Polyhydroxyalkanoates (PHAs) are synthesized by a wide variety of bacteria as storage and reserve materials. PHAs are linked by ester bonds and today >150 different types of monomers can be combined resulting in a wide range of different types of bioplastics (Verlinden, 2007, Jendrossek, 2013). PHAs are synthesized from 3-hydroxy fatty acid monomers. During the synthesis the carboxyl group of one monomer forms an ester bond with the hydroxyl group of the next monomer. While these polymers are inpriciple based on natural resources they can be built from fossil fuels; and in fact more than half of the globally proiduced poylmers are not based on renewable resources.

Since PHAs are 'natural polymers', degradation is possible. However, some PHA variants are not biodegradable. The biodegradation of PHA has been observed in many different bacteria and fungi from marine and terrestrial sites (Jendrossek, 2002, Suzuki, 2021, Viljakianen, 2021) . The enzymes involved are either secreted extracellular PHA depolymerases (e-PHA depolymerases) or internal PHA (i-PHA) depolymerases. The known depolymerases are carboxyesterases (EC 3.1.1.75 / EC 3.1.1.76) and the best characterized enzymes are derived from Cuprivadus nector (Ralstonia eutropha) and Paucimonas lemoignei. Both are bacterial model organisms for PHA (PHB) metabolism. Measurements of enzyme activities are technically difficult (Jendrossek, 2007). Notably, only a few enzymes were characterized in detail. The best-studied and verified enzymes are listed below: