Optimizing Electroactive Organisms: The Effect of Orthologous Proteins

Extracellular electron transfer pathways allow bacteria to transfer electrons from the cellmetabolism to extracellular substrates, such as metal oxides in natural environmentsand electrodes in microbial electrochemical technologies (MET). Studies of electroactivemicroorganisms and mainly of Shewanella oneidensis MR-1 have demonstrated thatextracellular electron transfer pathways relies on several multiheme c-type cytochromes.The small tetraheme cytochrome c (STC) is highly conserved among Shewanellaspecies and is one of the most abundant cytochromes in the periplasmic space. Ittransfers electrons from the cell metabolism delivered by the inner-membrane tetrahemecytochrome CymA, to the porin-cytochrome complex MtrCAB in the outer-membrane,to reduce solid electron acceptors outside the cell, or electrodes in the case of MET.In this work knock-out strains of STC of S. oneidensis MR-1, expressing STC fromdistinct Shewanella species were tested for their ability to perform extracellular electrontransfer, allowing to explore the effect of protein mutations in living organisms. Thesestudies, complemented by a biochemical evaluation of the electron transfer properties ofthe individual proteins, revealed a considerable plasticity in the molecular componentsinvolved in extracellular electron transfer. The results of this work are pioneering andof significant relevance for future rational design of cytochromes in order to enhanceextracellular electron transfer and thus contribute to the practical implementation of MET.