hydroxyalkanoates

Hydroxyalkanoates, commonly called polyhydroxyalkanoates (PHAs), are a diverse class of biopolyesters produced by many bacteria as intracellular carbon and energy reserves. They form insoluble granules in the cytoplasm when microbes experience excess carbon and limitation of other nutrients, such as nitrogen or phosphorus. PHAs can be homopolymers or copolymers with varying hydroxyalkanoate units, yielding a range of material properties. The best known PHA is polyhydroxybutyrate (PHB); copolymers such as polyhydroxybutyrate-co-hydroxyvalerate (PHBV) broaden processing options, and medium-chain-length PHAs offer more flexible elastomeric behavior.

Biosynthesis involves enzymes encoded by pha operons. The pathway typically starts from acetyl-CoA, with PhaA (β-ketothiolase) condensing two acetyl-CoA to acetoacetyl-CoA, PhaB reducing it to hydroxybutyryl-CoA, and PhaC (PHA synthase) polymerizing hydroxyacyl-CoA to polyhydroxyalkanoate. Granule metabolism is regulated by nutritional status; PHA depolymerases can degrade stored polymer when carbon becomes limiting.

Industrial production relies on bacterial fermentation using organisms such as Cupriavidus necator or mixed cultures, with renewable feedstocks. PHAs are valued as biodegradable, biocompatible plastics and have applications in packaging, agriculture, electronics, and biomedicine. Their properties can be tuned by monomer composition and processing conditions, although production costs and process optimization remain challenges. PHAs degrade in soil, compost, and marine environments, with degradation rates depending on composition and environmental conditions.