Engineering topics
Harris, Dvir
Publications and source records attributed to Harris, Dvir.
Energetic robustness to large scale structural fluctuations in a photosynthetic supercomplex
Abstract Photosynthetic organisms transport and convert solar energy with near-unity quantum efficiency using large protein supercomplexes held in flexible membranes. The individual proteins position chlorophylls to tight tolerances considered critical for fast and efficient energy transfer. The variability in protein organization within the supercomplexes, and how efficiency is maintained despite variability, had been unresolved. Here, we report on structural heterogeneity in the 2-MDa cyanobacterial PSI-IsiA photosynthetic supercomplex observed using Cryo-EM, revealing large-scale variances in the positions of IsiA relative to PSI. Single-molecule measurements found efficient IsiA-to-PSI energy transfer across all conformations, along with signatures of transiently decoupled IsiA. Structure based calculations showed that rapid IsiA-to-PSI energy transfer is always maintained, and even increases by three-fold in rare conformations via IsiA-specific chls. We postulate that antennae design mitigates structural fluctuations, providing a mechanism for robust energy transfer in the flexible membrane.
Elucidating interprotein energy transfer dynamics within the antenna network from purple bacteria
In photosynthesis, absorbed light energy transfers through a network of antenna proteins with near-unity quantum efficiency to reach the reaction center, which initiates the downstream biochemical reactions. While the energy transfer dynamics within individual antenna proteins have been extensively studied over the past decades, the dynamics between the proteins are poorly understood due to the heterogeneous organization of the network. Previously reported timescales averaged over such heterogeneity, obscuring individual interprotein energy transfer steps. Here, we isolated and interrogated interprotein energy transfer by embedding two variants of the primary antenna protein from purple bacteria, light-harvesting complex 2 (LH2), together into a near-native membrane disc, known as a nanodisc. We integrated ultrafast transient absorption spectroscopy, quantum dynamics simulations, and cryogenic electron microscopy to determine interprotein energy transfer timescales. By varying the diameter of the nanodiscs, we replicated a range of distances between the proteins. The closest distance possible between neighboring LH2, which is the most common in native membranes, is 25 Å and resulted in a timescale of 5.7 ps. Larger distances of 28 to 31 Å resulted in timescales of 10 to 14 ps. Corresponding simulations showed that the fast energy transfer steps between closely spaced LH2 increase transport distances by ∼15%. Overall, our results introduce a framework for well-controlled studies of interprotein energy transfer dynamics and suggest that protein pairs serve as the primary pathway for the efficient transport of solar energy.
Ultrafast Dynamics of Photosynthetic Light Harvesting: Strategies for Acclimation Across Organisms
Photosynthetic light harvesting exhibits near-unity quantum efficiency. The high efficiency is achieved through a series of energy and charge transfer steps within a network of pigment-containing proteins. Remarkably, high efficiency is conserved across many organisms despite differences in the protein structures and organization that allow each organism to respond to its own biological niche and the stressors within. In this review, we highlight recent progress toward understanding how organisms maintain optimal light-harvesting ability by acclimating to their environment. First, we review the building blocks of photosynthetic light harvesting, energy transfer, and time-resolved spectroscopic techniques. Then, we explore how three classes of photosynthetic organisms—purple bacteria, cyanobacteria, and green plants—optimize their light-harvesting apparatuses to their particular environment. Overall, research has shown that photosynthetic energy transfer is robust to changing environmental conditions, with each organism utilizing its own strategies to optimize photon capture in its particular biological niche.
Photoenzymatic Catalysis in a New Light: Gluconobacter “Ene”-Reductase Conjugates Possessing High-Energy Reactivity with Tunable Low-Energy Excitation
Non-natural light-driven enzymatic reactivity was recently developed to perform the highly stereoselective reactions required for pharmaceutical synthesis. However, photoenzymes require high-intensity light to function because of the poor absorption properties of their photoactive intermediates. Inspired by the modular architecture of photosynthesis, we designed a conjugate composed of a covalently linked photoenzyme and a light antenna to separate light capture from catalysis. Spectroscopic characterization of the conjugate showed the presence of efficient energy transfer from the light-harvesting components to the photoenzyme. In the presence of energy transfer, a ~4-fold increase in product yield was observed for intramolecular hydroalkylation of alkenes, and reactivity was enabled for intermolecular hydroalkylation of alkenes. As a result, these improvements establish the power of incorporating nature’s design into non-natural photoenzymatic catalysis.
Comparison of the Energy-Transfer Rates in Structural and Spectral Variants of the B800–850 Complex from Purple Bacteria
Photosynthetic light harvesting can occur with a remarkable near-unity quantum efficiency. The B800–850 complex, also known as light-harvesting complex 2 (LH2), is the primary light-harvesting complex in purple bacteria and has been extensively studied as a model system. The bacteriochlorophylls of the B800–850 complex are organized into two concentric rings, known as the B800 and B850 rings. However, depending on the species and growth conditions, the number of constituent subunits, the pigment geometry, and the absorption energies vary. While the dynamics of some B800–850 variants have been exhaustively characterized, others have not been measured. Furthermore, a direct and simultaneous comparison of how both structural and spectral differences between variants affect these dynamics has not been performed. In this work, we utilize ultrafast transient absorption measurements to compare the B800 to B850 energy-transfer rates in the B800–850 complex as a function of the number of subunits, geometry, and absorption energies. The nonameric B800–850 complex from Rhodobacter (Rb.) sphaeroides is 40% faster than the octameric B800–850 complex from Rhodospirillum (Rs.) molischianum, consistent with structure-based predictions. In contrast, the blue-shifted B800–820 complex from Rs. molischianum is only 20% faster than the B800–850 complex from Rs. molischianum despite an increase in the spectral overlap between the rings that would be expected to produce a larger increase in the energy-transfer rate. These measurements support current models that contain dark, higher-lying excitonic states to bridge the energy gap between rings, thereby maintaining similar energy-transfer dynamics. Altogether, these results demonstrate that energy-transfer dynamics in the B800–850 complex are robust to the spectral and structural variations between species used to optimize energy capture and flow in purple bacteria.