The kinase enzyme, which forms the inhibitor, is activated by Fru-6-P and Pi, the two products of the
FBPase reaction. The same two products also inactive the phosphatase enzyme, which degrades the in-
hibitor. Thus, the FBPase can indirectly inhibit its own activity, inasmuch as increased activity of the FB-
Pase will lead eventually to increased levels of Fru-1,2-bis-P through increased synthesis and a slower rate
of degradation. Conversely, the kinase, which forms the inhibitor, is inactivated by PGA and triose-P (in
the form of dihydroxyacetone phosphate), so that when high levels of triose-P are being exported to the cy-
tosol, the inhibition of FBPase by F-2,6-bis-P is relieved and carbon flow to sucrose can continue [10,11].
REGULATION OF SPS ACTIVITY Regulation of sucrose phosphate synthase (SPS, reaction 3, Fig-
ure 3) occurs by a number of different mechanisms. One mechanism is via coarse control through en-
hanced or decreased synthesis of SPS protein. A second mechanism is the fine control by metabolites act-
ing as allosteric effectors. It is now generally accepted that SPS is activated by a high Glu-1-P/Piratio, a
form of allosteric feed-forward control. The SPS enzyme may also be inactivated by high levels of su-
crose, a form of feedback control [14].
Finally, light-dark modulation of SPS activity can be regulated by covalent modification through re-
versible protein phosphorylation. The major regulatory phosphorylation site of SPS has been identified
as Ser158. Phosphorylation of Ser158 is both necessary and sufficient for the inactivation of SPS activ-
ity, and dephosphorylation of this site causes its activation. SPS kinase, which deactivates the enzyme, is
strictly Ca^2 dependent. Thus, when cytosolic Ca^2 levels are reduced in the light, SPS kinase is deacti-
vated. At the same time, phospho-SPS is dephosphorylated/activated by SPS protein phosphatase, which
is inhibited by Pi[14].
CARBOHYDRATE SYNTHESIS AND CROP METABOLISM 471
Figure 3 Pathway of sucrose synthesis in the cytoplasm of photosynthetic leaf cells. Reaction 1, cytoplas-
mic fructose-1,6-bisphosphatase (FBPase); reaction 2, UDPG pyrophosphorylase; reaction 3, sucrose phos-
phate synthase (SPS); reaction 4, sucrose phosphate phosphatase.