Biology subjects

Fantuzzi, A.

Publications and source records attributed to Fantuzzi, A..

7 recordsLinked to original sources

bioRxiv2025

Far-red light absorption strategies and their structural basis in Photosystem I of Acaryochloris marina NIES-2412

The marine cyanobacterium, Acaryochloris marina, uses the red-shifted chlorophyll d as its primary pigment, allowing it to absorb photons >700 nm. However, the widely studied type strain, A. marina MBIC11017, is atypical compared to most cyanobacteria, due to the absence of low energy chlorophylls ( red forms) within its Photosystem I complex. Consequently, Photosystem I and Photosystem II in the MBIC11017 strain share similar absorption spectra and are incapable of absorbing photons >740 nm. Recently, it has been discovered that the absorption and emission spectra from other A. marina strains are significantly more red-shifted than the MBIC11017 strain. Here, we have combined advanced spectroscopy and high-resolution cryo-EM to characterize Photosystem I from Acaryochloris marina NIES-2412, a red-shifted strain that is more representative of the A. marina species. The structure resolves all 96 chlorophylls and cofactors and indicates the location of the red chlorophyll forms. Spectroscopic analysis reveals two distinct types of red forms: one arising from the classical mechanism of charge transfer-exciton mixing, and the other from purely excitonic interactions. Furthermore, we have identified PsaX2 as a critical subunit that fine-tunes the pigment geometries and energies to enable the formation of these red forms. Together, these findings reveal how NIES-2412 PSI balances far-red light harvesting and energy trapping, highlighting its distinct strategy for adaptation in far-red light environments and redefining A. marina MBIC11017 as an atypical representative of the species.

biophysics↗

bioRxiv2025

Structure of far-red allophycocyanin: stripped down and tuned up for low energy photosynthesis.

A diverse subset of cyanobacteria is capable of transiently modifying their photosynthetic machinery in a process known as far-red light photoacclimation to drive photosynthesis with less energetic photons (700 nm - 800 nm). To achieve this all the main light-driven components of the photosynthetic apparatus, including their allophycocyanin antenna, are replaced with red-shifted paralogues. Recent studies based on the structure of an incomplete complex provided some insights into the tuning of the far-red phycobiliproteins. Here, we solved the structure of the intact bicylindrical allophycocyanin complex from the cyanobacterium Chroococcidiopsis thermalis PCC 7203 at a resolution of 2.61 [A] determined by Cryo-electron microscopy single particle analysis. A comparison between far-red and white light allophycocyanin cores provides insight on the evolutionary adaptations needed to optimize excitation energy transfer in the far-red light adapted photosynthetic apparatus. The reduction in antenna size in far-red photosynthesis, suggests a need to optimize membrane packing to increase the number of photosystems, while the wider spread in the absorption range of the bilins suggests faster and more efficient excitation energy transfer to far-red Photosystem II by limiting backflow of excitation from the reaction centres to the far-red bilin pigments.

biophysics↗

bioRxiv2024

Locating the Missing Chlorophylls f in Far-red Photosystem I

The discovery of chlorophyll f-containing oxygenic photosynthesis, with its long-wavelength photochemistry, represented a new low-energy paradigm. However, subsequent structural studies on chlorophyll f-containing Photosystem I (PSI) found five chlorophylls f but none among the photochemically active pigments and concluded that chlorophyll f plays no photochemical role. Here we report a cryo-EM structure (2.01 [A]) of far-red PSI from Chroococcidiopsis thermalis PCC 7203, showing all eight chlorophylls f, including the redox active A-1B. Simulations of absorption difference spectra induced by charge separation indicate that the A-1B chlorophyll f absorbs at 755 nm. The chlorophyll f sites, some wavelength assignments, and conserved far-red-specific amino acids, provide functional insights, including redox tuning of chlorophyll f as the primary donor and far-red excitation energy-sharing over the PSI trimer.

biochemistry↗

bioRxiv2024

Modified Chlorophyll Pigment at ChlD1 Tunes Photosystem II Beyond the Red-Light Limit

Photosystem II (PSII) is powered by the light-capturing properties of chlorophyll a pigments that define the spectral range of oxygenic photosynthesis. Some photosynthetic cyanobacteria can acclimate to growth in longer wavelength light by replacing five chlorophylls for long wavelength pigments in specific locations, including one in the reaction center (RC). However, the exact location and the nature of this long wavelength pigment still remain uncertain. Here we have addressed the color-tuning mechanism of the farred light PSII (FRL-PSII) by excited state calculations at both the ab initio correlated (ADC2) and linear-response time-dependent density functional theory (LR-TDDFT) levels in combination with large-scale hybrid quantum/classical (QM/MM) simulations and atomistic molecular dynamics. We show that substitution of a single chlorophyll pigment (ChlD1) at the RC by chlorophyll d leads to a spectral shift beyond the far-red light limit, as a result of the protein electrostatic, polarization and electronic coupling effects that reproduce key structural and spectroscopic observations. Pigment substitution at the ChlD1 site further results in a low site energy within the RC that could function as a sink for the excitation energy and initiate the primary charge separation reaction, driving the water oxidation. Our findings provide a basis for understanding color-tuning mechanisms and bioenergetic principles of oxygenic photosynthesis at the far-red light limit.

