12
or poly(3HB-co-3HP). The increase in the 3-HP fraction was found to result in an
increase of thermal degradation, a reduction of the glass transition temperature,
and, at up to 60–70 mol % 3-HP, in a reduction of the melting point of the copolymer. Furthermore, the enzymatic degradation of 3-HP-containing copolymers has
been previously shown, particularly in poly(3HP-co-3HB), in reason of the activity of poly(3-hydroxyalkanoate) depolymerases PhaZ from Ralstonia pichettii and
Acidovorax sp. strain TP4. The metabolically controlled transformation of CO 2 by
cyanobacteria, in the presence of a lightening source, has been shown feasible.
3-HP is an intermediate of the carbon-fixing 3HP/4-hydroxybutyrate cycle
whereby 3-HP is derived from reduction of malonyl-CoA through an intermediate
malonate semialdehyde (MSA) in the malonyl-CoA-dependent pathway. This
pathway is interesting since the precursor acetyl-CoA originates directly from the
Calvin–Benson–Bassham cycle. Another potential pathway for 3-HP production is
through β-alanine which bypasses malonyl-CoA. Since the latter is significantly
regulated in the cells’ by-product feedback inhibition, engineering interventions
on the β-alanine pathway could be advantageous. Lan et al. (2015) expressed both
the malonyl-CoA-dependent pathway (malonyl-CoA reductase and malonate
semialdehyde dehydrogenase) and the β-alanine-dependent pathway (PEP carboxylase, aspartate transaminase, aspartate decarboxylase, and β-alanine transaminase) to connect PEP to malonate semialdehyde. The strains expressing the
malonyl-CoA reduction pathway were able to produce a titer of up to 659 mg/L
3-HP in 16 days. The 3-HP production generally increased in the strains expressing both the malonyl-CoA- dependent and β-alanine-dependent pathways, even
though the effect depends on the choice of the parental strain harboring only the
former pathway. Wang et al. (2016) modified Synechocystis sp. PCC 6803 to
ensure high expression level of the mcr gene encoding for the malonyl-CoA reductase, increased supplies of malonyl-CoA and NADPH, and inactivated competing
pathways to achieve 837.2 mg/L 3-HP in a 6-daylong cultivation Finally, cyanobacteria have been modified to introduce the glycerol biosynthetic pathway and
the enzymes catalyzing glycerol transformation into 3-HP (including the glycerol
dehydration to 3-hydroxypropionaldehyde (3-HPA) and the oxidation of 3-HPA to
3-HP). The recombinant strains in dark, anaerobic, and nitrate−/phosphate-limited
conditions produced 31.7 mg/L 3-HP (Wang et al. 2015). A few microorganisms
other than cyanobacteria have been metabolically engineered for acquiring the
ability to fix carbon dioxide into 3-HP up to now. A study (Hanko et al. 2017)
characterized a 3-HP-inducible system from Pseudomonas putida, consisting of a
transcriptional regulator and a corresponding 3-HP-responsive promoter, which
proved to be exploitable for orthogonal gene expression control in Cupriavidus
necator, a chemolithoautotrophic betaproteobacterium with the potential to
improve the production of 3-HP from carbon dioxide. Finally, Keller et al. (2013)
described a unique temperature-dependent approach that confers on the archaeon
Pyrococcus furiosus, which grows optimally on carbohydrates at 100 °C, the
capacity to incorporate carbon dioxide to produce 150 μM 3-HP using hydrogen
as electron donor after 2 h incubation 72 °C.
A. A. Azim et al.
or poly(3HB-co-3HP). The increase in the 3-HP fraction was found to result in an
increase of thermal degradation, a reduction of the glass transition temperature,
and, at up to 60–70 mol % 3-HP, in a reduction of the melting point of the copolymer. Furthermore, the enzymatic degradation of 3-HP-containing copolymers has
been previously shown, particularly in poly(3HP-co-3HB), in reason of the activity of poly(3-hydroxyalkanoate) depolymerases PhaZ from Ralstonia pichettii and
Acidovorax sp. strain TP4. The metabolically controlled transformation of CO 2 by
cyanobacteria, in the presence of a lightening source, has been shown feasible.
3-HP is an intermediate of the carbon-fixing 3HP/4-hydroxybutyrate cycle
whereby 3-HP is derived from reduction of malonyl-CoA through an intermediate
malonate semialdehyde (MSA) in the malonyl-CoA-dependent pathway. This
pathway is interesting since the precursor acetyl-CoA originates directly from the
Calvin–Benson–Bassham cycle. Another potential pathway for 3-HP production is
through β-alanine which bypasses malonyl-CoA. Since the latter is significantly
regulated in the cells’ by-product feedback inhibition, engineering interventions
on the β-alanine pathway could be advantageous. Lan et al. (2015) expressed both
the malonyl-CoA-dependent pathway (malonyl-CoA reductase and malonate
semialdehyde dehydrogenase) and the β-alanine-dependent pathway (PEP carboxylase, aspartate transaminase, aspartate decarboxylase, and β-alanine transaminase) to connect PEP to malonate semialdehyde. The strains expressing the
malonyl-CoA reduction pathway were able to produce a titer of up to 659 mg/L
3-HP in 16 days. The 3-HP production generally increased in the strains expressing both the malonyl-CoA- dependent and β-alanine-dependent pathways, even
though the effect depends on the choice of the parental strain harboring only the
former pathway. Wang et al. (2016) modified Synechocystis sp. PCC 6803 to
ensure high expression level of the mcr gene encoding for the malonyl-CoA reductase, increased supplies of malonyl-CoA and NADPH, and inactivated competing
pathways to achieve 837.2 mg/L 3-HP in a 6-daylong cultivation Finally, cyanobacteria have been modified to introduce the glycerol biosynthetic pathway and
the enzymes catalyzing glycerol transformation into 3-HP (including the glycerol
dehydration to 3-hydroxypropionaldehyde (3-HPA) and the oxidation of 3-HPA to
3-HP). The recombinant strains in dark, anaerobic, and nitrate−/phosphate-limited
conditions produced 31.7 mg/L 3-HP (Wang et al. 2015). A few microorganisms
other than cyanobacteria have been metabolically engineered for acquiring the
ability to fix carbon dioxide into 3-HP up to now. A study (Hanko et al. 2017)
characterized a 3-HP-inducible system from Pseudomonas putida, consisting of a
transcriptional regulator and a corresponding 3-HP-responsive promoter, which
proved to be exploitable for orthogonal gene expression control in Cupriavidus
necator, a chemolithoautotrophic betaproteobacterium with the potential to
improve the production of 3-HP from carbon dioxide. Finally, Keller et al. (2013)
described a unique temperature-dependent approach that confers on the archaeon
Pyrococcus furiosus, which grows optimally on carbohydrates at 100 °C, the
capacity to incorporate carbon dioxide to produce 150 μM 3-HP using hydrogen
as electron donor after 2 h incubation 72 °C.
A. A. Azim et al.