Engineering of an alternative electron transfer path in photosystem II

Abstract

The initial steps of oxygenic photosynthetic electron transfer occur within photosystem II, an intricate pigment/protein transmembrane complex. Light-driven electron transfer occurs within a multistep pathway that is efficiently insulated from competing electron transfer pathways. The heart of the electron transfer system, composed of six linearly coupled redox active cofactors that enable electron transfer from water to the secondary quinone acceptor Q B , is mainly embedded within two proteins called D1 and D2. We have identified a site in silico, poised in the vicinity of the Q A intermediate quinone acceptor, which could serve as a potential binding site for redox active proteins. Here we show that modification of Lysine 238 of the D1 protein to glutamic acid (Glu) in the cyanobacterium Synechocystis sp. PCC 6803, results in a strain that grows photautotrophically. The Glu thylakoid membranes are able to perform light-dependent reduction of exogenous cytochrome c with water as the electron donor. Cytochrome c photoreduction by the Glu mutant was also shown to significantly protect the D1 protein from photodamage when isolated thylakoid membranes were illuminated. We have therefore engineered a novel electron transfer pathway from water to a soluble protein electron carrier without harming the normal function of photosystem II. cyanobacteria | energy conversion | proinhibition | photosynthesis | protein engineering P hotosynthesis is the major source of useful chemical energy in the biosphere. All photosynthetic processes require efficient electron transfer (ET) pathways that are utilized for proton gradient formation (to be used for the production of ATP) and/or accumulation of reducing equivalents (1, 2). Light energy, absorbed by light-harvesting antenna complexes, is transferred to photochemical reaction centers (RC), initiating charge separation in specific chlorophyll molecules bound to the RC proteins. Following charge separation, electrons are transferred sequentially to a series of acceptor molecules, each with a redox potential determined by its immediate environment (3, 4). The source of electron replenishment differs according to the reaction center type. For instance in purple non-sulfur bacteria, electrons are cycled back to the oxidized reaction center by a soluble cytochrome c (cyt c) type protein (5, 6). Oxygenic photosynthetic organisms (cyanobacteria, red and green algae and plants) contain two photosystems: photosystem I (PSI) and photosystem II (PSII) that work linearly (1, 7), and the source of electrons is water. One common facet of all ET pathways is the requirement for insulation of the redox active cofactors from potentially reducing/oxidizing molecules within the RC or in the surrounding media. Insulation provides the system with maximal ET rates and efficiencies and also prevents damage to the RC.PSII has a redox potential of up to 1.2 V (8, 9), required to abstract electrons from water (Fig. 1A). The photoexcited P 680 reaction center chlorophyll a primary donor transfers electron...

Operon flv4-flv2 Provides Cyanobacterial Photosystem II with Flexibility of Electron Transfer

“…Binding of the Flv4/Flv2 heterodimer to PSII likely occurs via charge interaction between the heterodimer and the PSII cytoplasmic side (Larom et al, 2010). The role of Flv2/Flv4 in interaction of PBS with PSII is likely to be linked to their electron transfer properties.…”

Photosynthetic Energy Conversion: Recent Advances and Future Perspective

“…Modification of this lysine to glutamate (Glu, E), i.e., K238E resulted in alternative electron transfer pathway to soluble electron acceptor protein (cytochrome c) near the Q A -site. 42 This redirection along with the addition of an herbicide that blocks the electron flow at Q B site resulted in decreased oxidative damage. In another attempt, cationic redox-active metal complexes …”

“…Electrons from Q A -in PSII were redirected to engineered collection sites approximately 13 Å away on the stromal side of the thylakoid membrane. 42 The positively charged amino acid lysine (Lys, K) at position 238 of D1 protein in PSII is important for the insulation of the PSII electron flow from external oxidation by soluble species and is also highly conserved in PSII of higher plants and algae. Modification of this lysine to glutamate (Glu, E), i.e., K238E resulted in alternative electron transfer pathway to soluble electron acceptor protein (cytochrome c) near the Q A -site.…”

“…Accordingly various efforts have been undertaken to collect more electrons from PSII. 42 Certain genetic engineering approaches have been used to redirect the electrons from the photosystem complexes, thereby manipulating P-ETC for generating higher photocurrent. Electrons from Q A -in PSII were redirected to engineered collection sites approximately 13 Å away on the stromal side of the thylakoid membrane.…”

Photoelectrochemical cells based on photosynthetic systems: a review

“…This limitation can be overcome by redirecting the electrons from the inner part of the protein to the surface (Sekar and Ramasamy, 2015). For instance, Larom et al (2010) successfully used an artificial mediator to redirect electrons from QA -to an artificial acceptor at a distance of about 1.3 nm from the stromal side of the membrane. This change in the direction of electron flow along with additional blocking of QB led to a reduction of oxidative damages at the expense of reducing the time of the intermediate electron transfer at the stage of QA/QB.…”