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The three dehydrogenase enzymes involved in the CO₂ to methanol conversion are: Formate dehydrogenase, Formaldehyde dehydrogenase and Alcohol dehydrogenase.

CO2 reduction
cofactor regeneration
enzyme immobilization

1. The Dehydrogenases

The three dehydrogenase enzymes involved in the CO₂ to methanol conversion are: Formate dehydrogenase, Formaldehyde dehydrogenase and Alcohol dehydrogenase. Dehydrogenases are enzymes that catalyze the “proton coupled to electron transfer” (PCET, H⁺ + e⁻) from a molecule that acts as “electron + proton” donor (reductant) to another one that acts as an acceptor (oxidant). During the redox reaction, NADPH/NADP⁺ or NADH/NAD⁺ (nicotinamide adenine dinucleotide) is employed as an essential cofactor. The three enzymes above manage the reduction in the oxidation state of carbon dioxide to carbon-based energy-carrier resources. ^[1]

2. Formate Dehydrogenase

Formate dehydrogenases are categorized as metal-independent and metal-dependent based on the presence of metals (molybdenum-Mo or tungsten-W) in the active sites. The former catalyzes the reaction from HCO₂H to CO₂ irreversibly; the latter catalyzes the reduction of CO₂ to HCO₂H reversibly and, because of this, is also employed in the enzymatic CO₂ to HCO₂H conversion. ^[2] The redox potential for the enzymatic reduction of CO₂ to HCO₂⁻ is E°ʹ = −420 mV, but both types of Formate dehydrogenase (F_ateDH) mainly catalyze HCO₂⁻ oxidation, hence their dehydrogenase designation. The difference in catalytic reaction reversibility is due to differences in the catalytic reaction mechanism and the enzyme structure governing the energy reorganization during catalysis. ^[3] Among these F_ateDHs, the one extracted from Candida boidinii (EC 1.2.1.2, CbF_ateDH) is commercially available and can be easily handled as a catalyst for CO₂ reduction. CbF_ateDH is a homodimer [79 kDa (6 nm × 6 nm × 10 nm)] with two independent active sites catalyzing the NAD⁺-dependent oxidation of formate to CO₂ via an irreversible hydride transfer from formate to NAD⁺. ^[4]

The conversion of CO₂ to formate is a process thermodynamically unfavorable and which somehow needs to be “helped” to happen. Table 1 shows the Km values with reference to the substrates of the reaction in the two possible directions. The Km is the index of the affinity between enzyme and substrate: the lower its value, the higher the affinity of the substrate for the enzyme. As reported in Table 1, such value is lower for CO₂ than for HCO₂⁻, confirming that the formate oxidation reaction is favored. Nevertheless, the formate reduction reaction can be “forced” to take place by working with an excess of substrate and cofactor, i.e., with high amounts of NADH and CO₂.

Table 1. Km values for the CO₂ reduction–formate oxidation reactions.

Enzyme	Reaction	Km	Ref.
F_ateDH	CO₂ → HCO₂⁻	30–50 mM	^[5]
	HCO₂⁻ → CO₂	0.5 mM	^[5]

While for NADH, the excess is reached by adding the cofactor in the solution, for CO₂, it is not so simple due to its low solubility and its interaction with water. Moreover, one must consider that depending on the pH of the solution, CO₂ in water is found in three forms: carbon dioxide, hydrogencarbonate, and carbonate, as shown in Figure 1.

Figure 1. Distribution of species derived from CO₂ according to pH in water solution. ^[6]

There are conflicting opinions in the literature on the role of the CO₂ hydration reaction. Sato et al. ^[7] affirmed that as carbonate and hydrogencarbonate concentrations increase in solution, there is a suppression of the formate formation reaction due to the fact that such species are competitive inhibitors of carbon dioxide for the formate production with CbF_ateDH. Other authors, see Wang et al.,^[8] consider hydrogencarbonate to be the active species, and in some cases (Cazelles et al. ^[5]), KHCO₃ is dissolved in water rather than bubbling CO₂ for better reproducibility. In order to increase the presence of CO₂ in the solution, this is left to bubble for at least half an hour before starting the reaction, and in some cases, a pressurized system is also used. However, the low solubility of CO₂ remains one of the reasons for the low yields of such a reaction. In order to improve the yield, a fourth enzyme is added to the system, i.e., Carbonic anhydrase, which very quickly (k_Cat = 10⁶ s⁻¹) catalyzes the hydration–dehydration of CO₂. (Equation (1))

C O 2 + H 2 O ⇋ H + + H C O 3 - (1)

According to Wang et al., ^[8] with the catalysis of CA, CO₂ molecules are rapidly hydrated and transformed into hydrogencarbonate, which is converted into formate, while the cofactor NADH is oxidized. Another possibility to increase CO₂ availability is to use materials such as MOFs able to both immobilize enzymes and absorb CO₂.

