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De novo design of energy transfer proteins housing excitonically coupled chlorophyll special pairs.

ABSTRACT: Natural photosystems couple light harvesting to charge separation using a "special pair" of chlorophyll molecules that accepts excitation energy from the antenna and initiates an electron-transfer cascade. To investigate the photophysics of special pairs independent of complexities of native photosynthetic proteins, and as a first step towards synthetic photosystems for new energy conversion technologies, we designed C₂-symmetric proteins that precisely position chlorophyll dimers. X-ray crystallography shows that one designed protein binds two chlorophylls in a binding orientation matching native special pairs, while a second positions them in a previously unseen geometry. Spectroscopy reveals excitonic coupling, and fluorescence lifetime imaging demonstrates energy transfer. We designed special pair proteins to assemble into 24-chlorophyll octahedral nanocages; the design model and cryo-EM structure are nearly identical. The design accuracy and energy transfer function of these special pair proteins suggest that de novo design of artificial photosynthetic systems is within reach of current computational methods.

SUBMITTER: Ennist N

PROVIDER: S-EPMC10153362 | biostudies-literature | 2023 Apr

REPOSITORIES: biostudies-literature

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<i>De novo</i> design of energy transfer proteins housing excitonically coupled chlorophyll special pairs.

Ennist Nathan N Wang Shunzhi S Kennedy Madison M Curti Mariano M Sutherland George G Vasilev Cvetelin C Redler Rachel R Maffeis Valentin V Shareef Saeed S Sica Anthony A Hua Ash A Deshmukh Arundhati A Moyer Adam A Hicks Derrick D Swartz Avi A Cacho Ralph R Novy Nathan N Bera Asim A Kang Alex A Sankaran Banumathi B Johnson Matthew M Reppert Mike M Ekiert Damian D Bhabha Gira G Stewart Lance L Caram Justin J Stoddard Barry B Romero Elisabet E Hunter C Neil CN Baker David D

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Natural photosystems couple light harvesting to charge separation using a "special pair" of chlorophyll molecules that accepts excitation energy from the antenna and initiates an electron-transfer cascade. To investigate the photophysics of special pairs independent of complexities of native photosynthetic proteins, and as a first step towards synthetic photosystems for new energy conversion technologies, we designed C<sub>2</sub>-symmetric proteins that precisely position chlorophyll dimers. X- ...[more]

PMID: 37131790

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Project description:The proton-coupled electron transfer (PCET) reactions of tyrosine (Y) are instrumental to many redox reactions in nature. This study investigates how the local environment and the thermodynamic properties of Y influence its PCET characteristics. Herein, 2- and 4-mercaptophenol (MP) are placed in the well-folded α3C protein (forming 2MP-α3C and 4MP-α3C) and oxidized by external light-generated [Ru(L)3]3+ complexes. The resulting neutral radicals are long-lived (>100 s) with distinct optical and EPR spectra. Calculated spin-density distributions are similar to canonical Y˙ and display very little spin on the S-S bridge that ligates the MPs to C32 inside the protein. With 2MP-α3C and 4MP-α3C we probe how proton transfer (PT) affects the PCET rate constants and mechanisms by varying the degree of solvent exposure or the potential to form an internal hydrogen bond. Solution NMR ensemble structures confirmed our intended design by displaying a major difference in the phenol OH solvent accessible surface area (≤∼2% for 2MP and 30-40% for 4MP). Additionally, 2MP-C32 is within hydrogen bonding distance to a nearby glutamate (average O-O distance is 3.2 ± 0.5 Å), which is suggested also by quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations. Neither increased exposure of the phenol OH to solvent (buffered water), nor the internal hydrogen bond, was found to significantly affect the PCET rates. However, the lower phenol pKa values associated with the MP-α3C proteins compared to α3Y provided a sufficient change in PT driving force to alter the PCET mechanism. The PCET mechanism for 2MP-α3C and 4MP-α3C with moderately strong oxidants was predominantly step-wise PTET for pH values, but changed to concerted PCET at neutral pH values and below when a stronger oxidant was used, as found previously for α3Y. This shows how the balance of ET and PT driving forces is critical for controlling PCET mechanisms. The presented results improve our general understanding of amino-acid based PCET in enzymes.

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De novo design of energy transfer proteins housing excitonically coupled chlorophyll special pairs.

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<i>De novo</i> design of energy transfer proteins housing excitonically coupled chlorophyll special pairs.

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De novo design of energy transfer proteins housing excitonically coupled chlorophyll special pairs.

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<i>De novo</i> design of energy transfer proteins housing excitonically coupled chlorophyll special pairs.