Fatty acids with the hydroxy group at position 4, 5 or 6 and pendant groups containing substituents or unsaturations are also known. Within bacterial metabolism,carbon substrates are converted into hydroxyacyl-CoA thioesters. As seen in Fig. 2, the carboxyl group of one monomer forms an ester bond with the hydroxyl group of the neighbouring monomer. This polymerization reaction is catalysed by the host’s PHA synthase.