Can extracellular enzymes functionalize electrodes?

Yoel Cortes-Pena, Georgia Institute of Technology

A microbial electrosynthesis cell (MEC) takes in electrical energy and converts it to chemical energy as fuels, but what supports that initial step of taking the electrons off the cathode so that they become available to the microbes? Are small redox molecules acting as intermediates between the electrode and the microbes? Or is there a more direct way?

Culutres

Photo of anaerobic cultures grown in a pressurized mixture of hydrogen and carbon dioxide. The tube on the far left is a sterile control of inoculated media that has no hydrogen inside. The other three tubes are biological triplicates.

Previous studies have shown that hydrogenases, enzymes that catalyze the conversion between hydrogen cations to hydrogen gas, are able to attach themselves to an electrode and remain active (1). Additionally, a study by Deutzmann el. al. showed that extracellular hydrogenases support the production of hydrogen on a cathode in an MEC that utilizes methanogenic bacteria (2). Therefore, It is possible that active hydrogenases are able to attach themselves onto an electrode, uptake the electrons directly off the cathode, and produce hydrogen which will later be consumed by the microbes!

Over this summer internship, I took the first step to test this hypothesis by asking the question “Are active hydrogenases released from the microbes?”. Active extracellular hydrogenases in the reactor suspension would support the hypothesis that hydrogenases are also active when attached to the cathode.

Hydrogenase_assayPhoto of hydrogenase activity as measured spectroscopically by methyl viologen. Oxidized methyl viologen is clear and reduced methyl viologen is blue. The test involves inserting the sample that may contain hydrogenases into a reaction mixture with oxidized methyl viologen and pressurized hydrogen. Hydrogenases would proceed to oxidize hydrogen gas and reduce methyl viologen, turning the mixture blue

The main experiment I am working on involves testing hydrogenase activity in anaerobic cultures that originated from the reactor suspension. These cultures are grown in a pressurized 80:20 mixture of hydrogen and carbon dioxide, which mimics the reactor environment.

By the end of the summer I hope to have hydrogenase activity data of spent cell-free culture medium that will answer our question  “Are active hydrogenases released from the microbes?”

References

(1) Baffert C, Sybirna K, Ezanno P, Lautier T, Hajj V, Meynial-Salles I, Soucaille P, Bottin H, Léger C(2012). Covalent attachment of FeFe hydrogenases to carbon electrodes for direct electron transfer. Anal Chem 84(18):7999-8005. doi: 10.1021/ac301812s.

(2) Deutzmann JS, Sahin M, Spormann AM. (2015) Extracellular enzymes facilitate electron uptake in biocorrosion and bioelectrosynthesis. mBio 6(2):e00496-15. doi:10.1128/mBio.00496-15.

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