Fossil fuel use remains prevalent for the production of chemicals, but with increasing environmental concern and limited availability, alternative sustainable methods are being developed. Starch-derived feedstocks like glucose have been used to produce chemicals such as lactic acid, but this method requires expensive downstream processing and puts the food industry in competition with the chemical industry. Non-edible lignocellulosic feedstock has a more complex composition and requires expensive pretreatment. With abundant quantities, methane and carbon dioxide can serve as an alternative feedstock. Dehydrodimerization of methane affords acetylene, a high energy gas that is difficult to transport. Subsequent carboxylation provides acetylenecarboxylic acid (ACA), a stable easy-to-handle liquid. The RuCl3-catalyzed 1,3-hydration of ACA results in pyruvic acid, while the 1,4-hydration to malonic semialdehyde is catalyzed by tautomerase enzyme Cg10062(E114N). Subsequent stereoselective enzymatic reduction of pyruvic acid using L- and D-lactate dehydrogenase (LDH) results in L- and D-lactic acids respectively. Hydrogen gas is utilized to enzymatically drive the recycling of NAD+ as cofactor to NADH using O2-tolerant and soluble NAD+-reducing hydrogenase (SH) from Ralstonia eutropha, allowing the possibility of using H2 formed during the dehydrodimerization of methane. The subsequent reduction of malonic semialdehyde to 3-HP can be achieved using NADH-dependent 3-hydroxyisobutyrate dehydrogenase (MmsB) from Pseudomonas putida. This system is also coupled with soluble hydrogenase for cofactor recycling. A separate system is compared using phosphite dehydrogenase (PTDH) to recycle cofactor.
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