Bacterial tyrosinases as extracellular sources of quinone-based electron shuttles in soil

Abstract

Extracellular electron transfer in soils provides microorganisms with a powerful tool to obtain energy for the sustenance of life processes under oxygen scarcity. Electron shuttles provided by natural organic matter (NOM) are widely considered the ubiquitous mediators that carry electrons from the cell to an acceptor that is particulate or physically distant. Specifically, quinone moieties associated with NOM are seen as the rechargeable molecular entities that make this process possible. For more than 3 decades, the hypothetical, abiotic recondensation of organic residues into polymeric macromolecules called humic substances has served as an explanation for the presence of quinone moieties in soils. As this so-called ‘humification’ model has been superseded, an alternative explanation for the origin of quinoid electron shuttles in natural systems is called for. Here we present a literature survey to show that prokaryotic microorganisms are capable of excreting tyrosinases, an enzyme class with the ability to synthesize soluble and mobile orthoquinones in extracellular soil environments. We argue that this mechanism represents a key pathway for microorganisms to keep their immediate environment stocked with electron shuttles featuring electrochemical specifications tailored to their individual needs.