Polyhydroxyalkanoates (PHAs) are biodegradable polyesters synthesized from beta-hydroxy coenzyme A esters by many bacteria using a PHA synthase. Under conditions of nutrient limitation in the presence of an appropriate carbon source, up to 90 percent of the dry weight of the bacteria can be present as these polymers that are located in granules within the cells. These polyesters are of interest as they possess properties ranging from thermoplastics to elastomers depending on the substrate(s) and bacteria. A copolymer of beta-hydroxybutyrate and beta-hydroxyvalerate is presently used commercially. Efforts are underway to optimize the production of these and other polyesters in bacteria and plants to make them economically competitive with the environmentally unfriendly polypropylenes. The present proposal is focused on understanding the mechanism of the polymerization process: the requirement of post-translational modification for enzymatic activity, the priming or initiation, the elongation, the termination, and the control of polymer size. An understanding of the polymerization process in vitro is essential for optimization of production of polymers in vivo. The synthases from Alcaligenes eutrophus, a class I enzyme, and Chromatium virosum, a class III enzymes, are the focus of the present proposal. While both proteins make polyesters of beta-hydroxybutyrate, their subunit composition and molecular weight, and the kinetics of polymerization are markedly different. Experiments are outlined to elucidate the mechanisms of these two classes of synthases. The long range goal is to understand in sufficient detail the proteins and their specificities such that new specialty polymers can be synthesized with novel properties that are biodegradable and produced in an economically competitive fashion.
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