256 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
synthesis in all species. With the exceptions of the
cereal species described above, which also have a
cytosolic isoform of AGPase, these reactions are
confined to the plastid. The activities of AGPase and
starch synthase are sufficient to account for the rates
of starch synthesis in a wide variety of photosynthet-
ic and heterotrophic tissues (Smith et al. 1997).
Furthermore, changes in the activities of these en-
zymes correlate with changes in the accumulation of
starch during development of storage organs (Smith
et al. 1995). AGPase from a range of photosynthetic
and heterotrophic tissues is well established to be
inhibited by phosphate, which induces sigmoidal
kinetics, and to be allosterically activated by 3-phos-
phoglycerate (3PGA), which relieves phosphate
inhibition (Preiss 1988; Fig. 11.4). There is clear
evidence that allosteric regulation of AGPase is
important in vivo to adjust the rate of starch synthe-
sis to changes in the rate of respiration that go along
with changes in the levels of 3PGA, and an impres-Figure 11.2.Pathways of sucrose to starch conversion in plants: 1 , ADP-glucose transporter, 2 , ATP/ADP translo-
cator, 3 , glucose-1-phoshate (Glc1P) translocator, 4 , glucose-6-phosphate (Glc6P) translocator, 5 , cytosolic ADP-
glucose pyrophosphorylase (AGPase), 6 , cytosolic phosphoglucomutase, 7 , plastidial AGPase, 8 , plastidial phospho-
glucomutase. In growing potato tubers, incoming sucrose is degraded by sucrose synthase to fructose and
UDPglucose and subsequently converted to fructose-6-phosphate (Fru6P) and Glc1P by fruktokinase and
UDPglucose pyrophosphorylase, respectively (not shown in detail). The conversion of Fru6P to Glc6P in the cytosol
is catalyzed by phosphoglucoisomerase, and the cleavage of PPito 2 Piin the plastid is catalyzed by inorganic
pyrophosphatase. In potato tubers, there is now convincing evidence that carbon enters the plastid almost exclusive-
ly via the Glc6P translocator, whereas in cereal endosperm the predominant form of uptake is as ADPglucose.
Figure 11.3.Routes of amylose and amylopectin syn-
thesis within potato tuber amyloplasts. For simplicity,
the possible roles of starch-degrading enzymes in trim-
ming amylopectin have been neglected in this scheme.