a terpolyester with two primary monomers, 3-hydroxybutyrate (3hb) and 3-hydroxyvalerate(3hv), and one minor monomer, 4-hydroxyvalerate(4hv), is formed on 4kva. the composition varies depending on co-substrates such as glucose and 4kva. The minor monomer 4hv does not increase with increased supply of precursors, 4-hydroxyvaleric acid or 4-ketovaleric acid. the presence of 4hv in pha backbone is attributed to random errors of a reductase and a synthase, two key enzymes involved in pha biosynthesis