Polyhydroxyalkanoates(PHA) : Bio-based and biodegradable promising polymer

Polyhydroxyalkanoates(PHA) : Bio-based and biodegradable promising polymer

대학생신재생에너지기자단 21기 길민석

 

[Introduction]

The invention of plastic was a major milestone that had a significant impact on the human lifestyle. Because of their diverse applications, durability, cost-effectiveness and easy production, plastics have been an excellent substitute for various types of materials such as metal, wood and ceramics. However, their indiscriminate use and accumulation of conventional synthetic plastic resulted in the severe environmental pollution problem. The consistent accumulation of conventional synthetic plastic in the environment has compelled us to think that there is an urgent need to replace it with biodegradable plastics. Among the natural polymers, polyhydroxyalkanoate (PHA) is the most promising solution to this major ecological problem. Polyhydroxyalkanoates (PHAs)  comprise a family of biodegradable polyesters that are produced by an extensive variety of microorganisms for intracellular carbon and energy storage purposes.

[자료 1. Granules of PHB accumulated in the cell of the bacterium Azotobacter chroococcum (a) / Structure of the granule of PHB (b)]

출처 : Redalyc

 

[Structure, classification and properties of PHA]

[자료 2. The general structure of PHA]

출처 : NIH

PHA is a linear polyester which contains 3-hydroxy fatty acid monomers. Hydroxyl group from one monomer forms an ester bond with the carboxyl group of another monomer unit. The R group in the structure represents alkyl group. 

Based on the number of carbons chain length, polyhydroxyalkanoates are classified into three classes on the basis of chain length, short chain length (scl) PHA, medium chain length (mcl) PHA and long chain length (lcl) PHA. Short chain length (scl) PHA consists of three to five carbon atoms. Some bacteria which produce scl-PHA are Azohydromonas lata and Cupriavidus necator. Medium chain length (mcl) PHA consists of 6–14 carbon atoms in the chain. Pseudomonas mendocina and Pseudomonas putida are two examples for mcl-PHA accumulating strains. Long chain length (lcl) PHA consists of more than 14 carbon atoms and some bacteria known for producing lcl-PHA are Aureispira marina and Shewanella oneidensis.

PHAs are flexible, crystalline, elastic and have thermoplastic properties similar to synthetic plastics when extracted from the cell. The mechanical properties of PHA like the tensile strength (40MPa), Young’s modulus (3.5 GPA) are similar to synthetic plastics. PHAs are soluble in chloroforms and other chlorinated hydrocarbons but insoluble in water. They are non-toxic, which makes them biocompatible.

 

[Biosynthesis of PHA]

[자료 3. Three basic synthesis pathways of PHA]

출처 : SpringerLink

PHA synthase is the main enzyme responsible for PHA production. A set of genes consisting of PHA synthase (PhaC), β-ketothiolase (PhaA) and NADPH-acetoacetyl-CoA reductase(PhaB) are co-localized and organized into PHA biosynthetic operon. The biosynthetic pathway of PHB(Polyhydroxybutyrate), a scl-PHA starts when two molecules of acetyl-Co-A get condensed to form acetoacetyl-CoA with the help of the enzyme 3-ketothiolase (PhaA). The acetoacetyl-CoA is then reduced to (R)-3-hydroxybutyryl-CoA which is catalyzed by (R)-specific acetoacetyl-CoA reductase (PhaB). (R)-3-Hydroxybutyryl-CoA is finally polymerized to PHB by the enzyme PHA synthase (PhaC) 

On the other hand, for mcl-PHA production, the intermediates of fatty acid metabolism are converted to (R)-3-hydroxyacyl-CoA. If the fatty acid β oxidation pathway is not followed, acetyl CoA is generated by the oxidation of carbon source and the intermediates of fatty acid de novo pathway are directed towards PHA biosynthesis with the help of the enzyme transacylase (PhaG). The (R)-3-hydroxyacyl moiety of the respective ACP (acyl carrier protein) thioester is transferred to CoA by this enzyme. Whereas if the carbon source is oxidized by β-oxidation pathway, then enoyl-CoA is oxidized to (R)-3-hydroxyacyl-CoA by the enzyme enoyl-CoA hydratase (PhaJ). The PhaG and PhaJ encoding genes are co-regulated but not present within the PHA synthase operon.

