Involved in the production of polyhydroxyalkonic acids (PHAs), which are water-insoluble biopolymers used as intracellular energy reserve material when cells grow under conditions of nutrient limitation. PHAs are composed primarily of 3-hydroxybutyric acid (3HB) and 3-hydroxyvaleric acid (3HV). Required for the production of poly-beta-hydroxybutyrate (PHB) and poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) (PHBV). Biopolymer metabolism; poly-(R)-3-hydroxybutanoate biosynthesis. Heterodimer with PhaC. Depletion of both phaC and phaE genes leads to complete loss of the PHA synthase activity. PHB and PHBV are desirable bioplastic due to their biodegradability, biocompatibility, and mechanical properties. However, PHBV has better mechanical properties than PHB. Belongs to the PHA/PHB synthase family.