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Lowering the Km of carbonic anhydrase to facilitate capture of CO2

Published onNov 16, 2023
Lowering the Km of carbonic anhydrase to facilitate capture of CO2
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This problem statement is a community effort, and we invite feedback via inline comments or email to the lead contact.

Origin: This problem statement originates from a workshop held by Homeworld Collective on Oct 10, 2023, where speakers presented ideas for how protein engineering could be applied for climate and sustainability.

Contributors: Sonja Salmon, Paul Reginato, Ariana Caiati (and you?)

Lead contact: Paul Reginato ([email protected])

Context

Atmospheric CO2 removal (CDR) and point-source capture (PSC) of CO2 are well-accepted as being necessary for successfully decarbonizing within climate goals (IPCC, 2022). Direct air capture (DAC) is a CDR pathway with ideal verifiability and durability. Both DAC and PSC are cost constrained, primarily by the CapEx of the gas contactor and the energy required to drive large swings in temperature or pH to regenerate CO2 from the capture material (Erans, 2022).

Those high cost and energy requirements are driven by a thermodynamic trade-off between the rate of CO2 absorption and the CO2 regeneration energy: CO2 capture materials with high absorption rate, which reduce cost by reducing the gas contactor size, typically have high CO2 regeneration energy, and vice versa (Reginato, 2023).

Significance

Carbonic anhydrases (CAs) catalyze fast CO2 absorption in solvents with low CO2 regeneration energy, resolving the tradeoff described above (Talekar, 2022). CA could reduce DAC and PSC cost by reducing parameter swing size or gas contactor size. Efforts are ongoing to engineer CA that can be used under the extreme physical conditions often found in DAC or PSC processes, with some success. E.g., thermostable CA via protein engineering (PE) can already reduce PSC cost >30% (Reardon, 2015), and Saipem and Novozymes collaboratively commercialized a CA-enhanced PSC technology in 2021 (Saipem, 2021). 

Greater CO2 absorption enhancement and economic benefit are achieved when CA acts with a higher catalytic rate. CA is famously a diffusion-limited enzyme, i.e., its Vmax is determined by the substrate diffusion rate when [CO2] >> KM. However, the KM of CA for CO2 is ~3-30 mM (De Luca, 2012, Salmon, 2015), which is high. The equilibrium concentration of CO2 in water at equilibrium with 100% CO2 at 1 atm is ~30 mM (NIST, 2023), and is within a few-fold of that value across common CO2 capture solvents (Li, 2017, Altway, 2016) (see *). CO2 constitutes ~10% and ~0.04% of flue gas and atmospheric air respectively, and CO2 reacts quickly with capture solvents, so [CO2]<<30 mM and [CO2]<<KM for PSC and DAC. The catalytic rate of CA is thus strongly influenced by its KM under practical conditions (see **). Lowering the KM of CA would therefore increase the utility of CA for PSC, and especially for DAC. 

Goals

CA should be engineered to lower its KM. CO2 absorption rate is expected to be proportional to 1/KM down to at least KM ~ 0.3 mM for PSC and KM ~ 0.001 mM for DAC. To be useful, the Vmax of such modified CA should not be reduced by a similar factor as the factor of KM reduction. The CA should have high stability and activity under desired process conditions, which may include high temperature, pH, and/or salinity. 

These goals would be further clarified by the following modeling analyses:

  • Description of the expected concentration of CO2 in capture solvents as a function of solvent composition and CO2 partial pressure. Those determine the KM value at which further KM reduction would yield diminishing returns.

  • Description of co-optimal process conditions and CA properties, which would specify engineering targets for CA catalytic activity and the physical conditions under which it should be active and stable.

Additional information 

Open Questions

  • What is the expected concentration of CO2 in capture solvents? That concentration determines the CA KM value at which further KM reduction would yield diminishing returns.

  • What are the optimal process conditions and CA stability and target activity under those conditions for economically competitive DAC or PSC using CA? Those parameters should be present in a CA engineered for reduced KM. (See problem statement here).

  • For experimental validation of whether the low KM  of CA is a limiting factor in CO2 absorption enhancement by CA, CO2 absorption rate or CA catalytic rate should be tested for multiple CAs with differing kcat and KM, to confirm that KM indeed impacts absorption rate.

Assumptions

  • This problem statement assumes that it will be possible to prevent biofouling of the industrial system by organisms that eat the enzyme.

  • This problem statement assumes that CA is acting in a system limited by CO2 hydration rate (which is true of current DAC and PSC using aqueous solvents) and that CA is positioned close enough to the gas-solvent interface that the rate of diffusion of CO2 to CA is not limited by diffusion through bulk solvent

Related problem statements

Other information 

*Henry’s constant for CO2 is the same order of magnitude across water (NIST, 2023), 30% K2CO3 (Li, 2017), MEA solutions (Altway, 2016), and solutions of K2CO3 and MEA (Li, 2017).

**Michaelis-Menten kinetics for enzymes say v = Vmax/(KM+[S]), where v = catalytic rate, Vmax= maximum catalytic rate,  [S] = substrate concentration, and KM = Michaelis constant. Note that v~Vmax when [S]>>KM and v~Vmax/KM when [S]<<KM

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