Abstract
Nowadays a growing demand for green chemicals and cleantech solutions is motivating the industry to strive for biobased building blocks. We have identified the tertiary carbon atom-containing 2-hydroxyisobutyric acid (2-HIBA) as an interesting building block for polymer synthesis. Starting from this carboxylic acid, practically all compounds possessing the isobutane structure are accessible by simple chemical conversions, e. g. the commodity methacrylic acid as well as isobutylene glycol and oxide. During recent years, biotechnological routes to 2-HIBA acid have been proposed and significant progress in elucidating the underlying biochemistry has been made. Besides biohydrolysis and biooxidation, now a bioisomerization reaction can be employed, converting the common metabolite 3-hydroxybutyric acid to 2-HIBA by a novel cobalamin-dependent CoA-carbonyl mutase. The latter reaction has recently been discovered in the course of elucidating the degradation pathway of the groundwater pollutant methyl tert-butyl ether (MTBE) in the new bacterial species Aquincola tertiaricarbonis. This discovery opens the ground for developing a completely biotechnological process for producing 2-HIBA. The mutase enzyme has to be active in a suitable biological system producing 3-hydroxybutyryl-CoA, which is the precursor of the well-known bacterial bioplastic polyhydroxybutyrate (PHB). This connection to the PHB metabolism is a great advantage as its underlying biochemistry and physiology is well understood and can easily be adopted towards producing 2-HIBA. This review highlights the potential of these discoveries for a large-scale 2-HIBA biosynthesis from renewable carbon, replacing conventional chemistry as synthesis route and petrochemicals as carbon source.
Highlights
Building-block chemical 2-hydroxyisobutyric acid (2-HIBA) In the future, the feedstock for the chemical industry will not any longer be delivered from petroleum but will depend on renewable biomass
The methyl ester of methacrylic acid, which can be synthesized by dehydration of 2-HIBA or via the corresponding amide, is polymerized to polymethylmethacrylate (PMMA) for the production of acrylic glass, durable coatings and inks [4,5]
In the bacterial methyl tert-butyl ether (MTBE) degradation pathway, this enzyme catalyzes the reversible conversion of the branched 2-HIBA into the linear 3-hydroxybutyric acid, more precisely, the Coenzyme A (CoA)-activated thioesters are the substrates of the mutase
Summary
Building-block chemical 2-HIBA In the future, the feedstock for the chemical industry will not any longer be delivered from petroleum but will depend on renewable biomass. In the bacterial MTBE degradation pathway, this enzyme catalyzes the reversible conversion of the branched 2-HIBA into the linear 3-hydroxybutyric acid, more precisely, the Coenzyme A (CoA)-activated thioesters are the substrates of the mutase. The so-called 2-hydroxyisobutyryl-CoA mutase is a new representative of the CoA-carbonyl mutase family These enzymes catalyze the 1,2-rearrangement of the CoA-activated carboxyl group in the carbon skeleton of their substrates [40,41] (Figure 4). Having a central position in the MTBE degradation pathway makes the new 2-hydroxyisobutyryl-CoA mutase an important tool in developing for the first time a completely biotechnological process for producing 2-HIBA. In case of running a suitable biological system not along the MTBE degradation path, but in the opposite direction and feeding simple substrates which are metabolized via acetyl-CoA, 3-hydroxybutyryl-CoA can be produced by well-known enzymatic reaction sequences (Figure 3c). Considering the advanced stage of development of the latter biotechnology [48,49], the new route to 2-HIBA is
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