Characterisation of OmcA From Shewanella oneidensis MR-1: Biophysical and Mineral Reduction Properties
Several!Shewanella(spp!are!versatile!in!the!respiratory!substrates!they!use.! A! novel! set! of! respiratory! substrates! implicated! are! insoluble! Fe(III)! and! Mn(III,IV)! oxides.! OmcA! from! Shewanella( oneidensis( MR?1! plays! a! role! in! the! terminal!electron!transfer!during!respiratory!mineral!reduction,!but!has!minimal! or! contrasting! properties! in! the! literature.! A! suite! of! biophysical! techniques! confirmed!the!bis?histidine!axial!ligation!of!all!OmcA’s!haem!content!in!the!crystal! structure! and! in! solution.!The!paramagnetic! resonance! feature! designated! “LS3”! was!identified!to!be!unique!to!OmcA.!However!this!resonance!signal!is!modelled!to! be! produced! spin?coupling! and! not! unique! haem! ligation.! OmcA’s! electroactive! coverage!is!comparable!to!UndA!and!the!other!major!outer!membrane!multihaem! cytochrome!(OMMC)!clades!MtrC!and!MtrF!(i.e.!+0.08!V!to!?0.47!V!vs!S.H.E.).!The! crystal! structure! of! OmcA! shows! domain! fold,! domain! organisation! and! haem! orientation!conservation!with!MtrF!and!UndA.!Comprehensive!solution?structure! studies!of!OmcA!provided!contrasting!experimental!data!on!the!oligomeric!state!of! OmcA,!such!that!it!is!unresolved!whether!OmcA!forms!an!ion?sensitive!dimer.!The! crystal!structure!shows!a!predicted!mineral!interaction!peptide!(i.e.!T725P726S727)! is!solvent!exposed!and!would!putatively!bind!substrate!within!electron!tunneling! distance! of! a! terminal! haem.! Site?directed! mutagenesis! indicates! Thr725! is! significant! to! maintenance! of! the!molecular! environment! of! haem! 10.! Although! T725G!mutation!produces!a!≈80%!decrease!in!whole!cell!reduction!of!synthesised! hematite! over! 120! hours;! secondary! effects! of! change! in! haem! reduction! potentials! or!widespread! conformational! effects!were! ruled! out.!OmcA! has! thus! been!shown!to!share!a!common!OMMC?fold!and!exist!in!S.(oneidensis(MR?1!outer! membranes! as! a! functioning! mineral! reductase! cytochrome! with! unique! paramagnetic!resonance!properties!in!this!set!of!studies.!
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