Abstract
Biopolyester (PHAs = polyhydroxyalkanoates) composed of hydroxy fatty acids represent a rather complex class of storage polymers synthesized by various eubacteria and archaea and are deposited as water-insoluble cytoplasmic nano-sized inclusions. These spherical shell-core particles are composed of a polyester core surrounded by phospholipids and proteins. The key enzymes of polyester biosynthesis and polyester particle formation are the polyester synthases, which catalyze the enantio-selective polymerization of (R)-hydroxyacyl-CoA thioesters to polyesters. Various metabolic routes have been identified and established in bacteria to provide substrate for polyester synthases. The role of the polyester synthases in morphogenesis and presumably self-assembly of these natural polyester particles will be described. Although not essential for particle formation, non-covalently attached proteins, the so-called phasins, can be found at the particle surface and are considered as structural proteins. A multiple alignment of 88 polyester synthases indicated an identity varying from 8% to 96% with eight strictly conserved amino acid residues. Protein engineering of polyester synthases and phasins was used to functionalize the polyester particle surface. The current knowledge enables the microbial and biocatalytic production of particles with controlled size, polyester core composition and surface functionality, which suggested numerous potential applications of these biocompatible and biodegradable nanostructures particularly in the medical field.
Highlights
Microorganisms are capable of forming a variety of intracellular and spherical inclusions
The self-assembly of the polyester particles requires that the soluble polyester synthases are transformed into amphipathic enzymes upon elongation of water-insoluble polyester chains, which remain covalently attached to the enzyme
The functional role of these non-covalently attached proteins will be discussed with respect to granule structure. Another class of non-covalently attached proteins is represented by regulator proteins involved in transcriptional regulation of PHA biosynthesis genes
Summary
Microorganisms are capable of forming a variety of intracellular and spherical inclusions. The C terminus (about 40 amino acid residues) appears to be rather conserved and hydrophobic among all class I and II polyester synthases, suggesting that this region might function as binding domain attaching the synthase to the hydrophobic polyester core (Rehm et al, 2002).
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