JXB Advance Access published online on March 26, 2009
Journal of Experimental Botany, doi:10.1093/jxb/erp054
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REVIEW-ARTICLE |
Eukaryotic starch degradation: integration of plastidial and cytosolic pathways
1Institute of Biochemistry and Biology, Mass Spectrometry of Biopolymers, University of Potsdam, Karl-Liebknecht-Str. 24-25, Building 20, D-14476 Potsdam-Golm, Germany
2Institute of Biochemistry and Biology, Department of Plant Physiology, University of Potsdam, Karl-Liebknecht-Str. 24-25, Building 20, D-14476 Potsdam-Golm, Germany
* To whom correspondence should be addressed: E-mail: msteup{at}rz.uni-potdam.de
Starch is an important plant product widely used as a nutrient, as a source of renewable energy, and for many technological applications. In plants, starch is the almost ubiquitous storage carbohydrate whereas most heterotrophic prokaryotes and eukaryotes rely on glycogen. Despite close similarities in basic chemical features, starch and glycogen differ in both structural and physicochemical properties. Glycogen is a hydrosoluble macromolecule with evenly distributed branching points. Starch exists as a water-insoluble particle having a defined (and evolutionary conserved) internal structure. The biochemistry of starch requires the co-operation of up to 40 distinct (iso)enzymes whilst approximately 10 (iso)enzymes permit glycogen metabolism. The biosynthesis and degradation of native starch include the transition of carbohydrates from the soluble to the solid phase and vice versa. In this review, two novel aspects of the eukaryotic plastidial starch degradation are discussed: Firstly, biochemical reactions that take place at the surface of particulate glucans and mediate the phase transition of carbohydrates. Secondly, processes that occur downstream of the export of starch-derived sugars into the cytosol. Degradation of transitory starch mainly results in the formation of neutral sugars, such as glucose and maltose, that are transported into the cytosol via the respective translocators. The cytosolic metabolism of the neutral sugars includes the action of a hexokinase, a phosphoglucomutase, and a transglucosidase that utilizes high molecular weight glycans as a transient glucosyl acceptor or donor. Data are included on the transglucosidase (disproportionating isozyme 2) in Cyanophora paradoxa that accumulates storage carbohydrates in the cytosol rather than in the plastid.
Key words: Arabidopsis thaliana L., cytosolic heteroglycans, cytosolic transglucosidase, glucan, water dikinase (GWD), maltose metabolism, phosphoglucan, water dikinase (PWD), Solanum tuberosum L., starch metabolism, starch phosphorylation, Zea mays L
Received 23 December 2008; Revised 8 February 2009 Accepted 10 February 2009