The Biosynthesis Of Starch

Plants are unique in many respects but one feature of their biochemistry and metabolism in particular is the synthesis of starch. It comes really from that other unique ability:- the capture of light energy from the Sun coupled with the fixing of carbon dioxide and water to form triose sugars. These are the precursors of more complex molecules called carbohydrates and the polymer most frequently encountered as a storage molecule of energy is starch.

Photosynthesis occurs in a unique structure, the chloroplast. The principle function of the chloroplast is three fold, the production of triose-phosphates, various reducing equivalents and ATP (adenosine triphosphate). The triose phosphates are either transported by their own carriers, the triose-phosphate transporters into the cytosol surrounding the chloroplast or they are converted to phosphorylated compounds in the plastid. One such compound produced is fructose-6-phosphate.

During daylight hours, chloroplasts are producing starch molecules which are then broken down at night into various sugars to be transported around the plant structure for storage. There is a unique cell structure called the amyloplast which does the opposite- it synthesizes starch purely for storage.

The amyloplasts are interesting plant structures. Unlike chloroplasts which are present in green tissues, these storage organs are actually albino plastids with no internal membrane structure. This is a considerable specialization because all they do is process starch and store it in plant cells.

In the amylopast, the fructose-6-phosphate which is produced in the chloroplast is converted into ribulose-1,5-bisphosphate or for the production of glucose-1-phosphate through glucose-6-phosphate. The glucose-1-phosphate is converted with ATP to produce ADP-glucose using the enzyme ADP-glucose pyrophosphorylase (AGPase). It is the first key committed step in starch biosynthesis.

As well as AGPase, other enzymes involved in the starch, especially amylopectin, biosynthetic cascade include starch synthases (SS), starch branching enzymes (SBE) and debranching enzymes (DBE). Amylose is synthesized exclusively by granule-bound starch synthase-I (GBSSI). The glucose moiety from ADPglucose is used to elongate an already existing glucan chain. Starch synthases catalyze the formation of a-1,4 glucosidic linkage between the glucose units to form a linear chain. SS require a primer for elongation of glucose chain.

The initiation of glucan polymerization reaction is poorly understood. One hypothesis suggests the presence of glycogenin-like self glycosylating protein as primer for amylopectin synthesis and addition of D-glucose occurs to the non-reducing end of a growing glucan chain.

Another hypothesis is the de novo synthesis of glucan chains mediated by a two-site insertion mechanism. Two glucose units from ADP-glucose complex with the active site of starch synthase, and are subsequently added to the reducing end of glucan chain.

Four starch synthase isoforms (SSI, SSII, SSIII, SSIV) play important role in elongating different regions of amylopectin. Therefore, alterations in SS activities would affect the amylopectin fine structure. Branches in amylopectin and amylose are introduced by SBE, which catalyze the cleavage of an a-1,4 linkage and join the cleaved chain to another glucan chain through a-1,6 glucosidic linkage. Two classes of SBE (i.e. SBEI and SBEII) exist, which have different substrate specificities.

Finally, debranching enzymes (isoamylase and pullulanase) act to trim the outer branches of amylopectin molecule to form ordered branch structure and packaging of the molecule into starch granules. Since multiple isoforms of starch biosynthetic enzymes exist in the endosperm and have specific functions, mutations in any of these genes would therefore lead to a change in starch content, structure and functional properties.

In addition to the core enzymes, other enzymes, such as phosphorylases, disproportionating enzymes and dikinases (glucan water dikinase, phosphoglucan water dikinase) also play important roles in starch metabolism.

Starch phosphorylation involves dikinases such as glucan water dikinase (GWD, mol wt 155 kDa) and phosphoglucan water dikinase (PWD, mol wt 130 kDa), which phosphorylate the C6 and C3 positions of glucose units of amylopectin, respectively – an important factor in starch degradation.

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