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Membrane Protein Modified Electrodes in Bioelectrocatalysis catalysts Review Membrane Protein Modified Electrodes in Bioelectrocatalysis Huijie Zhang y, Rosa Catania y and Lars J. C. Jeuken * School of Biomedical Sciences and the Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK; [email protected] (H.Z.); [email protected] (R.C.) * Correspondence: [email protected]; Tel.: +44-113-343-3829 These authors have contributed equally. y Received: 18 November 2020; Accepted: 3 December 2020; Published: 6 December 2020 Abstract: Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis- based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis. Keywords: membrane protein; bioelectrocatalysis; electrode modification; biofuel cells; photosynthesis; liposomes; hybrid vesicles; microbial electrosynthesis 1. Introduction Membrane proteins constitute 20–30% of all proteins encoded by both prokaryotic and eukaryotic cells. They perform a wide variety of functions, including material transport, signal transduction, catalysis, proton and electron transport (Figure1)[ 1]. They are also key to a number of earth’s most fundamental reactions, such as respiration and photosynthesis [2,3]. Redox enzymes in the respiratory chain catalyse a variety of fundamental processes for energy conversion and fuel production, including H2 oxidation, O2 reduction, and carbon and nitrogen cycling. Membrane proteins that are involved in the light reaction of photosynthesis harvest light and facilitate electron transfer essential for solar energy conversion. The amphiphilic nature of membrane proteins makes them difficult to isolate, study and manipulate. Despite these challenges, membrane proteins have been widely advocated and studied for applications in bioelectrocatalysis, such as biofuel cells [4] and semi-artificial photosynthesis [5]. Here, we will review electrochemical studies of membrane proteins with the view to using these systems for bioelectrocatalysis. To aid discussion later on, we will briefly introduce a small selection of membrane enzymes active in bioenergy conversion, although this is far from a comprehensive list. We will then summarise the main strategies to immobilise membrane proteins on electrodes and discuss common techniques used to characterise these electrodes, including electrochemistry, spectroscopy, spectroelectrochemistry, microscopy and quartz crystal microbalance. Finally, some critical application challenges and potential future research directions will be highlighted that might find application in bioelectrocatalysis. Specifically, we will focus on two emerging directions. One is the reconstitution of membrane proteins into hybrid vesicles to extend their functional lifetime. The other is the use of microorganisms for microbial electrosynthesis and semi-artificial photosynthesis. Catalysts 2020, 10, 1427; doi:10.3390/catal10121427 www.mdpi.com/journal/catalysts Catalysts 2020, 10, x FOR PEER REVIEW 2 of 28 Catalystsvesicles2020 to, 10extend, 1427 their functional lifetime. The other is the use of microorganisms for microbial2 of 29 electrosynthesis and semi-artificial photosynthesis. Figure 1. Representative functions of transmembrane proteins. Figure 1. Representative functions of transmembrane proteins. 2. Membrane Proteins in Biofuel Conversion and Photosynthesis 2. Membrane Proteins in Biofuel Conversion and Photosynthesis 2.1. Membrane Enzymes in Biofuel Conversion 2.1. MembraneNature offers Enzymes highly in specialised Biofuel Conversion enzyme machineries that can be exploited for (bio)fuel conversion. AmongNature them, offers membrane-bound highly specialised hydrogenases, enzyme found machineries in many bacteria,that can archaea be exploited and lower for eukaryotes, (bio)fuel areconversion. metalloenzymes Among capablethem, membrane-bound of catalysing the reversiblehydrogenases, oxidation found of Hin2 manyto protons bacteria, and electronsarchaea and [6]. Depending on the metal located in the active site, three phylogenetically unrelated classes can be lower eukaryotes, are metalloenzymes capable of catalysing the reversible oxidation of H2 to protons identified:and electrons [NiFe], [6]. Depending [FeFe] and, on less the common, metal located [Fe] hydrogenases. in the active site, Most three hydrogenases phylogenetically are rapidly unrelated and almostclasses completelycan be identified: