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Fulltext A review on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] and genetic modifications that affect its production

Tang HJ, Neoh SZ, Sudesh K

Front Bioeng Biotechnol, 2022;10:1057067.
PMID: 36545679 DOI: 10.3389/fbioe.2022.1057067

Polyhydroxyalkanoates (PHAs) have garnered global attention to replace petroleum-based plastics in certain applications due to their biodegradability and sustainability. Among the different types of PHAs, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] copolymer has similar properties to commodity plastics, making them a suitable candidate to replace certain types of single-use plastics, medical devices, and packaging materials. The degradation rate of P(3HB-co-3HHx) is faster than the commercial petroleum-based plastics which take a very long time to be degraded, causing harmful pollution to both land and marine ecosystem. The biodegradability of the P(3HB-co-3HHx) is also dependent on its 3HHx molar composition which in turn influences the crystallinity of the material. Various metabolic pathways like the common PHA biosynthesis pathway, which involves phaA, phaB, and phaC, β-oxidation, and fatty acids de novo synthesis are used by bacteria to produce PHA from different carbon sources like fatty acids and sugars, respectively. There are various factors affecting the 3HHx molar composition of P(3HB-co-3HHx), like PhaCs, the engineering of PhaCs, and the metabolic engineering of strains. It is crucial to control the 3HHx molar composition in the P(3HB-co-3HHx) as it will affect its properties and applications in different fields.
Complete genome sequence of Aquitalea pelogenes USM4 (JCM19919), a polyhydroxyalkanoate producer

Wan JH, Ng LM, Neoh SZ, Kajitani R, Itoh T, Kajiwara S, et al.

Arch Microbiol, 2023 Jan 16;205(2):66.
PMID: 36645481 DOI: 10.1007/s00203-023-03406-1

Polyhydroxyalkanoate (PHA) is a type of biopolymer produced by most bacteria and archaea, resembling thermoplastic with biodegradability and biocompatibility features. Here, we report the complete genome of a PHA producer, Aquitalea sp. USM4, isolated from Perak, Malaysia. This bacterium possessed a 4.2 Mb circular chromosome and a 54,370 bp plasmid. A total of 4067 predicted protein-coding sequences, 87 tRNA genes, and 25 rRNA operons were identified using PGAP. Based on ANI and dDDH analysis, the Aquitalea sp. USM4 is highly similar to Aquitalea pelogenes. We also identified genes, including acetyl-CoA (phaA), acetoacetyl-CoA (phaB), PHA synthase (phaC), enoyl-CoA hydratase (phaJ), and phasin (phaP), which play an important role in PHA production in Aquitalea sp. USM4. The heterologous expression of phaC1 from Aquitalea sp. USM4 in Cupriavidus necator PHB-4 was able to incorporate six different types of PHA monomers, which are 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), 4-hydroxybutyrate (4HB), 5-hydroxyvalerate (5HV), 3-hydroxyhexanoate (3HHx) and isocaproic acid (3H4MV) with suitable precursor substrates. This is the first complete genome sequence of the genus Aquitalea among the 22 genome sequences from 4 Aquitalea species listed in the GOLD database, which provides an insight into its genome evolution and molecular machinery responsible for PHA biosynthesis.
Fulltext Statistical optimization of P(3HB-co-3HHx) copolymers production by Cupriavidus necator PHB-4/pBBR_CnPro-phaCRp and its properties characterization

Trakunjae C, Boondaeng A, Apiwatanapiwat W, Janchai P, Neoh SZ, Sudesh K, et al.

Sci Rep, 2023 Jun 02;13(1):9005.
PMID: 37268758 DOI: 10.1038/s41598-023-36180-7

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] is a bacterial copolymer in the polyhydroxyalkanoates (PHAs) family, a next-generation bioplastic. Our research team recently engineered a newly P(3HB-co-3HHx)-producing bacterial strain, Cupriavidus necator PHB-4/pBBR_CnPro-phaCRp. This strain can produce P(3HB-co-2 mol% 3HHx) using crude palm kernel oil (CPKO) as a sole carbon substrate. However, the improvement of P(3HB-co-3HHx) copolymer production by this strain has not been studied so far. Thus, this study aims to enhance the production of P(3HB-co-3HHx) copolymers containing higher 3HHx monomer compositions using response surface methodology (RSM). Three significant factors for P(3HB-co-3HHx) copolymers production, i.e., CPKO concentration, sodium hexanoate concentration, and cultivation time, were studied in the flask scale. As a result, a maximum of 3.6 ± 0.4 g/L of P(3HB-co-3HHx) with 4 mol% 3HHx compositions was obtained using the RSM optimized condition. Likewise, the higher 3HHx monomer composition (5 mol%) was obtained when scaling up the fermentation in a 10L-stirrer bioreactor. Furthermore, the produced polymer's properties were similar to marketable P(3HB-co-3HHx), making this polymer suitable for a wide range of applications.
N-terminal truncation of PhaCBP-M-CPF4 and its effect on PHA production

Neoh SZ, Tan HT, Trakunjae C, Chek MF, Vaithanomsat P, Hakoshima T, et al.

Microb Cell Fact, 2024 Feb 15;23(1):52.
PMID: 38360657 DOI: 10.1186/s12934-024-02329-w

BACKGROUND: Among the polyhydroxyalkanoate (PHA), poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] [P(3HB-co-3HHx)] is reported to closely resemble polypropylene and low-density polyethylene. Studies have shown that PHA synthase (PhaC) from mangrove soil (PhaCBP-M-CPF4) is an efficient PhaC for P(3HB-co-3HHx) production and N-termini of PhaCs influence its substrate specificity, dimerization, granule morphology, and molecular weights of PHA produced. This study aims to further improve PhaCBP-M-CPF4 through N-terminal truncation.
RESULTS: The N-terminal truncated mutants of PhaCBP-M-CPF4 were constructed based on the information of the predicted secondary and tertiary structures using PSIPRED server and AlphaFold2 program, respectively. The N-terminal truncated PhaCBP-M-CPF4 mutants were evaluated in C. necator mutant PHB-4 based on the cell dry weight, PHA content, 3HHx molar composition, molecular weights, and granule morphology of the PHA granules. The results showed that most transformants harbouring the N-terminal truncated PhaCBP-M-CPF4 showed a reduction in PHA content and cell dry weight except for PhaCBP-M-CPF4 G8. PhaCBP-M-CPF4 G8 and A27 showed an improved weight-average molecular weight (Mw) of PHA produced due to lower expression of the truncated PhaCBP-M-CPF4. Transformants harbouring PhaCBP-M-CPF4 G8, A27, and T74 showed a reduction in the number of granules. PhaCBP-M-CPF4 G8 produced higher Mw PHA in mostly single larger PHA granules with comparable production as the full-length PhaCBP-M-CPF4.
CONCLUSION: This research showed that N-terminal truncation had effects on PHA accumulation, substrate specificity, Mw, and granule morphology. This study also showed that N-terminal truncation of the amino acids that did not adopt any secondary structure can be an alternative to improve PhaCs for the production of PHA with higher Mw in mostly single larger granules.