Food Biochemistry and Food Processing (2 edition)

BLBS102-c32 BLBS102-Simpson March 21, 2012 14:2 Trim: 276mm X 219mm Printer Name: Yet to Come

618 Part 5: Fruits, Vegetables, and Cereals

Figure 32.5.Outline of the pathway of plastidial starch degradation. The scheme is mainly based on recent molecular studies inArabidopsis

leaves (Ritte et al. 2002, Smith et al. 2003, Chia et al. 2004). After phosphorylation of glucans on the surface of the starch granule via glucan

water dikinase (GWD; R1-protein), starch granules are attacked most probably byα-amylase and the resulting branched glucans

subsequently converted to unbranchedα-1,4-glucans via debranching enzymes (isoamylase and pullulanase). Linear glucans are

metabolized by the concerted action ofβ-amylase and disproportionating enzyme (D-enzyme, glucan transferase) to maltose and glucose.

The phosphorolytic degradation of linear glucans to Glc-1-P (glucose-1-phosphate) byα-1-4-glucan phosphorylase (α-1-4-GP) is also

possible, but seems of minor importance under normal conditions. Most of the carbon resulting from starch degradation leaves the chloroplast

via a maltose transporter in the inner membrane. Subsequent cytosolic metabolism of maltose involves the combined action of glucan

transferase (D-enzyme),α-1-4-glucan phosphorylase (α-1-4-GP), and hexokinase (HK). Glucose transporter (GTP) and

triose-P/Pi-translocator (TPT) are also shown.

be very stable and relatively resistant against enzymatic action

in vitro.

More recently, molecular and genetic approaches have al-

lowed rapid progress in clarifying the route of starch degra-

dation in leaves of the model speciesArabidopsis thaliana

(Fig. 32.5). The phosphorylation of starch granules by glucan

water dikinase (GWD, R1) has been found to be essential for the

initiation of starch degradation in leaves and tubers (Ritte et al.

2002, Ritte et al. 2004). Transgenic potato plants (Lorberth et al.

1998) andArabidopsismutants (Yu et al. 2001) with decreased

GWD activity showed a decrease in starch-bound phosphate and

were severely restricted in their ability to degrade starch, lead-

ing to increased starch accumulation in leaves. The underlying

mechanisms are still unknown. Phosphorylation of starch may

change the structure of the granule surface to make it more sus-

ceptible to enzymatic attack or may regulate the extent to which

degradative enzymes can attack the granule.

It is generally assumed that the initial attack on the starch

granule is catalyzed byα-amylase (endoamylase). This enzyme

catalyzes the internal cleavage of glucan chains from amylose or

amylopectin, yielding branched and unbranchedα-1,4-glucans,

which are then subject to further digestion (Fig. 32.5). De-

branching enzymes (isoamylase and pullulanase) are needed

to convert branched glucans into linear glucans by cleaving the

α-1,6 branch points. Further metabolism of linear glucans could

involve phosphorolytic or hydrolytic routes. In the first case,

α-glucan phosphorylase leads to the phosphorolytic release of

Glc1-P, which can be further metabolized to triose-P within the

chloroplast and subsequently exported to the cytosol via the

triose-P/Pitranslocator. Recent results show that the contribu-

tion ofα-glucan phosphorylase to plastidial starch degradation

is relatively small. Removal of the plastidial form of phospho-

rylase inArabidopsisdid not affect starch degradation in leaves

ofArabidopsis(Zeeman et al. 2004) and potato (Sonnewald

et al. 1995). It has been suggested that the phosphorolytic path-

way could be more important to degrading starch under certain

stress conditions (i.e., water stress; Zeeman et al. 2004). How-

ever, no regulatory properties have been described for glucan