobic domain near B850 BChl a is rigid and plays a major role in the structural formation of molischianum-LH2.

Impact of the lipid bilayer on energy transfer kinetics in the photosynthetic protein LH2

Photosynthetic purple bacteria convert solar energy to chemical energy with near unity quantum efficiency. The light-harvesting process begins with absorption of solar energy by an antenna protein called Light-Harvesting Complex 2 (LH2). Energy is subsequently transferred within LH2 and then through a network of additional light-harvesting proteins to a central location, termed the reaction center, where charge separation occurs. The energy transfer dynamics of LH2 are highly sensitive to intermolecular distances and relative organizations. As a result, minor structural perturbations can cause significant changes in these dynamics. Previous experiments have primarily been performed in two ways. One uses non-native samples where LH2 is solubilized in detergent, which can alter protein structure. The other uses complex membranes that contain multiple proteins within a large lipid area, which make it difficult to identify and distinguish perturbations caused by protein-protein interactions and lipid-protein interactions. Here, we introduce the use of the biochemical platform of model membrane discs to study the energy transfer dynamics of photosynthetic light-harvesting complexes in a near-native environment. We incorporate a single LH2 from Rhodobacter sphaeroides into membrane discs that provide a spectroscopically amenable sample in an environment more physiological than detergent but less complex than traditional membranes. This provides a simplified system to understand an individual protein and how the lipid-protein interaction affects energy transfer dynamics. We compare the energy transfer rates of detergent-solubilized LH2 with those of LH2 in membrane discs using transient absorption spectroscopy and transient absorption anisotropy. For one key energy transfer step in LH2, we observe a 30% enhancement of the rate for LH2 in membrane discs compared to that in detergent. Based on experimental results and theoretical modeling, we attribute this difference to tilting of the peripheral bacteriochlorophyll in the B800 band. These results highlight the importance of well-defined systems with near-native membrane conditions for physiologically-relevant measurements.