Cap­tur­ing CO2 with elec­tri­city: A mi­cro­bial en­zyme in­spires elec­tro­chem­istry

Humanity continuously emits greenhouse gases and thereby worsens global warming. Increasing research efforts go into developing strategies to convert these gases, such as carbon dioxide (CO₂), into valuable products. CO₂ accumulates dramatically over the years and is chemically very stable, thus challenging to transform. Yet, for billions of years, some microbes have actively captured CO₂ using highly efficient enzymes. Scientists from the Max Planck Institute for Marine Microbiology in Bremen together with the Universities of Geneva and Radboud isolated one of these enzymes. When the enzyme was electronically branched on an electrode, they observed the conversion of CO₂ to formate with perfect efficiency. This phenomenon will inspire new CO₂-fixation systems because of its remarkable directionality and rates. The results have now been published in the journal “Angewandte Chemie”.

Seek­ing mi­croor­gan­isms that ef­fi­ciently cap­ture the green­house gas CO₂

“The en­zymes em­ployed by the mi­croor­gan­isms rep­res­ent a fant­astic play­ground for sci­ent­ists as they al­low highly spe­cific re­ac­tions at fast rates”, says Tristan Wag­ner, head of the Max Planck Re­search Group Mi­cro­bial Meta­bol­ism at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy (MPIMM).

Some of these en­zymes have an in­ter­est­ing way of cap­tur­ing CO₂: They trans­form it into form­ate, a stable and safe com­pound that can be used to store en­ergy or to syn­thes­ize vari­ous mo­lecules for in­dus­trial or phar­ma­ceut­ical pur­poses. One ex­ample is Methermicoccus shengliensis, a meth­ano­gen (a mi­crobe pro­du­cing meth­ane) isol­ated from an oil­field and grow­ing at 50 °C. It has been cul­tiv­ated and stud­ied over the past years by Ju­lia Kurth and Cor­ne­lia Welte at Rad­boud Uni­versity in the Neth­er­lands. At the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, Olivier Lemaire, Mélissa Bel­hamri and Tristan Wag­ner dis­sec­ted the mi­crobe to find its CO₂-cap­tur­ing en­zyme and meas­ure how fast and ef­fi­ciently it can trans­form CO₂.

A CO₂-con­vert­ing en­zyme with great po­ten­tial

The Max Planck-sci­ent­ists un­der­took the chal­len­ging task to isol­ate the mi­cro­bial en­zyme. “Since we knew that such en­zymes are sens­it­ive to oxy­gen, we had to work in­side an an­aer­obic tent devoid of am­bi­ent air to sep­ar­ate it from the other pro­teins – quite com­plic­ated, but we suc­ceeded”, says Olivier Lemaire.

Once isol­ated, the sci­ent­ists char­ac­ter­ized the en­zyme’s prop­er­ties. They showed that it ef­fi­ciently gen­er­ates form­ate from CO₂ but per­forms the re­verse re­ac­tion at very slow rates and poor yield.

“Sim­ilar en­zymes be­long­ing to the fam­ily of form­ate de­hyd­ro­genases are well known to op­er­ate in both dir­ec­tions, but we showed that the en­zyme from Methermicoccus shengliensis is nearly uni­direc­tional and could not ef­fi­ciently con­vert the form­ate back into CO₂”, re­ports Mélissa Bel­hamri. “We were quite thrilled by this phe­nomenon, oc­cur­ring only in the ab­sence of oxy­gen”, she adds. “Since the form­ate gen­er­ated from CO₂-fix­a­tion can­not be trans­formed back and there­fore ac­cu­mu­lates, such a sys­tem would be a highly in­ter­est­ing can­did­ate for CO₂-cap­ture, es­pe­cially if we could branch it on an elec­trode”, Tristan Wag­ner points out.

The ad­vant­age of that: With the en­zyme nat­ur­ally or chem­ic­ally at­tached to an elec­trode, the “en­ergy” re­quired to cap­ture the CO₂ will be dir­ectly de­livered by the elec­trode, without elec­tric cur­rent loss or the need for ex­pens­ive or toxic chem­ical com­pounds as re­lays. Con­sequently, the en­zyme-bound elec­trodes are ef­fi­cient and at­tract­ive sys­tems for gas con­ver­sion pro­ced­ures. Thus, the pur­i­fied en­zyme was sent to the Uni­versity of Geneva to set up an elec­trode-based CO₂-cap­ture sys­tem.

