3-hydroxypropionate cycle

Strauss G. and Fuchs G. (1993) Enzymes of a novel autotrophic CO2 fixation pathway in the phototrophic bacterium Chlorofiexus aurantiacus, the 3-hydroxypropionate cycle. Euro. J. Biochem. 215, 633-643. 

As already mentioned, cyanobacteria and most of the chemolithoautotrophic bacteria fix carbon dioxide through the Calvin-Benson cycle, but some litho-autotrophic bacteria fix carbon dioxide through other pathways. When the green phototrophic bacterium Chloroflexus aurantiacus grows lithoautotrophically, the bacterium fixes carbon dioxide through the 3-hydroxypropionate cycle (Ivanovsky... 

Ishii M, Chuakrut S, Aral H, Igarashi Y. (2004). Occurrence, biochemistry and possible biotechnological application of the 3-hydroxypropionate cycle. Appl Microbiol Biotechnol, 64, 605-610. 

Alber B, Olinger M, Rieder A, Kockelkom D, Jobst B, Hugler M, Fuchs G (2006) Malonyl-coenzyme A reductase in the modified 3-hydroxypropionate cycle for autotrophic carbon fixation in Archaeal Metallosphaera and Sulfolobus spp. J Bacterid 188 8551-8559... 

Menendez, C., Bauer, Z., Huber, H., Gad on, N., Stetter, K.O., and Fuchs, G. (1999) Presence of acetyl coenzyme A (CoA) carboxylase and propionyl-CoA carboxylase in autotrophic Crenar-chaeota and indication for operation of a 3-hydroxypropionate cycle in autotrophic carbon fixation. /. Bacte-riol, 181 (4), 1088-1098. 

G. (2007) Properties of R-citramalyl-coenzyme A lyase and its role in the autotrophic 3-hydroxypropionate cycle of Chloroflexus aurantiacus. /. Bacteriol,... 

Hydroxypropionate / malyl-CoA cycle 10 7 NAD(P)H, but 1 FAD is reduced in the cycle Acetyl-CoA/propionyl-CoA carboxylase HCOJ Acetyl-CoA, pyruvate, succinyl-CoA Malonyl-CoA reductase, propionyl-CoA synthase, malyl-CoA lyase... 

The pathway can be divided into two metabolic cycles (Figure 3.4). In the first cycle, acetyl-CoA is carboxylated to malonyl-CoA, which is subsequently reduced and converted into propionyl-CoA via 3-hydroxypropionate as a free intermediate. Propionyl-CoA is carboxylated to methylmalonyl-CoA, which is subsequently converted to succinyl-CoA the latter is then used to activate L-malate by succinyl-CoA L-malate coenzyme A transferase, which forms L-malyl-CoA and succinate. Succinate is oxidized to L-malate via conventional steps. L-Malyl-CoA, the second characteristic intermediate of this cycle, is cleaved by L-malyl-CoA/P-methylmalyl-CoA lyase, thus regenerating the starting molecule acetyl-CoA and releasing gly-oxylate as a first carbon-fixation product [27]. 

Figure 3.4 3-Hydroxypropionate/malyl-CoA cycle, as studied and proposed in Chloroflexus aurantiacus.

The energy costs of the 3-hydroxypropionate/malyl-CoA cycle are high, with ten ATP required per triose phosphate (see Table 3.1). However, bicarbonate rather than C02 is the actual inorganic carbon species used by acetyl-CoA/propionyl-CoA carboxylase (this is discussed in Chapter 4). Moreover, as this enzyme is virtually irreversible and has a high affinity for bicarbonate, this cycle is expensive although kinetically effective. 

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 3-hydroxypropionate/4-hydroxybutyrate cycle functions in autotrophic Sul-folobales (Crenarchaeota) [35-37]. These are extreme thermoacidophiles from volcanic areas which grow best at a pH of about 2 and temperatures of 60 to 90 °C. 

Figure 3.5 3-Hydroxypropionate/4-hydroxybutyrate cycle, as studied in Metaiiosphaera sedula.

The active C02 species in the 3-hydroxypropionate/4-hydroxybutyrate cycle is bicarbonate (see Table 3.1). The use of bicarbonate as a substrate may be advantageous for organisms using this cycle in comparison with, for example, the CBB... 

This cycle resembles the 3-hydroxypropionate/4-hydroxybutyrate cycle, but with pyruvate ferredoxin oxidoreductase (pyruvate synthase) and phosphoenolpyruvate (PEP) carboxylase as the carboxylating enzymes (Figure 3.6). 

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). 

The only cultivated AOA, Nitrospumilus maritimus, depends on CO2 as its only carbon source and the presence of even low levels of organic carbon were inhibitory to growth. The pathway of CO2 fixation is, however, unknown. Hyperthermophilic Crenarchaeota generally utilize a 3-hydroxypropionate pathway or a reductive TCA cycle for autotrophic carbon fixation. Another cultivated marine Crenarchaeota strain, Cenarchaeum symbiosum, a sponge symbiont, appears to use the 3-hydroxypropionate pathway. It cannot be concluded on this basis which pathway is used by the AOA, but it very likely that is not the Calvin cycle. N. maritimus had a minimal generation time of 21 h, longer but roughly on the same scale as AOB. 

But, both polymers, PHB and PLA, lead to conflicts in the context of assigning them as biodegradable polymers, because these are naturally occurring polymers, which like PHB were evolved in natural material cycles. However, in the case of PLA only the monomer, L-2-hydroxypropionic acid, (l-Lactic acid) can be found in the natural material cycle. On the other hand, both of them can be generated on a natural nonpetrochemical basis, with renewable resources, and they are degradable. This common denominator should justify the discussion of both polymers herein and makes a comparison meaningful. 

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