1. The Dehydrogenases
The three dehydrogenase enzymes involved in the CO
2 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 HCO
2H to CO
2 irreversibly; the latter catalyzes the reduction of CO
2 to HCO
2H reversibly and, because of this, is also employed in the enzymatic CO
2 to HCO
2H conversion.
[2] The redox potential for the enzymatic reduction of CO
2 to HCO
2− is E°ʹ = −420 mV, but both types of
Formate dehydrogenase (
FateDH) mainly catalyze HCO
2− 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
FateDHs, the one extracted from
Candida boidinii (EC 1.2.1.2,
CbFateDH) is commercially available and can be easily handled as a catalyst for CO
2 reduction.
CbFateDH is a homodimer [79 kDa (6 nm × 6 nm × 10 nm)] with two independent active sites catalyzing the NAD
+-dependent oxidation of formate to CO
2 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.
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 CO
2 according to pH in water solution.
[6]
There are conflicting opinions in the literature on the role of the CO
2 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
CbFateDH. Other authors, see Wang et al.,
[8] consider hydrogencarbonate to be the active species, and in some cases (Cazelles et al.
[5]), KHCO
3 is dissolved in water rather than bubbling CO
2 for better reproducibility. In order to increase the presence of CO
2 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 CO
2 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 (
kCat = 10
6 s
−1) catalyzes the hydration–dehydration of CO
2. (Equation (1))
According to Wang et al.,
[8] with the catalysis of
CA, CO
2 molecules are rapidly hydrated and transformed into hydrogencarbonate, which is converted into formate, while the cofactor NADH is oxidized. Another possibility to increase CO
2 availability is to use materials such as MOFs able to both immobilize enzymes and absorb CO
2.
Although
Formate dehydrogenase from
C. boidinii is the most widely used enzyme for testing the reduction reaction,
FateDHs are also derived from other microorganisms and have been tested for their ability to reduce CO
2. Nielsen et al.
[9] compared the catalytic efficiency of
Formate dehydrogenase of various organisms by highlighting the type of electron donor, the
Km for CO
2, and the working conditions. Amongst the various enzymes, the activity of
DdFateDH (
Desulfovibrio desulfuricans) is particularly interesting, with a
Km of 0.02 mM and a catalytic efficiency
Kcat/
Km of 2968.
[10]
Other
FateDHs such as, for example,
FateDH from
Thiobacillus sp. KNK65MA,
[11] FateDH from
Myceliophthora thermophila,
[12] FateDH from
Clostridium carboxidivorans,
[13] on the other hand, have a low
Km (0.95, 0.44, and 0.05 mM, respectively) but present a not very high catalytic efficiency (
Kcat/
Km = 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
FaldDH for the reduction reaction (HCO
2H → 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 (
FaldDH, 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.
It was shown that the formate reduction step in the enzymatic cascade from CO
2 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 CO
2 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 CO
2 to methanol. However,
ADH is not a very stable enzyme for industrial use. Indeed, when the three
dehydrogenases catalyzing the cascade reaction from CO
2 to CH
3OH 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.