Sweetpotato - Bioenergy Crop Plants - page 742

Adenosine-5 ′ -diphosphate glucose (ADPGlc) pyrophosphorylase (AGPase, EC 2.7.7.27) is the
enzyme responsible for the production of ADPGlc, the soluble precursor and substrate for starch
synthases in all plant tissues capable of starch biosynthesis. The AGPase reaction is the first
committed step in the biosynthesis of starch (Tetlow 2006). AGPase catalyzes the rate-limiting step
in starch biosynthesis and exerts a high degree of control on the flux of carbon into this pathway.
In 1966, Ghosh and Preiss established that plant AGPases were regulated through allosteric
activation by 3-phosphoglyceric acid (3-PGA) and inhibition by orthophosphate. A review by Preiss
and Levi (1980) reported in vivo concentrations for the effectors and substrates of AGPase that
fully support the hypothesis that the 3-PGA/inorganic phosphate ratio regulates the rate of starch
biosynthesis in plant leaves.
Starch synthases (SS, EC 2.4.1.21) catalyze the transfer the transfer of the glucosyl moiety of the
soluble precursor ADP glucose to the reducing end of a pre-existing α-(1,4)-linked glucan primer
to synthesize the insoluble glucan polymers amylose and amylopectin (Tetlow 2006). Plants posses
multiple isoforms of SS, categorized according to conserved sequence relationships. The isoforms
within each of the major classes of SS genes are highly conserved in higher plants (Ball and Morell
2003). The major classes of SS genes are broadly split into two groups: the first group is primarily
involved in amylose synthesis, and the second group primarily confined to amylopectic biosynthesis
(Tetlow 2006). The first group of SS genes contains granule-bound starch synthase (GBSS) and
includes GBSSI and GBSSII, which function in the elongation of amylose (De Fekete et al. 1960;
Nelson and Rines 1962; Fujita and Taira 1998; Nakamura et al. 1998; Vrinten and Nakamura 2000).
The second group of SS genes, designated SSI, SSII, SSIII, and SSIV, are exclusively involved in
amylopectin biosynthesis (Ball and Morell 2003).
According to Tetlow (2006), starch-branching enzymes (SBEs, EC 2.4.1.18) generate α-(1,6)-
linkages by cleaving internal α-(1,4) bonds and transferring the released reducing ends to C 6
hydroxyls to form the branched structure of the amylopectin molecule. SBE activity is also a

Pi + G-G-G-G-G - .... degradative G-P G-P + G-G-G-G- ....

starch chain synthetic

α-Glc-1-P degraded starch chain

Phosphorylase (so-called primer)

function of multiple isoforms (Yamanouchi and Nakmura 1992; Gao et al. 1997; Morell et al. 1997;
Sun et al. 1998; Regina et al. 2005).
Starch phosphorylase (SP, EC 2.4.1.1) catalyzes the reversible transfer of glucosyl units from
glucose 1-phosphate to the nonreducing end of α-1,4-linked glucan chains and may be driven in a
synthetic or degradative direction by relative concentrations of the soluble substrates (Tetlow 2006).
Because a degraded starch chain is the product of the degradation reaction of SP, it is thus a
required substrate in the reverse synthetic reaction as shown in the reaction above. This was the
origin of the primer requirement for starch biosynthesis from the nonreducing end and has pretty
much been retained for 65 years (Bocca et al. 1997; Ball et al. 1998; Tomlinson and Denyer 2003).
However, Mukerjea and Robyt (2005) showed that three kinds of starch granules are capable of
incorporating d-glucose from ADPGlc into their starches in the absence of any added maltodextrin
primers to suggest that starch synthesis does not require a primer. They showed that biosynthesis of
starch chains occurs de novo with the addition of two glucose units from ADPGlc to the active site
of starch synthase in a nonprimer, reducing-end, two-site insertion mechanism.
Murata and Akazawa (1968) showed that starch synthesis in sweetpotato indispensably requires
K + for the starch synthesizing reaction catalyzed by the granule-bound enzyme of sweetpotato
roots—SS. It has long been known that root growth (starch accumulation) of sweetpotato is
most effectively stimulated by the K + fertilizer compared with some other plants. Later in 1969,
they demonstrated that the activity of starch synthetase in sweetpotato roots was also enhanced