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

32 Starch Synthesis in the Potato Tuber 617

Figure 32.4.Regulation of starch synthesis in potato tubers: ADPGlc-pyrophosphorylase (AGPase) is a key regulatory enzyme of starch

biosynthesis. It is regulated at different levels of control, involving allosteric regulation, regulation by posttranslational redox modification and

transcriptional regulation. Redox regulation of AGPase represents a novel mechanism regulating starch synthesis in response to changes in

sucrose supply (Tiessen et al. 2002). There are at least two separate sugar-sensing pathways leading to posttranslational redox activation of

AGPase, one involving an SNF1-like protein kinase (SnRK1), the other involving hexokinase (HK) (Tiessen et al. 2003).

to account for the measured rates of starch synthesis in barley

endosperm, suggesting that at least some of the ADP-glucose

required for this process is provided by cytosolic production

(Thorbjornsen et al. 1996).

It is interesting to note that the involvement of the various

isoforms of AGPase in starch biosynthesis is strictly species de-

pendent, whereas the various starch-polymerizing activities are

ever present and responsible for the formation of the two differ-

ent macromolecular forms of starch, amylose and amylopectin

(Fig. 32.3). SS catalyze the transfer of the glucosyl moiety from

ADP-glucose to the nonreducing end of anα-1,4-glucan and are

able to extendα-1,4-glucans in both amylose and amylopectin.

There are four different SS isozymes—three soluble and one

that is bound to the starch granule.

Starch branching enzymes (SBE), meanwhile, are responsible

for the formation ofα-1,6 branch points within amylopectin

(Fig. 32.3). The precise mechanism by which this is achieved is

unknown; however, it is thought to involve cleavage of a linear

α-1,4-linked glucose chain and reattachment of the chain to form

anα-1,6 linkage. Two isozymes of starch branching enzyme,

SBEI and SBEII, are present and differ in specificity. The former

preferentially branches unbranched starch (amylose), while the

latter preferentially branches amylopectin. Furthermore, in vitro

studies indicate that SBEII transfers smaller glucan chains than

does SBEI and would therefore be expected to create a more

highly branched starch (Schwall et al. 2000). The fact that the

developmental expression of these isoforms correlates with the

structural properties of starch during pea embryo development is

in keeping with this suggestion (Smith et al. 1997). Apart from

that, isoforms of isoamylase (E.C. 3.2.1.68) might be involved

in debranching starch during its synthesis (Smith et al. 2003).

While the pathways governing starch synthesis are relatively

clear, those associated with starch degradation remain somewhat

controversial (Smith et al. 2003). The degradation of plastidial

starch can proceed via phosphorolytic or hydrolytic cleavage

mechanisms involvingα-1-4-glucan phosphorylases or amy-

lases, respectively. The relative importance of these different

routes of starch degradation has been a matter of debate for

many years. The question is whether they are, in fact, inde-

pendent pathways, since oligosaccharides released by hydrol-

ysis can be further degraded by amylases or, alternatively, by

phosphorylases. In addition to this, the mechanisms responsi-

ble for the initiation of starch grain degradation in the plastid

remain to be resolved, since starch grains have been found to