Switching sides—Reengineered primary charge separation in the bacterial photosynthetic reaction center
Philip D. Laible, Deborah K. Hanson, James C. Buhrmaster, Gregory A. Tira, Kaitlyn M. Faries, Dewey Holten, and Christine Kirmaier
PNAS January 14, 2020 117 (2) 865-871; first published December 31, 2019 https://doi.org/10.1073/pnas.1916119117
Edited by Donald R. Ort, University of Illinois at Urbana–Champaign, Urbana, IL, and approved December 4, 2019 (received for review September 16, 2019)
Researchers solve a critical part of the mystery of photosynthesis, uncovering an engineered version of a protein complex that switches the use of photosynthetic pathways. This discovery could improve human-made devices such as solar panels and sensors. (Image by Shutterstock/Quality Stock Arts.) Source/Fonte
Significance
Photosynthetic organisms use pigment–protein complexes called reaction centers (RCs)—effectively nature’s solar cells—to convert the energy of sunlight into charge-separated species that power life processes. Whether from plants, algae, or photosynthetic bacteria, RCs feature 2 quasi-mirror-image arrangements of protein and pigment cofactors. In RCs from purple photosynthetic bacteria, photon-induced electron transfer (ET) reduces the A-branch bacteriopheophytin (BPh) with near-unity quantum yield, while ET to the symmetry-related B-branch BPh is suppressed. Amino acid changes made here in the Rhodobacter sphaeroides RC nearly completely reverse this design. Specifically, we report RCs wherein ET to the B-branch BPh occurs with 90% yield. We draw insight regarding architectural and mechanistic foundations of the primary photon-driven charge-separation events of photosynthesis.
Abstract
We report 90% yield of electron transfer (ET) from the singlet excited state P* of the primary electron-donor P (a bacteriochlorophyll dimer) to the B-side bacteriopheophytin (HB) in the bacterial photosynthetic reaction center (RC). Starting from a platform Rhodobacter sphaeroides RC bearing several amino acid changes, an Arg in place of the native Leu at L185—positioned over one face of HB and only ∼4 Å from the 4 central nitrogens of the HB macrocycle—is the key additional mutation providing 90% yield of P+HB−. This all but matches the near-unity yield of A-side P+HA− charge separation in the native RC. The 90% yield of ET to HB derives from (minimally) 3 P* populations with distinct means of P* decay. In an ∼40% population, P* decays in ∼4 ps via a 2-step process involving a short-lived P+BB− intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → P+HB− conversion takes place in ∼20 ps by a superexchange mechanism mediated by BB. An ∼10% population of P* decays in ∼150 ps largely by internal conversion. These results address the long-standing dichotomy of A- versus B-side initial charge separation in native RCs and have implications for the mechanism(s) and timescale of initial ET that are required to achieve a near-quantitative yield of unidirectional charge separation.
bacteriochlorophyll dimermutant reaction centerprotein distributionsprotein dynamicsultrafast transient absorption spectroscopy
To whom correspondence may be addressed. Email: kirmaier@wustl.edu
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Press Release Excerpt/Excerto da Comunicação à Imprensa:
As a result of their efforts, the scientists are now closer than ever to being able to design electron transfer systems in which they can send an electron down a pathway of their choosing.
“This is important because we are gaining the ability to harness the flow of energy to understand design principles that will lead to new applications of abiotic systems,” Laible said.
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