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), which allows it to produce a detectable current in a bioelectrochemical system. Utilization of quinone mediators by L. plantarum requires a type-II NADH dehydrogenase (Ndh2), however small structural variations in the core of 1,4-naphthoquinone EET mediators yields significantly different current outputs. Herein, we assembled a library of 30 naphthoquinone-based EET mediators in order to probe the important physicochemical properties and biochemical interactions that are responsible for Ndh2-dependent EET in L. plantarum. The library was designed with inspiration from naturally-occurring metabolites, and was assembled focused on structural modifications that diversified polarity (cLogP), reduction potential (Eo), and Ndh2 binding affinity as these properties are hypothesized to be driving EET activity. In general, Ndh2-dependent EET activity in an iron(III) oxide nanoparticle assay significantly correlates to the mediator's polarity and binding affinity. Five mediators were analyzed in BESs with L. plantarum and each generated Ndh2-dependent current with low background signal. Importantly, the amine containing mediators yielded incredibly stable current output over the course of the experiment (up to 5 days). These findings increase our understanding of structure-activity relationships for quinone-mediated EET and provide mediators for bioelectronic sensing.