Polyhydroxyalkanoate Granules Are Complex Subcellular Organelles (Carbonosomes)

Abstract

Polyhydroxyalkanoates (PHAs) such as poly(3-hydroxybutyrate) (PHB) or poly(3-hydroxyoctanoate), are universal prokaryotic storage compounds of carbon and energy. PHAs are accumulated intracellularly in form of inclusion bodies (PHA granules) during times of oversupply with carbon sources (for reviews, see references 2, 54, 64, 76, 86, and 100). PHAs can consist of short-chain-length hydroxyalkanoic acids (PHA SCL ) or medium-chain-length monomers (PHA MCL ), depending on strain and culture conditions. Given a number of more than 150 identified hydroxyalkanoates as potential constituents (101) the theoretical number of different PHA copolymers is incredibly high. A few PHAs, such as PHB and copolymers of 3-hydroxybutyrate (3HB), 3-hydroxyvalerate, and/or 4-hydroxybutyrate, are produced by industry (Biocycle, Biomer, Biopol, Enmat, Mirel, and Nodax).The number of publications on PHA metabolism has considerably increased in the last two decades, and many aspects of the biosynthesis, molecular architecture, intracellular mobilization, extracellular degradation, and commercial applications of PHAs have been addressed. One exiting outcome of these studies was the finding that many polypeptides are specifically present on the surface of PHA granules, much more than would be essential for PHA synthesis. These proteins constitute a particular surface layer on PHA granules that is essential for PHA metabolism. In conclusion, PHA granules are complexly organized subcellular structures and appear to be more than simple polymer inclusions. The current knowledge of the biochemical functions of PHA-associated proteins will be reviewed here. This will be done using the example of Ralstonia eutropha H16, which is the model organism of PHA research and has become an important prokaryotic strain for several biotechnological applications (82). It should be noted that several other taxonomic names of R. eutropha are found in literature. These include Hydrogenomonas eutropha, Alcaligenes eutrophus, Wautersia eutropha, and Cupriavidus necator. We used here the most commonly accepted name, R. eutropha, which has been also used for description of the recently sequenced genome (71). When appropriate, the properties of PHA-bound proteins of other organisms are discussed afterward. PHA SYNTHASESPHA synthase is the key enzyme of PHA synthesis and catalyzes the polymerization of hydroxyacyl-coenzyme A (CoA) to PHA and free CoA. The first PHA synthase gene (phaC) was cloned 20 years ago. Remarkably, this was done independently in three labs with the same bacterial strain, R. eutrophus H16 (66, 95, 97). Meanwhile, several PHA synthases from different sources have been biochemically investigated, and many more PHA synthase genes have been cloned or have been determined by genome sequencing. PHA synthases currently are divided into four classes depending on their subunit composition and substrate specificity. The R. eutropha and Pseudomonas putida (previously Pseudomonas oleovorans) PHA synthases represent class I (producing PHA SCL ) and class ...

Localization of Poly(3-Hydroxybutyrate) (PHB) Granule-Associated Proteins during PHB Granule Formation and Identification of Two New Phasins, PhaP6 and PhaP7, in Ralstonia eutropha H16

“…Poly(3-hydroxybutyrate) (PHB) is the most abundant PHA type among bacteria, and PHB contents of 90% of the cellular dry weight have been reported. Biosynthesis and biodegradation of PHB and related PHAs have been studied in several labs during the last 25 years (6,8,11,12,16,17,24,27,33,44,45). It turned out that PHB granules in vivo consist of an amorphous polymer core and a complexly organized proteinaceous surface layer.…”

EliA is required for inducing the stearyl alcohol-mediated expression of secretory proteins and production of polyester in Ralstonia sp. NT80

“…Ralstonia eutropha, which is closely related to NT80, is known to accumulate PHB granules in cells grown in the presence of excess carbon supply and/or in the absence of one or more essential nutritional elements (Jendrossek, 2009). Therefore, we presumed that the observed granules in NT80 cells might be PHAs because of their shapes and sizes.…”

“…(Tanadchangsaeng et al, 2009;Lopes et al, 2011;Raberg et al, 2011). This polymer is formed in the presence of excess supply of carbon and/or in the absence of one or more essential nutritional elements, and it can account for as much as 85 % of the cell dry weight under optimized growth conditions (Yamada et al, 2007;Jendrossek, 2009;Beeby et al, 2012). The synthesized PHA is reutilized when carbon and energy sources are depleted from the surrounding environment (Jendrossek & Handrick, 2002).…”

Poliphasic Screening of Polyhydroxyalkanoate-Producing Microorganisms

“…4 Vale ressaltar que, na maioria das vezes, esse polímero biodegradável é produzido em condições não balanceadas pela limitação de algum nutriente (como fósforo e nitrogênio) e rico na fonte de carbono. 5 Cupriavidus necator (também conhecida como Wautersia eutropha, Ralstonia eutropha e Alcaligenes eutrophus) é a espécie bacteriana mais estudada na produção desses biopolímeros, 6 mas a busca por outros isolados em amostras ambientais diversas que favoreçam o mesmo tem se intensificado. Do ponto de vista econômico, embora os PHAs apresentem vantagens ambientais sobre os plásticos de origem petroquímica, seu alto custo de produção, em especial com utilização de fontes de carbono puras, e também de metodologias de extração, ainda limita a aplicação comercial.…”