Ironies in photosynthetic electron transport: a personal perspective

Cramer, William A.

doi:10.1007/1-4020-3324-9_48

Discoveries in Photosynthesis

DOI: 10.1007/1-4020-3324-9_48

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Ironies in photosynthetic electron transport: a personal perspective

William A. Cramer

Abstract: In this brief personal perspective, studies, mainly in my laboratory, directed toward understanding of the structure and function of the Fe-metalloprotein electron transport carriers of oxygenic photosynthesis are discussed. This level of understanding and the description of the events that led to this level are inevitably incomplete. The Fe-proteins considered are those, cytochromes b-559 (560), f, b 6 , and x, and the Rieske [2Fe-2S] iron-sulfur protein, which have now been placed in a structural context as … Show more

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“…Cyanobacteria evolve O 2 at a rate of approximately 400 μmol mg Chl −1 h −1 (2, 3) in a process limited by diffusiongoverned electron transfer steps (Fig. 1A), in particular the slow interaction of plastoquinol-9 with the cytochrome b 6 f complex (4). Once electrons leave PSI, diffusionally governed electron transfer steps constrain the rate of interaction of ferredoxin with other enzymes, including ferredoxin:NADP þ oxidoreductase.…”

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Solar hydrogen-producing bionanodevice outperforms natural photosynthesis

Lubner

Applegate

Knörzer

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

156

124

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Although a number of solar biohydrogen systems employing photosystem I (PSI) have been developed, few attain the electron transfer throughput of oxygenic photosynthesis. We have optimized a biological/organic nanoconstruct that directly tethers F B , the terminal þ þ 2 ferredoxin red is carried out in two separate photochemical half-reactions. Photosystem II (PSII) catalyzes the anodic half-cell reaction H 2 O þ plastoquinone-9 þ 2 hν → 1∕2 O 2 þ plastoquinol-9, while photosystem I (PSI) catalyzes the cathodic half-cell reaction cytochrome c 6ðredÞ þ ferredoxin ox þ 1 hν → cytochrome c 6ðoxÞ þ ferredoxin red . Visible photons provide the energy necessary to drive these otherwise thermodynamically unfavorable half-cell reactions to completion (1). Cyanobacteria evolve O 2 at a rate of approximately 400 μmol mg Chl −1 h −1 (2, 3) in a process limited by diffusiongoverned electron transfer steps (Fig. 1A), in particular the slow interaction of plastoquinol-9 with the cytochrome b 6 f complex (4). Once electrons leave PSI, diffusionally governed electron transfer steps constrain the rate of interaction of ferredoxin with other enzymes, including ferredoxin:NADP þ oxidoreductase. Were it possible to directly connect redox proteins through their redox centers, electrons could be vectored preferentially thereby eliminating any dependence on diffusional electron transfer (5, 6, 7). Here we report that by engineering a nanoconstruct in which both the electron donor (cytochrome c 6 ) and acceptor (here: ½FeFe-H 2 ase) are tethered to PSI in vitro, rate-limiting, diffusion-based electron transfer reactions are eliminated (Fig. 1B), resulting in electron transfer rates that exceed those of natural photosynthesis.The approach connects PSI to an ½FeFe-H 2 ase (8) using a molecular wire, which separates the [4Fe-4S] clusters of each enzyme by a defined distance (5). By introducing an exchangeable sulfhydryl ligand to the most solvent-exposed iron atom of PSI, the molecular wire can be attached by a ligand exchange mechanism. This is achieved by site-specific conversion of a ligating Cys residue (C13) of F B , the terminal [4Fe-4S] cluster, to a Gly (9-11) and by chemically rescuing the cluster with a small sulfhydrylcontaining molecule (11). Because ½FeFe-H 2 ases also contain [4Fe-4S] clusters, which constitute an electron transfer pathway between the surface of the enzyme and its catalytic site (12, 13), a similar strategy is used to introduce an exchangeable ligand at the distal [4Fe-4S] cluster (C97G). A tether that contains two sulfhydryl groups serves as a chemical rescue agent for both the F B cluster of PSI and the distal [4Fe-4S] cluster of ½FeFe-H 2 ase, thereby providing a pathway for electrons to quantum mechanically tunnel between the two proteins.

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mentioning

confidence: 99%

Solar hydrogen-producing bionanodevice outperforms natural photosynthesis

Lubner

Applegate

Knörzer

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

156

124

View full text Add to dashboard Cite

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Ironies in photosynthetic electron transport: a personal perspective

Cited by 1 publication

References 65 publications

Solar hydrogen-producing bionanodevice outperforms natural photosynthesis

Solar hydrogen-producing bionanodevice outperforms natural photosynthesis

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