Bio-degradation of Polyhydroxyalkanoates

The naturally synthesized polymers (biopolymers) have limited stability and sustainability, which is a major contrast from the properties of the synthetic polymers (polypropylene). The biodegradable property of PHAs helps in the maintenance of ecological balance. Bacterial biopolymers are synthesized when the bacteria anticipate the ‘stressful’ conditions indicating the exhaustion of growth media. The accumulated biopolymers are then metabolized (degraded) to maintain cellular activities when such starvation conditions are eventually encountered. The carbon reserve stored in the form of PHAs is degraded for energy production after the absolute depletion of growth media. 

Under starvation conditions, the phbZ (phb depolymerase) enzyme initiates polymer degradation in bacteria by de-polymerizing the accumulated PHB biopolymer. Enzymatic degradation of PHB results in the formation of oligomers, which subsequently break down into 3-HB (hydroxyl butyrate) monomers. PHA depolymerase enzyme degrades PHAs into different fractions, comprising of 3-HB, 3-HV, and 3-HHx, depending on the primary polymer composition.

PHAs are organic molecules that can be degraded completely into carbon dioxide and water with no residual solid waste. Therefore, PHAs are considered to be completely environmentally safe. Alteration in the physicochemical properties of the biopolymers also makes them susceptible to biodegradation; however, this property often causes the loss of structural and functional integrity of the polymer. 

Biological life cycle of PHAs

The degradation process can be triggered by several environmental factors (such as temperature, light, humidity), chemical, mechanical, or biological factors. The rate and extent of biodegradation depend on the intrinsic properties of the biopolymer, such as the surface area, length, and polymer composition. The degradable property of PHAs has made them suitable candidates for various applications; however, this property also hinders the extent of their applications. Any competitor of petroleum-derived plastics has to be durable, elastic, thermo-stable, chemical-resistant, moisture-resistant, and resistant to microbial attack.

Various organisms produce extracellular PHA degrading enzymes, which can degrade PHA aggregates in the environment under aerobic and anaerobic conditions. Carbon dioxide and water molecules are formed after complete aerobic degradation, while methane or other methyl intermediates are generated as a result of anaerobic degradation. Several PHA degrading strains such as BacillusPseudomonas, and Streptomyces absorb the decomposed products for growth (oligomers and monomers) and nourishment. 

Moreover, extracellular PHB depolymerase has been isolated from A. faecalis T1, which has been reported to degrade PHB films under different aqueous solutions at a temperature of 37º C and pH of 7.4. However, the rate of biodegradation of bioplastics is variable, especially due to the difference in the laboratory conditions from that of the natural compost environment, which provides a niche to different consortia of microbes. The PHA depolymerases (hydrolyzing) enzymes have also been isolated from several fungi such as Aspergillus. PHA chains present in the environment are generally partially hydrolyzed (degraded) and, therefore, they are soluble in water, unlike the intracellular PHA granules, which are insoluble in water. Thus, the intracellular and extracellular PHA depolymerases have different substrate (structural) specificity. Moreover, the crystalline structure of PHAs is resistant to microbial hydrolyzing enzymes. 

The life assessment cycle (LCA) is calculated to identify the ecological significance of substituting fossil fuel-based plastics with biopolymers. LCA accounts for the negative and positive consequential impact on several key environmental issues, including eco-toxicity, acidification, eutrophication, climate change, ozone depletion potential, and energy utilization arising due to replacement of the petroleum-derived plastics with biopolymers. 

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