Biosynthesis of latex-like polyhydroxyalkanoates

Abstract

A wide variety of microorganisms accumulate polyhydroxyalkanoic acids as metabolic storage materials. Most of these prokaryotes synthesize poly(3-hydroxybutyric acid) PHB and other polyhydroxyalkanoic acids (PHAs) as storage compounds and deposit these polyesters as insoluble inclusions in the cytoplasm. These water insoluble, biodegradable PHAs exhibit interesting material properties as thermoplastics and may be used for many applications. PHAs represent a rather complex and diverse class of bacterial storage compounds with more than 150 different hydroxyalkanoic acids identified as constituents of these polyesters. The latex-like PHAs (PHAMCL) consisting of various saturated 3-hydroxy fatty acids with carbon chain length ranging from 6 to 14 carbon atoms (MCL = medium chain length) as carbon and energy storage compound are mainly produced by fluorescent pseudomonads belonging to the rRNA homology group I (STEINBÜCHEL ET AL. 1997). Only a few exceptions of the fluorescent pseudomonads e.g. Pseudomonas fragi were not able to accumulate PHA either from fatty acids or non-related carbon sources (TIMM & STEINBÜCHEL 1990). The latex-like PHAs exhibit physical properties, which differ significantly from the short-chain-length PHAs (PHASCL), such as PHB. Particularly with respect to the melting temperature and the extension to break value the two types of PHAs showed, mainly based on the lower cristallinity of PHAMCL, striking differences (Table 1). In contrast to PHB or the commercially produced Biopol™ (copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate) the PHAMCL is less brittle and more elastic, which imposes a different range of potential applications (Table 2). The PHAMCL are synthesized in a weight average molar mass range of 5 x 104 to 2 x 105 g mol-1, whereas PHB can be produced with a weight average molar mass of >1 x 106 g mol-1.The PHA composition depends on the PHA synthases, the carbon source, and the involved metabolic routes (REHM & STEINBÜCHEL 1999). In Pseudomonas aeruginosa, for instance, two PHA synthase genes phaC1 and phaC2 which were separated by the PHA depolymerase encoding phaZ gene were identified and characterized (Figure 1). These PHAMCL synthases are related to type II PHA synthases which prefer 3-hydroxyacyl-CoA thioester with a chain length of 6-14 carbon atoms as substrate. At least three different metabolic routes were found in Pseudomonas putida for the synthesis of 3-hydroxyacyl coenzyme A thioesters, which are the substrates of the PHA synthase (HUIJBERTS ET AL. 1994, REHM ET AL. 1998) (Figure 1): (i) ß-Oxidation is the main pathway when fatty acids are used as carbon source. (ii) Fatty acid de novo biosynthesis is the main route during growth on carbon sources which are metabolized to acetyl-CoA, like gluconate, acetate or ethanol. (iii) Chain elongation reaction, in which acetyl-CoA moieties are condensed to 3-hydroxyacyl- CoA, is involved in the PHA synthesis during growth on hexanoate. In this review, we provide the state of the art of the biosynthetic pathways involved in the synthesis of latex-like PHAs (Figure 2). Furthermore, biosynthetic enzymes as well as the metabolic engineering of PHAMCL biosynthesis with respect of PHAMCL design, i.e. production of novel PHAs exhibiting material properties useful for specific applications, will be described