Biosynthesis of 2-hydroxyisobutyric acid (2-HIBA) from renewable carbon

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.

Figure 1

Various substances that can easily be derived from 2-HIBA by standard chemical conversions.

Figure 2

Pathway of aerobic MTBE degradation. In bacteria, MTBE and also other ethers used as fuel oxygenates are initially attacked by various monooxygenases [20]. The resulting tert-butoxy methanol may spontaneously dismutate to tert-butanol and formaldehyde or is further oxidized to tert-butyl formate. The latter ester can be hydrolyzed to tert-butanol and formic acid. A second monooxygenase catalyzes the hydroxylation of tert-butanol to 2-methyl-1,2-propanediol which is oxidized via the corresponding aldehyde to 2-HIBA. The latter is then isomerized to the common metabolite 3-hydroxybutyryl-CoA [39]. For further details see also Figure 3b and c.

Figure 3

Proposed biotechnological routes to 2-HIBA. (a) Biohydrolysis of acetone cyanohydrin, (b) biooxidation of tert-butanol and (c) bioisomerization of 3-hydroxybutyryl-CoA that is synthesized from acetyl-CoA. The known biosynthesis sequence of the linamarin pathway leading from valine to acetone cyanohydrin is also indicated.

Figure 4

Carbon skeleton rearrangement catalyzed by cobalamin-dependent CoA-carbonyl mutases. Conversions for carboxylic acids having residues R1 to R4 out of H, CH₃, OH and COOH have already been described.

Figure 5

Connection of PHB biosynthesis steps to the proposed whole-cell production of 2-HIBA employing the bioisomerization route of aerobic MTBE degradation. In the absence of PHB polymerase activity, a biological system having the ability for overflow metabolism will accumulate 3-hydroxybutyryl-CoA from any suitable carbon source. This common metabolite will be converted to 2-hydroxyisobutyryl-CoA which is hydrolyzed and excreted.