The consumption of conventional petroleum based plastics far exceeds its recycle, although the latter has significantly increased since the 90s. The durability of plastics was originally viewed as a virtue; however, this durability has created environmental problems, and led to the early research and development of the first biodegradable plastics. Pure aromatic polyesters like PET are traditionally considered as quite insensitive to hydrolytic biodegradation. Under drastic chemical conditions (e.g. sulphuric acid at 150°C) hydrolysis of such polyesters can be used for recycling purposes. Due to the requirement of these energy-consuming conditions for chemical recycling, the application of polyester hydrolases has been proposed as an environmentally friendly alternative for PET recycling . All polyester hydrolases from actinomycetes described so far are members of the α/β hydrolase fold superfamily of enzymes, similar to the well-known polyhydroxyalkanoate (PHA) depolymerases for the degradation of PHA. Enzymatic degradation of PHA can involve two different processes, carried out by different enzymes; Intracellular degradation of PHAs, is the process by which the PHA accumulating cell mobilizes its reserves of carbon and energy in order to grow when the environmental conditions are favourable after a period of unbalance. This is carried out by intracellular PHA depolymerases, hydrolytic α/β hydrolase bound to the surface of the PHA granule. Extracellular PHA degradation is a different process, by which a microorganism (not necessarily PHA producer) releases soluble PHA depolymerases to the extracellular medium in order to hydrolyze PHA, present in the environment as a result of the death and lysis of PHA producer bacteria, using the resulting monomers and oligomers as an alternative source of energy and carbon in absence of more suitable substrates.
Degradation of PHAs is an interesting process, not only with the purpose of relieving the excessive accumulation of plastic waste in the environment, but also because the possibility of using the enantiopure monomers (HAs) and oligomers, by-products of PHA hydrolysis, as chiral compounds with high added value. These reaction products can be employed for the synthesis of polymers with tailor-made physicochemical properties, and also as starting materials for the synthesis of a wide range of enantiopure pharmaceuticals.
These PHA degradation studies are also part of the European H2020 project Mix-Up. Details of this project can be seen under “Plastic value from plastic waste (EU-projects: P4SB and Mix-Up)” or directly on the official project website at www.mix-up.eu.