Photoreduction of outer membrane cytochromes : solution and proteoliposome nanocompartment studies

• Main Author: Ainsworth, Emma
• Published: University of East Anglia 2017
• Subjects: 541
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.716453

Due to ever increasing global energy demand and depletion of fossil fuels, interest has grown in developing systems which can utilise the near limitless supply of energy provided to the Earth by the sun. Artificial photosynthetic systems mimic natural photosynthesis by using photosensitisers that absorb solar energy to generate high-energy electrons. These photoexcited electrons are passed to catalysts which drive reductive transformations to produce fuels and valued organic compounds. Sacrificial electron donors regenerate photosensitisers to complete the photocatalytic cycle. To date artificial photosynthetic systems are unable to fully mimic and reap the full benefits of natural photosynthesis as they lack spatially separating reaction compartments, and/or compartment-linking transmembrane electron transport chains. A model for the biochemistry that supports such transmembrane electron transfer is provided by the outer membrane cytochromes of Shewanella oneidensis MR-1. In this Gram-negative bacterium, trans-outermembrane electron transfer to extracellular electron acceptors is performed by the extracellular cytochromes MtrC and OmcA acting together with the porin:cytochrome complex (MtrCAB), which provides an outer membrane spanning conduit that moves electrons from the periplasm to the cell surface. Liposomes offer reaction compartments which are separated from the surrounding external environment by a phospholipid membrane. Incorporation of MtrCAB in the membranes of liposomes containing methyl viologen facilitates transmembrane electron transfer from an external reductant to the compartmentalised electron acceptor. In this study the potential for MtrCAB proteoliposomes to act as a modular artificial photosynthetic system, which performs reductive transformations, was investigated. To explore this potential we have studied the ability of several photosensitisers to photoreduce: i) outer membrane cytochromes of S. oneidensis; ii) methyl viologen in solution and within liposomes; iii) MtrCAB in liposome membranes; and iv) reactive red 120 in solution and within (proteo)liposomes. In addition the sacrificial electron donor(s), photoreduction mechanisms and interaction mechanisms used by each photosensitiser have been identified.