Abstract
Shewanella oneidensis MR-1 is quickly becoming a synthetic biology workhorse for bioelectrochemical technologies due to a high level of understanding of its interaction with electrodes. Transmembrane electron transfer via the Mtr pathway has been well characterized, however, the role of NADH dehydrogenases in feeding electrons to Mtr has been only minimally studied in S. oneidensis MR-1. Four NADH dehydrogenases are encoded in the genome, suggesting significant metabolic flexibility in oxidizing NADH under a variety of conditions. A strain lacking the two dehydrogenases essential for aerobic growth exhibited a severe growth defect with an anode (+0.4 VSHE) or Fe(III)-NTA as the terminal electron acceptor. Our study reveals that the same NADH dehydrogenase complexes are utilized under oxic conditions or with a high potential anode. Our study also supports the previously indicated importance of pyruvate dehydrogenase activity in producing NADH during anerobic lactate metabolism. Understanding the role of NADH in extracellular electron transfer may help improve biosensors and give insight into other applications for bioelectrochemical systems.
Highlights
Bioelectrochemical systems (BESs) interface electrochemically active bacteria with electrodes for biotechnological applications including biosensing and electricity production[1,2]
Beyond the importance of NADH dehydrogenases in anaerobic lactate metabolism, S. oneidensis uses other substrates that must be oxidized by NAD+-linked dehydrogenases, such as N-acetylglucosamine (NAG)
In-frame deletions of each of the four NADH dehydrogenases in S. oneidensis MR-1 were generated in a previous study (Fig. 1)[18]
Summary
Bioelectrochemical systems (BESs) interface electrochemically active bacteria with electrodes for biotechnological applications including biosensing and electricity production[1,2]. Hunt et al.[13] proposed that anaerobic lactate oxidation by S. oneidensis MR-1 utilizes only quinone-linked dehydrogenases to bring electrons into the quinone pool, making NADH dehydrogenases theoretically unnecessary under these conditions. Nuo couples NADH oxidation with proton translocation (4 H+/2 e−), Nqr[1] and Nqr[2] act as sodium-ion translocators (2 Na+/2 e−), and Ndh acts as a type II ‘uncoupling’ dehydrogenase that does not translocate ions[16,17] Ion translocation by these complexes generates a membrane potential that powers flagellar motors and ATP synthesis. We used mutant strains lacking single or multiple NADH dehydrogenases and studied their electron transfer capability in BESs under anoxic conditions[18] We utilized both NAG and d,l-lactate as substrates to investigate the effect of differing levels of NADH generation on phenotypic differences between the strains. We learned that under the conditions tested here, S. oneidensis requires either Nuo or Nqr[1], the same dehydrogenases needed under aerobic conditions
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