biophysics↗

bioRxiv2022

Impact of energy limitations on function and resilience in long-wavelength Photosystem II

Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two kinds of cyanobacterial PSII can use Chl-d and Chl-f to perform the same reactions using lower energy, far-red light. PSII from Acaryochloris marina has Chl-d replacing all but one of its 35 Chl-a, while PSII from Chroococcidiopsis thermalis, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII and Chl-a-PSII. We show that: i) all types of PSII have a comparable efficiency in enzyme turnover; ii) the modified energy gaps on the acceptor side of Chl-d-PSII favor recombination via PD1+Phe- repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; ii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favoring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and QA and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.

biochemistry↗

bioRxiv2022

Bicarbonate Activation of Monomeric Photosystem II-PsbS/Psb27 Complex

In thylakoid membranes, Photosystem II monomers from the stromal lamellae contain the subunits PsbS and Psb27 (PSIIm-S/27), while Photosystem II monomers from granal regions (PSIIm) lack these subunits. Here, we have isolated and characterised these two types of Photosystem II complexes. The PSIIm-S/27 showed enhanced fluorescence, the near-absence of oxygen evolution, as well as limited and slow electron transfer from QA to QB compared to the near-normal activities in the granal PSIIm. However, when bicarbonate was added to the PSIIm-S/27, water splitting and QA to QB electron transfer rates were comparable to those in granal PSIIm. The findings suggest that the binding of PsbS and/or Psb27 inhibits forward electron transfer and lowers the binding affinity for the bicarbonate. This can be rationalized in terms of the recently discovered photoprotection role played by bicarbonate binding via the redox tuning of the QA/QA*- couple, which controls the charge recombination route, and this limits chlorophyll triplet mediated 1O2 formation (Brinkert K et al. (2016) Proc Natl Acad Sci U S A. 113(43):12144-12149). These findings suggest that PSIIm-S/27 is an intermediate in the assembly of PSII in which PsbS and/or Psb27 restrict PSII activity while in transit, by using a bicarbonate-mediated switch and protective mechanism. One sentence summaryA photosystem II monomer with PsbS and Psb27 as additional subunits, is inactive as isolated but activated by bicarbonate, and is attributed to be a late-stage intermediate in photoassembly.

plant biology↗

bioRxiv2020

The primary donor of far-red Photosystem II: ChlD1 or PD2?

Far-red light (FRL) Photosystem II (PSII) isolated from Chroococcidiopsis thermalis is studied using parallel analyses of low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies in conjunction with fluorescence measurements. This extends earlier studies (Nurnberg et al 2018 Science 360 (2018) 1210-1213). We confirm that the chlorophyll absorbing at 726 nm is the primary electron donor. At 1.8 K efficient photochemistry occurs when exciting at 726 nm and shorter wavelengths; but not at wavelengths longer than 726 nm. The 726 nm absorption peak exhibits a 21 {+/-} 4 cm-1 electrochromic shift due to formation of the semiquinone anion, QA*-. Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: PD1, PD2 ChlD1, ChlD2, PheoD1 and PheoD2. Two of these chlorins, ChlD1 and PD2, are located at a distance and orientation relative to QA*- so as to account for the observed electrochromic shift. Previously, ChlD1 was taken as the most likely candidate for the primary donor based on spectroscopy, sequence analysis and mechanistic arguments. Here, a more detailed comparison of the spectroscopic data with exciton modelling of the electrochromic pattern indicates that PD2 is at least as likely as ChlD1 to be responsible for the 726 nm absorption. The correspondence in sign and magnitude of the CD observed at 726 nm with that predicted from modelling favors PD2 as the primary donor. The pros and cons of PD2 vs ChlD1 as the location of the FRL-primary donor are discussed. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=110 SRC="FIGDIR/small/021097v1_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@159ea46org.highwire.dtl.DTLVardef@23c04forg.highwire.dtl.DTLVardef@126c235org.highwire.dtl.DTLVardef@1982d58_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIPrimary Donor confirmed at 726 nm C_LIO_LIDetermination of far-red chl pigment Qy excitation positions, widths, CD and MCD amplitudes C_LIO_LIQuantification of electrochromic shifts and QA*- photoconversion yield C_LIO_LIElectrochromic shift consistent with primary donor at either ChlD1 or PD2 C_LIO_LIThe CD amplitude favors the primary donor at PD2 C_LI

biophysics↗