 

[PHA application]

The versatile application of PHA is due to their biocompatibility. Several applications include: bone tissue engineering, drug delivery or as drug itself, packaging, biofuels. For food packaging materials, PHB and PHBV bioplastics have a greater barrier to aromatic chemicals, allowing the packed food to keep its smell and taste quality for longer.

[자료 4. CJ CheilJedang first product with PHA/PLA blend biodegradable packaging film]

출처 :  모닝경제

CJ CheilJedang introduced food packaging materials using PHA and PLA(Polylactic acid), which are biodegradable materials, for the first time in the world. They announced that it has applied transparent vinyl made from materials that decompose in nature to its "Happy Bean Tofu" bundle.

 

[Conclusion]

Industrial production of polyhydroxyalkanoate is not yet on full scale because of high production cost as opposed to the synthetic plastics. Understanding the biosynthetic pathways and further manipulating the pathways would be key to achieving low and competitive production cost, as well as product design of the desired polymer.

 

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생분해 플라스틱에 대한 대학생신재생에너지기자단 기사 더 알아보기

1. "착한 플라스틱? PLA가 뭐길래!", 20기 황지영, https://renewableenergyfollowers.tistory.com/3507

착한 플라스틱? PLA가 뭐길래!

2. "Bio-resource tech vol.2 <Bio-material; Bio-plastic>", 13기 문한태, https://renewableenergyfollowers.tistory.com/2578

Bio-resource tech vol. 2 <Bio-material; Bio-plastic>

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참고문헌

[Introduction]

1) Sehgal R, Gupta R, Polyhydroxyalkanoate and its efficient production: an eco-friendly approach towards development. 3 Biotech. 2020 Dec;10(12):549. 

2) Shraddha Chavan, Bhoomika Yadav, R.D. Tyagi, Patrick Drogui, A review on production of polyhydroxyalkanoate (PHA) biopolyesters by thermophilic microbes using waste feedstocks, Bioresource Technology, Volume 341, 2021.

3) Li, Z., Yang, J. & Loh, X. Polyhydroxyalkanoates: opening doors for a sustainable future. NPG Asia Mater 8, e265 (2016). 

4) Escuela de Ciencias y Humanidades y Escuela de Ingeniería de la Universidad EAFIT, From Obtaining to Degradation of PHB: Material Properties. Part I, Ingeniería y Ciencia, vol. 13, no. 26, 2017.

[Structure, classification and properties of PHA]

1) Sehgal R, Gupta R. Polyhydroxyalkanoate and its efficient production: an eco-friendly approach towards development, 3 Biotech, 2020 Dec;10(12):549. 

2) A. B. Akinmulewo, O. C. Nwinyi, Polyhydroxyalkanoate: a biodegradable polymer (a mini review), Journal of Physics: Conference Series, Volume 1378, Issue 4, 2019.

[Biosynthesis of PHA]

1) Guo-Qiang Chen, Xiao-Ran Jiang, Yingying Guo, Synthetic biology of microbes synthesizing polyhydroxyalkanoates (PHA), Synthetic and Systems Biotechnology, Volume 1, Issue 4, 2016.

2) Kniewel, R., Revelles Lopez, O., Prieto, M. Biogenesis of Medium-Chain-Length Polyhydroxyalkanoates. In: Geiger, O. (eds) Biogenesis of Fatty Acids, Lipids and Membranes. Handbook of Hydrocarbon and Lipid Microbiology, Springer, Cham, 2017.

[PHA application]

1) 정선경 기자, "CJ제일제당, ‘생분해 소재’ 식품 포장에 적용… “친환경 경영 확대”", 모닝경제, 2021.04.02, https://www.menews.co.kr/news/articleView.html?idxno=20478

2) A. B. Akinmulewo, O. C. Nwinyi, Polyhydroxyalkanoate: a biodegradable polymer (a mini review), Journal of Physics: Conference Series, Volume 1378, Issue 4, 2019.

[Conclusion]

1) A. B. Akinmulewo, O. C. Nwinyi, Polyhydroxyalkanoate: a biodegradable polymer (a mini review), Journal of Physics: Conference Series, Volume 1378, Issue 4, 2019.