4HB monomer

The metabolic pathways involved in the synthesis of P(3HB-co-4HB) from 4-hydroxybutyric acid are shown in Fig. 2.4. Transferase or thiokinase catalyzes the conversion of 4-hydroxybutyric acid into 4HB-CoA, which is then used as the substrate by the PHA synthase in the polymerization reaction. The catabolism of 4-hydroxybutyric acid also leads to the formation of intermediates such as 3-hydroxybutyryl-CoA, resulting in the accumulation of P(3HB-co-4HB) copolymer. The main catabolic pathway for 4HB is probably via succinic acid semialdehyde and succinic acid pathways, which are catalyzed by 4HB dehydrogenase and succinic acid semialdehyde dehydrogenase (Valentin et al. 1995 Lutke-Eversloh and Steinbiichel 1999). All precursor substrates for the generation of 4HB monomers are first converted into 4HB-CoA, which is the immediate substrate for PHA synthase. 

Another precursor substrate that is also widely used for the generation of 4HB monomers is y-butyrolactone. The lactone is cleaved into 4-hydroxybutyric acid by the reaction of esterases or lactonases (Fig. 2.4). Subsequently, the resulting 4-hydroxybutyric acid is directly converted into 4HB-CoA either by a transferase or by a thiokinase. Again, 3HB-CoA may also be formed, thus resulting... 

HB is an interesting monomer in the family of PHA. This monomer contains similar number of carbon atoms as 3HB but does not possess an alkyl side group. The linear structure of 4HB monomer suggests that 4HB could not co-crystallize into the 3HB lattice. Mitomo and co-workers had suggested that 4HB lattice had to undergo deformation process in order to accommodate into the 3HB monomer lattice (Mitomo et al. 2001). The crystallinity of P(3HB-co-4HB) copolymer decreased from 60 to 15 % when the 4HB monomer composition was increased... 

Percentage of 4HB monomer (by NMR) Durometer hardness (shore A scale) Glass transition temperature, Tgi C)... 

Rennukka and AmiruP reported fabrication of P(3HB-co-4HB)/chitosan blends by adding chitosan powder (5, 10,15 and 20 wt%) to P(3HB-co-4HB) with various 4HB molar fractions (10,18 and 28 mol%). Since P(3HB-co-4HB) copolymer is hydrophobic in nature, it did not show any hydrophilicity regardless of various compositions of 4HB monomer. However, there was a significant improvement of the water adsorption capability when the chitosan content in the blends was increased from 5 to 20 wt%. Water uptake was influenced by the presence of the free hydrophilic groups in the blend films. The presence of chitosan induced the intra or/and inter-molecular network between the components in the blend films. Therefore, the hydrophilicity of the blend films increased. 

Besides the three main pathways mentioned above, there are several other metabolic pathways that can be manipulated to produce substrates for PHA biosynthesis. In recombinant E. coli, it has been shown that 4-hydroxybutyryl-CoA can be derived from the intermediates of the tricarboxylic acid (TCA) cycle [137]. By providing external precursor substrates such as 4-hydroxybutyric acid, 1,4-butanediol and y-butyrolactone to certain wild type [59, 60, 62, 63] and recombinant microorganisms [138], 4HB monomers can be incorporated more efficiently. 

The copolymer poly(3-hydroxybutyrate-co -hydroxybutyrate) (P(3HB-co-4HB)) has also been produced by R. eutropha and has potential medical applications. As with other copolymers, thermal properties of P(3HB-co-4HB) improve with increasing amounts of 4HB monomer (Ishida et al. 2001 Kasuya et aL 1996). There are many instances in the literature where P(3HB-co-4HB) is produced (Cavalheiro et al. 2012 Ishida et al. 2001 Kim et al. 2005 Volova et al. 2011), and in most cases, a 4HB precursor molecule (y-butyrolactone, 4-hydroxybutyrate, etc.) is typically added to the culture. Depending on the amount of precursor feeding, the 4HB fraction of PHA produced by R. eutropha can vary from 0 to 100 mol% (Cavalheiro et al. 2012 Ishida et al. 2001 Kim et al. 2005 Volova et al. 2011). 

Figure 3 also shows the pathway of 4-hydroxybutyrate synthesis from 4-hydroxybutyric acid (Pathway V) [81, 143]. 4-Hydroxybutyryl-CoA is first formed, and part of it is polymerized by PHA synthase. A portion of the hydroxyacyl-CoA is however dehydrated to the corresponding enoyl-CoA, which enters the pathway of 3HB synthesis via R-3-hydroxybutyryl-CoA. According to Valentin et al. [136] it is more likely that formation of 3-hydroxybutyryl-CoA occurs via succinate semialdehyde, succinate, pyruvate, and acetyl-CoA from 4-hydroxybutyrate. A copolymer of 3HB and 4HB is thus usually produced from 4HB acid. Presence of butyric acid in the medium somewhat inhibits the conversion of 4-hydroxybutyryl-CoA into R-3-hydroxybutyryl-CoA. The apparently complete inhibition of 3HB synthesis from 4HB acid by citrate and ammonium sulfate observed by Nakamura et al. [92] has to date not been explained. Substrates which can be converted to 4-hydroxybutyric acid, such as y-butyrolactone, 1, 4-butanediol, 1, 6-hexanediol, and 4-chlorobutyric acid, also lead to the formation of 4HB monomers [143]. 

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