28.02.2013 Views

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

into C3-metabolites, which then may be merged to form transient carbohydrates.<br />

The synthesis <strong>of</strong> starch from such metabolites involves interconversion <strong>of</strong> sugars,<br />

sugar-phosphates, <strong>and</strong> nucleotide-sugars (3–9).<br />

The substrate for one <strong>of</strong> the key metabolites <strong>of</strong> starch synthesis, adenosinediphosphate-glucose<br />

(ADP-glucose, Fig. 1a), glucose-1-phosphate is formed<br />

either by hydrolysis from UDP-glucose (E.2.7.7.9) or by isomerization from<br />

glucose-6-phosphate (E.5.4.2.2). ADP-glucose pyrophosphorylase (E.2.7.7.27) is<br />

the major controlling enzyme for the rate <strong>of</strong> starch synthesis <strong>and</strong> the amount <strong>of</strong><br />

amylose-type glucans in starch granules (10).<br />

Nonbranched (nb)/long-chain branched (lcb) starch glucans (amylose) <strong>and</strong><br />

short-chain branched (scb) starch glucans (amylopectin) are synthesized in<br />

the amyloplast from ADP-glucose, primarily by the catalytic action <strong>of</strong> starch<br />

synthases [E.2.4.1.21; granular bound starch synthase (GBSSx) or soluble forms<br />

<strong>of</strong> starch synthase (SSx) <strong>and</strong> branching enzymes (BEx)]. Additionally,enzymes<br />

such as debranching enzymes <strong>and</strong> disproportionating enzymes are involved<br />

(Table 4).<br />

Elongation <strong>of</strong> glucans by subsequential coupling <strong>of</strong> ADP-glucose to a<br />

Glc n-chain, probably starting with a maltodextrosyl-protein as primer, via<br />

a(1 !4)-glycosidic linkages, results in nonbranched (nb) glucans with high<br />

symmetry(helix)<strong>and</strong>complexingpotentialforhydrophobic<strong>and</strong>anionicmaterials<br />

within the helix. Catalytic action <strong>of</strong> branching enzymes (BE) introduces<br />

a(1 !6)-glycosidic linkages <strong>and</strong> results in long-chain branched (lcb) <strong>and</strong><br />

more or less short-chain branched (scb) starch glucans (Fig. 2b).<br />

Branches act as symmetry breakers when compared to nonbranched<br />

compounds: hydrophilic <strong>and</strong> hydrophobic domains become less pronounced with<br />

increasing scb-characteristics. In general, branches increase molecular packing<br />

density(masswithinoccupiedvolume)<strong>and</strong>enforceintramolecularstabilization(11).<br />

Intheinitialstateamix<strong>of</strong>nb-,lcb-,<strong>and</strong>scb-glucansformlooseamorphous<br />

clusters, which are soluble in aqueous media. By subsequent action <strong>of</strong><br />

disproportionating enzyme, these clusters are rearranged by increasing packing<br />

density<strong>and</strong>order(amorphous !crystallinity)<strong>and</strong>areprecipitatedingranulesfor<br />

temporary storage.<br />

Simultaneously, debranching enzymes provide nb-glucans, which are<br />

elongated by granular bound starch synthase (GBSS), yielding amylose-type<br />

starch glucans.The amount<strong>of</strong>nb-starch glucans<strong>of</strong> course depends ontheGBSSconcentration<br />

located in the amyloplast-matrix <strong>and</strong> local temperature (12–14).<br />

Although the formation <strong>of</strong> nb-glucans (amylose) by debranching <strong>of</strong> lcb- <strong>and</strong> scbglucansisgenerallyaccepted,itshouldbenotedthatthesenb-glucansarefoundin<br />

storage starch granules only <strong>and</strong> not in the transient cluster structures <strong>of</strong> leaves<br />

(15,16).<br />

Many<strong>of</strong> thekeyenzymes <strong>of</strong> starch biosynthesis havebeen cloned (Table5)<br />

from plant species, in particular from maize endosperm, rice endosperm, barley<br />

© 2004 by Marcel Dekker, Inc.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!