# Microbial and Enzymatic Biodegradation of Plastic Waste for a Circular Economy
## 3 Factors Affecting Plastic Biodegradation
### Polymer Properties  
The hydrophobicity of polymers is strongly influenced by the presence or absence of functional groups. When non-polar groups dominate in the polymer structure, the polymer is more hydrophobic and more resistant to microbial attacks [154]. For example, PE and PP, which consist of long chains of non-polar carbon and hydrogen atoms, are highly hydrophobic and resistant to biodegradation [158]. Another important factor is the polymer's structural complexity. For instance, crystalline polymers have an ordered structure at the molecular level and are therefore more difficult to degrade, as it is difficult for enzymes to access suitable regions of the substrate to begin degradation. The degree of crystallinity is determined by the functional groups, polymer chain length, degree of branching, and tacticity of the monomers. In general, branched and interlinked polymers are resistant to degradation compared with the same polymers without interlinking [125]. Similarly, ester bonds break down more quickly than ether or amide bonds because they have a lower bond dissociation energy [159]. The sequence of bond breakability is as follows: ester > ether > amide > urethane [160]. Higher-density polymers are often more resilient to microbial attacks as they have tighter molecular packing, which inhibits the entry of enzymes or microorganisms [161]. Along the same reasoning, amorphous plastic is more prone to degrade than crystalline plastic [162]. These factors significantly vary between polymers, influencing not only their biodegradability but also their susceptibility to abiotic degradation processes such as photo- and thermal- degradation, which rely on the presence of reactive sites.  
Engineering novel enzymes or microbial strains for more specificity and activity towards plastics requires a multidisciplinary approach. For studying and designing protein thermostability, the ProthermaDB database is a valuable online resource [163]. The database comprises over 14 million protein sequences which can be viewed according to theirthermal stability and protein family. These experimentally derived data, when combined with advancing machine learning and artificial intelligence, can be beneficial to increase our understanding of protein stability towards creating new mutational and evolutional strategies to make enhanced enzymes. Based on known, experimentally measured changes in temperature stability (\(\Delta\)T) mutations, the database recommends mutations for protein families to improve their thermostability. However, the application of this knowledge to plastic degrading enzymes is still in the early stages.  
Furthermore, protein glycosylation produced in eukaryotic microbial cells can also improve enzyme thermostability [164] There are some examples of successful glycosylation for enhancing enzyme stability. For example, the lipase from _Geobacillus zalihae_ was expressed in a yeast _Pichia_ sp. strain, producing a lipase with improved thermostability [165]. These more stable enzymes could potentially be more useful to degrade plastic compared to their native forms.  
Enzyme-mediated plastic degradation can be accelerated by altering the enzyme's surface properties. However, in this approach, the enzymes must first be identified based on their catalytic properties and may require mutation to improve substrate specificity or stability under specific conditions, such as pH or temperature [166].