Biology subjects

Leong, H. F.

Publications and source records attributed to Leong, H. F..

3 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↗