3-Hydroxypropionate/4-hydroxybutyrate pathway

For this reason (similar approach of conversion), it was assumed that both enzymes are identical [38]. In both cases a C2-unit is produced the acetyl-CoA in the 3-hydroxypropionate/malyl-CoA cycle and the glyoxylate in the 3-hydroxypropionate/4-hydroxybutyrate pathway. 

Acyl-CoA carboxylase pathways 3-hydroxypropionate/malonyl-CoA cycle [18, 19], 3-hydroxypropionate/4-hydroxybutyrate cycle [20], diearboxylate/ 4-hydroxybutyrate pathway [21], and the ethylmalonyl-CoA pathway [22]. 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

The dicarboxylate/4-hydroxybutyrate cycle starts from acetyl-CoA, which is reductively carboxylated to pyruvate. Pyruvate is converted to PEP and then car-boxylated to oxaloacetate. The latter is reduced to succinyl-CoA by the reactions of an incomplete reductive citric acid cycle. Succinyl-CoA is reduced to 4-hydroxybu-tyrate, the subsequent conversion of which into two acetyl-CoA molecules proceeds in the same way as in the 3-hydroxypropionate/4-hydroxybutyrate cycle. The cycle can be divided into part 1 transforming acetyl-CoA, one C02 and one bicarbonate to succinyl-CoA via pyruvate, PEP, and oxaloacetate, and part 2 converting succinyl-CoA via 4-hydroxybutyrate into two molecules of acetyl-CoA. This cycle was shown to function in Igrticoccus hospitalis, an anaerobic autotrophic hyperther-mophilic Archaeum (Desulfurococcales) [40]. Moreover, this pathway functions in Thermoproteus neutrophilus (Thermoproteales), where the reductive citric acid cycle was earlier assumed to operate, but was later disproved (W.H. Ramos-Vera et al., unpublished results). 

W., and Fuchs, G. (2007) A 3-hydroxypropionate/4-hydroxybutyrate autotrophic carbon dioxide assimilation pathway in Archaea. Science, 318, 1782-1786. 

Meng D-C, Shi Z-Y, Wu L-P, Zhou Q, Wu Q, Chen J-C, Chen G-Q. (2012). Production and characterization of poly(3-hydroxypropionate-co- 4-hydroxybutyrate) with fully controllable structures by recombinant Escherichia coli containing an engineered pathway. Metab Eng, 14,317-324. 

Deletion of the fabi gene can also be appUed in other malonyl-CoA based pathways, such as pathway 8, in order to increase the malonyl-CoA pool for 3-HP production. This malonyl-CoA mediated pathway was also used for the development of novel metabolic pathways for the production of P(3HB-co-3HP) from a structurally unrelated carbon source in Cupriavidus necator, in which malonyl-CoA reductase and the 3-HP-CoA synthetase domain of propionyl-CoA synthase from C. aurantiacus were employed [36]. Recombinant C. necator with malonyl-CoA mediated 3-HP synthetic activity could produce poly(3-hydroxybutyrate-co-3-hydroxypropionate) (P(3HB-co-3HP)), in which 3-HP monomer could be incorporated into copolymer up to 2.1 mol%, from fructose or even alkanoic acids [36]. 

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