Microbial Polyhydroxyalkanoates and Nonnatural Polyesters

Abstract

Microorganisms produce diverse polymers for various purposes such as storing genetic information, energy, and reducing power, and serving as structural materials and scaffolds. Among these polymers, polyhydroxyalkanoates (PHAs) are microbial polyesters synthesized and accumulated intracellularly as a storage material of carbon, energy, and reducing power under unfavorable growth conditions in the presence of excess carbon source. PHAs have attracted considerable attention for their wide range of applications in industrial and medical fields. Since the first discovery of PHA accumulating bacteria about 100 years ago, remarkable advances have been made in the understanding of PHA biosynthesis and metabolic engineering of microorganisms toward developing efficient PHA producers. Recently, nonnatural polyesters have also been synthesized by metabolically engineered microorganisms, which opened a new avenue toward sustainable production of more diverse plastics. Herein, the current state of PHAs and nonnatural polyesters is reviewed, covering mechanisms of microbial polyester biosynthesis, metabolic pathways, and enzymes involved in biosynthesis of short‐chain‐length PHAs, medium‐chain‐length PHAs, and nonnatural polyesters, especially 2‐hydroxyacid‐containing polyesters, metabolic engineering strategies to produce novel polymers and enhance production capabilities and fermentation, and downstream processing strategies for cost‐effective production of these microbial polyesters. In addition, the applications of PHAs and prospects are discussed.

Comparative Genomics of Marine Bacteria from a Historically Defined Plastic Biodegradation Consortium with the Capacity to Biodegrade Polyhydroxyalkanoates

“…PHAs are not only biodegradable, but also biobased polyesters, that have physicochemical properties comparable to conventional plastics [ 24 , 25 ]. With over 150 described monomers, they are classified according to the number of carbons in their monomers, the position of the hydroxyl group, and the presence of functional groups in their side chains (e.g., phenoxy, phenyl and acetoxy groups) [ 26 ]. PHAs that consist of monomers of 3–5 carbon atoms are referred to as short-chain length PHAs or PHAscl, while those comprised of 6–15 carbon atoms are called medium chain length PHAs or PHAmcl.…”

“…Many bacteria accumulate PHAs as storage compounds of both carbon and energy in response to carbon excess and/or nitrogen or phosphate stress [ 32 ]. With the first PHAs already being described in 1926 in Bacillus megaterium [ 33 ], it is now known that both Gram-positive and Gram-negative bacteria have PHA production capabilities, including members of the genera Aeromonas , Alcaligenes , Bacillus , Cupriavidus , and Pseudomonas [ 26 ]. These various microorganisms can be and are currently utilized and optimized to produce PHAs for the biobased plastics industry [ 34 ].…”

“…There are numerous metabolic pathways known for PHA biosynthesis, most-recently reviewed in Choi et al [ 26 ], which result in various PHA (co-)polymers. A simplified overview of the PHA biosynthetic and degradation pathways, the PHA cycle, is depicted in Figure 1 .…”

“…The hydroxyacyl-CoA can also be produced from other compounds like enoyl-CoA, also present in the β-oxidation pathway, by PhaJ ((R)-specific enoyl-CoA hydratase, EC 4.2.1.119) or from a hydroxyacyl-[acyl carrier protein] complex, a compound made in fatty acid biosynthesis, by PhaG (hydroxyacyl-CoA-[acyl-carrier-protein] transferase, EC 2.4.1.-). PhaC (PHA synthase, also called PHA polymerase, EC 2.3.1.-), polymerizes the hydroxyacyl-CoA into PHA, the last step in the PHA biosynthesis [ 26 ].…”

Biosynthetic Polymalic Acid as a Delivery Nanoplatform for Translational Cancer Medicine

“…In addition to functions in carbon storage and as an energy reservoir for other polyhydroxyalkanoates (PHAs) in their host microorganisms [ 3 ], PMLA acts as a molecular transporter to bind to and carry nuclear proteins related to DNA replication, demonstrating its unique role in maintaining protein homeostasis and assisting DNA synthesis [ 4 ]. Unlike bioderived water-insoluble polyhydroxybutyrate (PHB) , PMLA possesses a high capacity to attach hydrophobic molecules while remaining water-soluble owing to the exceptional hydrophilicity of abundant carboxylic acids that are present as pendant groups from its polymeric chain [ 5.…”