inactivated [NiFe], by [FeFe] O2 [7]. and, However, less common, aerobic or[Fe] facultative hydrogenases. aerobic Most H2-oxidising hydrogenases bacteria are have a particular subtype of O -tolerant [NiFe] hydrogenase, which withstands the presence of O and rapidly and almost completely2 inactivated by O2 [7]. However, aerobic or facultative aerobic2 H2- are membrane-bound hydrogenases (MBHs) [8]. The most studied O -tolerant [NiFe] hydrogenases oxidising bacteria have a particular subtype of O2-tolerant [NiFe] hydrogenase,2 which withstands the are found in Ralstonia eutropha, Ralstonia metallidurans, Aquifex aeolicus, Hydrogenovibrio marinus and presence of O2 and are membrane-bound hydrogenases (MBHs) [8]. The most studied O2-tolerant Escherichia[NiFe] hydrogenases coli. MBHs are are multimeric found in proteins Ralstonia with oneeutropha large, subunitRalstonia and metallidurans one small subunit,, Aquifex bound aeolicus to the, periplasmicHydrogenovibrio side ofmarinus the cytoplasmic and Escherichia membrane coli. throughMBHs are a transmembrane multimeric proteins protein with (b-type one cytochrome) large subunit [8]. Theand [NiFe]one small active subunit, site, with bound the same to the configuration periplasmi asc side other of [NiFe] the cytoplasmic hydrogenases, membrane is deeply through buried ina thetransmembrane large subunit. protein MBHs ( coupleb-type cytochrome) oxidation of hydrogen[8]. The [NiFe] in the acti largeve subunitsite, with to the the same reduction configuration of either menaquinone-7as other [NiFe] orhydrogenases, ubiquinone-8 is in thedeeplyb-type buried cytochrome in the inlarge the subunit. membrane. MBHs couple oxidation of hydrogenIn the in respiratory the large subunit chain, to terminal the reduction oxidases of either catalyse menaquinone-7 the reduction or ubiquinone-8 of molecular in oxygen the b-type to watercytochrome without in formationthe membrane. of reactive oxygen species (ROS). They oxidise quinones (ubiquinone and menaquinoneIn the respiratory oxidases) chain, or cytochromes terminal oxidases (cytochrome catalysec oxidases) the reduction and can of be molecular classed into oxygen haem-copper to water oxidaseswithout [formation9], bd oxidases of reactive [10] or alternativeoxygen species oxidases (ROS). [11]. They For instance, oxidise cytochromequinones (ubiquinonebo3 enzymes and are haem-coppermenaquinone oxidases oxidases) in or bacteria cytochromes such as (cytochromeE. coli and couple c oxidases) oxidation and ofcan ubiquinol-8 be classed tointo the haem-copper reduction of oxygen [12]. Cytochrome bd is a quinol-dependent terminal oxidase found exclusively in prokaryotes oxidases [9], bd oxidases [10] or alternative oxidases [11]. For instance, cytochrome bo3 enzymes are andhaem-copper is structurally oxidases unrelated in bacteria to haem-copper such as E. coli oxidases and couple [13]. oxidation of ubiquinol-8 to the reduction of oxygenNitrate [12]. reductases Cytochrome are molybdoenzymes bd is a quinol-dependent capable of reducing terminal nitrate oxidase (NO found3−) to nitriteexclusively (NO2 −in). Thisprokaryotes enzyme and family is structurally can be found unrelated in eukaryotic to haem-copper and prokaryotic oxidases cells. [13]. Eukaryotic nitrate reductases are present in plants, algae and fungi, and are involved in the assimilation of nitrate. Prokaryotic nitrate Nitrate reductases are molybdoenzymes capable of reducing nitrate (NO3−) to nitrite (NO2−). This reductasesenzyme family are classified can be found into threein eukaryotic classes: assimilatory and prokaryotic nitrate cells. reductases Eukaryotic (Nas), nitrate periplasmic reductases nitrate are reductasespresent in plants, (Nap) algae and respiratory and fungi, nitrateand are reductase involved in (Nar). the assimilation The latter are of nitrate. transmembrane Prokaryotic enzymes nitrate whichreductases use nitrate are classified as the electron into three acceptor classes: of assimilatory an anaerobic nitrate respiratory reductases chain [(Nas),14]. Facultative periplasmic anaerobic nitrate bacteriareductases use (Nap) these alternativeand respiratory respiratory nitrate enzymes reductase in oxygen (Nar). depletedThe latter environments, are transmembrane replacing enzymes oxygen withwhich di usefferent nitrate electron as the acceptorselectron acceptor [15]. For of
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