Identifying Key Properties that Drive Redox Mediator Activity in Lactiplantibacillus plantarum

Lactiplantibacillus plantarum is known to utilize exogenous small molecule quinone mediators to perform extracellular electron transfer (EET), allowing it to produce a detectable current in a bioelectrochemical system (BES). Utilization of quinone mediators by L. plantarum requires a type-II NADH dehydrogenase (Ndh2), however structural variations in the core of 1,4-naphthoquinone EET mediators has shown to yield significantly different current outputs. Herein, we assembled a library of 40 quinone-based EET mediators to probe the important physicochemical properties and biochemical interactions responsible for Ndh2-dependent EET in L. plantarum. The library was designed with inspiration from naturally-occurring metabolites, and assembly was focused on structural modifications that diversified polarity, reduction potential, and predicted free energy of binding to Ndh2 (ΔGcomp), as these properties are hypothesized to drive EET activity. In general, Ndh2-dependent EET activity in an iron(III) nanoparticle reduction assay significantly correlates to the mediator's polarity and ΔGcomp. Five mediators were analyzed in BESs with L. plantarum and each generated Ndh2-dependent current over background signal. Importantly, an amine containing mediator yielded incredibly stable current output over the course of the experiment (up to 5 days). These findings improve our understanding of structure-activity relationships for quinone-mediated EET and provide stable mediators for bioelectronic sensing applications.