Polyhydroxyalkanoate (PHA) Synthases: The Key Enzymes of PHA Synthesis

Abstract

Polyhydroxyalkanoic acids (PHAs) represent a rather complex class of polyesters that are synthesized by most genera of bacteria and members of the family Halobacteriaceae of the Archaea (Steinbuchel et al., 1997; Steinbuchel and Fuchtenbusch, 1998). Most of these prokaryotes synthesize poly(3-hydroxy-butyric acid), poly(3HB), and other PHAs as storage compounds and deposit these polyesters as insoluble inclusions in the cytoplasm. The number of 91 different constituents of PHAs that were recently compiled (Steinbuchel and Valentin, 1995) has meanwhile been outnumbered, and approximately 150 different hydroxyalkanoic acids are now known to occur as constituents of PHAs. These water-insoluble PHAs exhibit rather high molecular weights, thermoplastic and/or elastomeric features, and some other interesting physical and material properties. Therefore, and since they are biodegradable (Jendrossek et al., 1996), they are considered for several applications in the packaging industry, medicine, pharmacy, agriculture and food industry, or as raw materials for the synthesis of enantiomerically pure chemicals and the production of paints (Anderson and Dawes, 1990; Muller and Seebach, 1993; Hocking and Marchessault; 1994 Steinbuchel, 1996; Williams et al., 1999; van der Walle et al., 1999). Many prokaryotic and eukaryotic organisms are able to produce low-molecular weight poly(3HB) molecules that are complexed with other biomolecules and that occur at concentrations which are three to four orders of magnitude less than storage PHAs in the cells (Reusch and Sadoff, 1988). A few eukaryotic microorganisms such as for example Aureobasidium pullulans are able to synthesize the water-soluble polyester polymalic acid which is not synthesized by prokaryotes (Liu and Steinbuchel, 1996).