Although Formate dehydrogenase from C. boidinii is the most widely used enzyme for testing the reduction reaction, F_ateDHs are also derived from other microorganisms and have been tested for their ability to reduce CO₂. Nielsen et al.^[9] compared the catalytic efficiency of Formate dehydrogenase of various organisms by highlighting the type of electron donor, the Km for CO₂, and the working conditions. Amongst the various enzymes, the activity of DdF_ateDH (Desulfovibrio desulfuricans) is particularly interesting, with a K_m of 0.02 mM and a catalytic efficiency K_cat/K_m of 2968. ^[10]

Other F_ateDHs such as, for example, F_ateDH from Thiobacillus sp. KNK65MA,^[11] F_ateDH from Myceliophthora thermophila,^[12] F_ateDH from Clostridium carboxidivorans,^[13] on the other hand, have a low K_m (0.95, 0.44, and 0.05 mM, respectively) but present a not very high catalytic efficiency (K_cat/K_m = 0.34, 0.23, 1.6, respectively).

3. Formaldehyde Dehydrogenase

The second step of the reaction is the reduction of formate to formaldehyde. The enzyme involved in such a reaction is Formaldehyde dehydrogenase. Again, as observed in the first step, the enzyme has more affinity for formaldehyde than formate, and this affects the reaction yield (Table 2). The kinetic parameters of F_aldDH for the reduction reaction (HCO₂H → HCHO) have not been determined yet, mainly due to the difficulty of measuring the reaction rates at different formic acid concentrations while keeping the pH constant. ^[14] Formaldehyde dehydrogenase (F_aldDH, EC 1.2.1.46) used in the reduction of formic acid is extracted from Pseudomonas putida and is a homo-tetramer of approximately 168 kDa [4 × 42 kDa (5 nm × 6 nm × 10 nm)]. ^[5]

Table 2. Km values for the formate reduction–formaldehyde oxidation reactions.

Enzyme	Reaction	Km	Ref.
F_aldDH	HCO₂⁻ → HCOH	n.d	^[5]
	HCOH → HCO₂⁻	0.09 mM	^[5]

It was shown that the formate reduction step in the enzymatic cascade from CO₂ to methanol is the bottleneck of the reaction, as this enzyme has low activity and is sensitive to pH, substrate, and product concentration. ^[5]^,^[14]^,^[15] Luo et al. ^[16] reported that if a minimum concentration of 10 mM of formate is not reached, the reaction does not proceed, and higher concentrations at the same time do not improve the speed of the reaction. Therefore, they defined 10 mM as the optimum concentration for the reduction to occur. Such information is obtained by a step-by-step study of the cascade reaction. Given this, it is possible to state that in the cascade reaction, the slow accumulation of formate affects the low yield of the second reduction step from formate to formaldehyde. Starting from formate instead of CO₂ would, thus, help the process.

4. Alcohol Dehydrogenase

The third enzyme involved in this reaction is Alcohol dehydrogenase. Alcohol dehydrogenase is present in many organisms, but the mainly used is the one from Saccharomyces cerevisiae (Alcohol dehydrogenase (ADH), EC 1.1.1.1, a homo tetramer of about 141–151 kDa with a size of 7 nm × 10 nm × 11 nm), which is commercially available. ADH normally acts on primary or secondary alcohols, but the enzyme oxidizes methanol much more poorly than ethanol. In the cascade reaction, it is used in the final step of the multienzyme process, i.e., the reversible reduction of formaldehyde to methanol. ^[17] The forward reaction (formaldehyde → methanol) is much more efficient than the reverse one (methanol → formaldehyde) given the affinity of the substrates for the enzyme (see the km value in Table 3); the reduction in formaldehyde is considered almost irreversible. This step is the only one favored in the direction of the cascade reaction from CO₂ to methanol. However, ADH is not a very stable enzyme for industrial use. Indeed, when the three dehydrogenases catalyzing the cascade reaction from CO₂ to CH₃OH were employed as free enzymes, ADH was by far the least stable one. ^[18]

Table 3. Km values for the formaldehyde reduction–methanol oxidation reactions.

Enzyme	Reaction	Km	Ref.
ADH	HCOH → CH₃OH	6 mM	^[5]
	CH₃OH → HCOH	100 mM	^[5]

This entry is adapted from the peer-reviewed paper 10.3390/molecules27154913

References

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