Polyhydroxyalkanoate granules are complex subcellular organelles (carbonosomes)

  • Jendrossek D
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Abstract

Polyhydroxyalkanoates (PHAs) such as poly(3-hydroxybu-tyrate) (PHB) or poly(3-hydroxyoctanoate), are universal pro-karyotic 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-hy-droxybutyrate, are produced by industry (Biocycle, Biomer, Biopol, Enmat, Mirel, and Nodax). The number of publications on PHA metabolism has con-siderably increased in the last two decades, and many aspects of the biosynthesis, molecular architecture, intracellular mobi-lization, extracellular degradation, and commercial applica-tions of PHAs have been addressed. One exiting outcome of these studies was the finding that many polypeptides are spe-cifically 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 knowl-edge 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, Alcali-genes 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 se-quenced genome (71). When appropriate, the properties of PHA-bound proteins of other organisms are discussed after-ward.

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Authors

  • Dieter Jendrossek

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