Abstract
Extracellular cytochromes are hypothesized to facilitate the final steps of electron transfer between the outer membrane of the metal-reducing bacterium Geobacter sulfurreducens and solid-phase electron acceptors such as metal oxides and electrode surfaces during the course of respiration. The triheme c-type cytochrome PgcA exists in the extracellular space of G. sulfurreducens, and is one of many multiheme c-type cytochromes known to be loosely bound to the bacterial outer surface. Deletion of pgcA using a markerless method resulted in mutants unable to transfer electrons to Fe(III) and Mn(IV) oxides; yet the same mutants maintained the ability to respire to electrode surfaces and soluble Fe(III) citrate. When expressed and purified from Shewanella oneidensis, PgcA demonstrated a primarily alpha helical structure, three bound hemes, and was processed into a shorter 41 kDa form lacking the lipodomain. Purified PgcA bound Fe(III) oxides, but not magnetite, and when PgcA was added to cell suspensions of G. sulfurreducens, PgcA accelerated Fe(III) reduction similar to addition of FMN. Addition of soluble PgcA to ΔpgcA mutants also restored Fe(III) reduction. This report highlights a distinction between proteins involved in extracellular electron transfer to metal oxides and poised electrodes, and suggests a specific role for PgcA in facilitating electron transfer at mineral surfaces.
Highlights
Dissimilatory metal reducing bacteria such as Geobacter sulfurreducens have to transfer respiratory electrons to extracellular acceptors via direct contact to minerals such as iron and manganese oxides
A few G. sulfurreducens proteins are known to be secreted beyond the outer membrane where they could act as loosely bound or mobile mediators to facilitate the final steps of electron transfer, analogous to how secreted redox-active molecules accelerate reduction by Shewanella oneidensis (Lies et al, 2005; Marsili et al, 2008; Von Canstein et al, 2008), Geothrix fermentans (Nevin and Lovley, 2002; Mehta-Kolte and Bond, 2012) and Geobacter uraniireducens (Tan et al, 2016)
PTx-rich tandem repeats are found in many G. sulfurreducens relatives, while PAx-dominated repeats are found in strains such as G. uraniireducens and D. soudanensis
Summary
Dissimilatory metal reducing bacteria such as Geobacter sulfurreducens have to transfer respiratory electrons to extracellular acceptors via direct contact to minerals such as iron and manganese oxides These minerals exist as a heterogeneous mixture of insoluble particles in nature, with a range of redox potentials and surface charges that change during reduction (Nealson and Saffarini, 1994; Cutting et al, 2009; Byrne et al, 2011; Coker et al, 2012; Majzlan, 2012). A more elusive secreted cytochrome was described in Lloyd et al (1999), where a 41 kDa extracellular hemeprotein enriched from G. sulfurreducens supernatants was found to rapidly adsorb to Fe(III) oxides This protein has not been linked to a genetic locus
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