Elec­tri­city-based gas con­ver­sion

Selmi­han Sahin and Ross Milton from the Uni­versity of Geneva are spe­cial­ists in elec­tro­chem­istry. They use elec­trodes con­nec­ted to elec­tric cur­rent to per­form chem­ical re­ac­tions. The elec­trode-based form­ate gen­er­a­tion from CO₂ of­ten re­quires pol­lut­ing and rare metals, and that is why they tried to re­place these metals with the en­zyme ex­trac­ted in the group of Tristan Wag­ner at the MPIMM. The pro­ced­ure of en­zyme bind­ing on an elec­trode is not al­ways as ef­fi­cient as ex­pec­ted, but the en­zyme from Wag­n­er’s re­search group has spe­cific char­ac­ter­ist­ics that could fa­cil­it­ate the pro­cess. The sci­ent­ists from Switzer­land man­aged to fix the en­zyme on a graph­ite elec­trode, where it per­formed the gas con­ver­sion. The meas­ured rates were com­par­able to those ob­tained with clas­sic form­ate de­hyd­ro­genases.

“The strength of this bio­lo­gical sys­tem coupled to the elec­trode lies in its ef­fi­ciency in trans­fer­ring the elec­trons from the elec­tri­city to­wards CO₂ trans­form­a­tion”, high­lights Lemaire.

Sahin and Milton also con­firmed that the sys­tem per­forms the re­verse re­ac­tion poorly, as pre­vi­ously ob­served in the re­ac­tion tube. Con­sequently, the mod­i­fied elec­trode con­tinu­ously con­ver­ted the green­house gas to form­ate without any de­tect­able side-products gen­er­ated or elec­tric cur­rent loss.

To­wards a new solu­tion for at­mo­spheric CO₂ util­iz­a­tion

The col­lab­or­at­ive work provides a new mo­lecu­lar tool to the sci­entific com­munity: An en­zyme con­vert­ing CO₂ by trans­fer­ring elec­tri­city with high ef­fi­ciency. Re­new­able green en­ergy (e.g., wind or solar) could provide elec­tri­city to the elec­trode-based sys­tem that would turn CO₂ into form­ate, a mo­lecule dir­ectly us­able for ap­plic­a­tions or to store en­ergy.

“Be­fore us, no one ever tried to study an en­zyme from such a meth­ano­gen for an elec­trode-based gas con­ver­sion”, says Tristan Wag­ner. “Yet, meth­ano­gens are nat­ural out­stand­ing gas con­vert­ers”.

As power­ful as they could be, em­ploy­ing en­zymes for large-scale pro­cesses would also re­quire sim­ilar-scale en­zyme pro­duc­tion sys­tems, a con­sid­er­able in­vest­ment. There­fore, while the dis­covered strategy could, in the­ory, sig­ni­fic­antly im­prove CO₂trans­form­a­tion, a deep know­ledge of the en­zyme mech­an­ism is ne­ces­sary be­fore its ap­plic­a­tion, and the team of re­search­ers will now have to dis­sect in depth the mo­lecu­lar secrets of the re­ac­tion.

Ori­ginal pub­lic­a­tion

Selmi­han Sahin*, Olivier N. Lemaire*, Mélissa Bel­hamri, Ju­lia M. Kurth, Cor­ne­lia U. Welte, Tristan Wag­ner, Ross D. Milton (2023): Bio­elec­trocata­lytic CO₂ Re­duc­tion by Mo-De­pend­ent Formyl­meth­an­o­furan De­hyd­ro­genase. Angew. Chem. Int. Ed. (15 Sept 2023), e202311981.

DOI: https://doi.org/10.1002/anie.202311981

* Both au­thors con­trib­uted equally

Source

Max Planck Institute, press release, 2023-09-28.

Supplier

Angewandte Chemie (Journal)
Max Planck Institute for Marine Microbiology
Max Planck Society
Radboud University, Nijmegen (NL)
University of Geneva

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