Format Dehydrogenase

Format Dehydrogenase: An Alternative Solution for Formic Acid Synthesis from Carbon Dioxide Since the Industrial Revolution, waste gases containing substantial quantities of carbon dioxide (CO2) have been released into the atmosphere by industries, causing environmental problems such as air pollution and climate change.[1] For this reason, producing useful chemicals from CO2 has become a popular research topic in recent years. Among chemicals obtained from CO2, formate/formic acid (HCOO-/HCOOH) has attracted attention due to its widespread use in various industries.[2] Additionally, formate has recently gained importance as an H2 carrier. Considering that H2 has a greater gravimetric energy density (approximately 33 kWh/kg) than other transport fuels (such as gasoline and diesel), but has very low volumetric energy density, formate receives special attention as an organic liquid H2 carrier and storage medium.[3] Various methods, including electrochemical approaches, have been attempted to convert CO2 to formate. However, the high energy and high temperature requirements of these methods emerge as a significant disadvantage.[4] Enzymes are highly specific biocatalysts, mostly protein-structured, that enable complex reactions in biological systems to occur under mild conditions without harming living organisms. The ability of enzymes to demonstrate their effects when adequate in vitro conditions are provided, their high selectivity toward their substrates, their capacity to show high activity under mild reaction conditions, and their high turnover numbers create opportunities to utilize enzymes in many fields beyond their natural environments.[5] Oxidoreductases, particularly dehydrogenases, are a remarkable group of enzymes that catalyze various biological oxidation-reduction reactions.[6] Formate dehydrogenases (FDH, EC 1.2.1.2) catalyze the reduction of NAD(P)+ to NAD(P)H simultaneously with the oxidation of formate to CO2. Although CO2 formation itself is not of industrial interest, this activity of FDHs is important when used together with other oxidoreductases to regenerate the expensive reduced cofactor NAD(P)H.[7] Furthermore, when necessary conditions are provided, FDHs also possess the potential to reduce CO2 to formic acid, an important raw material for fuel and chemical production.[8] [caption id="attachment_134972" align="aligncenter"] Figure 1. Formic acid formation reaction from CO2 catalyzed by FDH[/caption] This situation presents FDHs as an alternative solution to reducing atmospheric CO2 and the greenhouse effect.[9] FDHs have been reported to be found in bacteria, archaea, yeasts, fungi, plants, and vertebrates.[10] Generally, it is possible to divide FDHs into two groups: metal-dependent and metal-independent.[11] Metal-dependent FDH enzymes are structurally very diverse. They are characterized by their complex quaternary structures, high molecular weights, and the presence of various prosthetic groups (selenocysteine, transition metal ions such as molybdenum or tungsten, and iron-sulfur clusters, among others). Metal-dependent FDHs are further divided into two groups: FDHs containing tungsten (W) or molybdenum (Mo). Despite their effectiveness in reducing CO2, it appears that most of these FDHs cannot be utilized in CO2 conversion systems because their active centers have complex structures and they are not stable against molecular oxygen (O2).[12,13] Recently, as an alternative to metal-dependent FDHs, FDHs that do not require a metal ion for their activities in CO2 conversion have been used. These types of FDHs are common in bacteria, yeasts, and fungi. Another characteristic of these FDHs is that their active centers do not have complex structures and they are stable against molecular oxygen (O2).[14,15] However, the lack of repeated or continuous use of soluble FDHs in reactor systems does not make the obtaining of formate/formic acid from CO2 economical. For this reason, ensuring the repeated or continuous use of soluble FDHs in reactor systems is an important necessity.[8] Enzyme immobilization methods are a widely used solution to enable the repeated or continuous use of soluble enzymes in reactor systems and can be defined, by definition, as binding the enzyme to an organic or inorganic support or wrapping it with a membrane to enable the repeated and continuous use of a dissolved enzyme and increase its stability.[5] To date, FDHs have been immobilized on various support materials using many different enzyme immobilization methods and have been used for formic acid synthesis from CO2. However, in these studies, formic acid synthesis has been performed on a small scale.[8,16] [caption id="attachment_134973" align="aligncenter"] Figure 2. FDH immobilization on various supports using different methods for obtaining formic acid from CO2.[/caption] In conclusion, immobilized FDHs can be an alternative to chemical methods for formic acid synthesis from CO2 under mild conditions, but they need to be developed for industrial-scale formic acid synthesis.

References

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Prof. Dr. Deniz Yıldırım Çukurova University Ceyhan Engineering Faculty Department of Chemical Engineering