Roles of Starch Debranching Enzymes in Amylopectin Biosynthesis

Two different types of starch debranching enzymes (DBE), isoamylase(ISA) and pullulanase (PUL), are present in higher plants. Pan and Nelson (1984) discovered that PUL is deficient in maize sugary-1 mutant endosperm where amylopectin is replaced by phytoglycogen, and proposed that PUL plays an important role in amylopectin biosynthesis. We (1992) also observed similar results with rice sugary-1 mutant endosperm.

James et al. (1995) directly determined that the Sugary-1 gene encodes the ISA gene of maize by using an elegant gene tagging. The result clearly indicated that ISA is essential for amylopectin synthesis. Considerable evidence has been accumulated to support the view using a variety of sugary-type mutants from various plant species (see below). In maize and rice, PUL activities are severely reduced in such mutant endosperm, while they are unaffected in Chlamydomonas, Arabidopsis and barley by the mutation.

(Plant species) (Tissue) (ISA) (PUL) (Branched polyglucans) (Researchers)
1. Maize endosperm greatly reduced reduced PG & AP James , Myers et al.
2. Rice endosperm greatly reduced reduced PG or AP Nakamura et al.
3. Chlamydomonas cell greatly reduced unaffected PG Ball et al.
4. Arabidopsis green leaves greatly reduced unaffected PG& AP in cell Smith et al.
5. Barley endosperm greatly reduced unaffected PG& AP in cell Denyer et al.

At present there seems to be agreement that DBE is involved in amylopectin biosynthesis. However, it remains to be proved firstly, how DBE is involved in amylopectin synthesis, and secondly, whether only ISA, the Sugary-1 gene product, plays a role, or other DBE such as PUL also plays a distinct part in amylopectin biosynthesis.

The author hypothesizes the possible role of DBEs. Amylopectin has a distinct structure in that a unit structure designated as a cluster is numerously linked (Fig. 1A). Glucan chains of different chain length are branched via alpha-1,6 glucosidic linkage, and the alpha-1,6 branches are localized at the basal region of the cluster, but not evenly distributed like glycogen. It is assumed that during the formation of the cluster short chains are transferred by starch branching enzyme (SBE) at the basal portion of the cluster, but a few chains are branched outside the portion. The most important role of DBE is to remove such chains which are branched at the wrong position in the cluster (Fig. 1B). If such chains remain undebranched, they will interfere in the formation of clusters of fixed size.
Fig. 1A Schematic Representation of Polyglucan Structure

Fig. 1B Possible role of starch debranching enzymes in amylopectin biosynthesis.The function of DBE is to remove unneccessary side chains which are branched at the wrong position in the cluster of amylopectin.
It is possible that the starch biosynthetic system has been improved during the process of evolution in higher plants, since the specific multiple cluster structure of amylopectin might better fit the plants for survival. The plants have modified the function of DBEs so that they are able to play important roles in amylopectin synthesis, although their function was originally restricted to hydrolysis it.
Yasunori Nakamura (Faculty of Bioresource Sciences, Akita Prefectural University)
References (1) Nakamura Y Plant Sci. 121, 1-18, 1996
(2) Pan D, Nelson OE Plant Physiol. 74, 324-328, 1984
(3) James MG, Robertson DS, Myers AM Plant Cell 7, 417-429, 1995
(4) Myers AM, Morell MK, James MG, Ball SG Plant Physiol. 122, 989-997, 2000
Sep. 15, 2000

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