Starch is synthesized via ADP-glucose
Fructose 6-phosphate, an intermediate of the Calvin cycle, is the precursor for starch synthesis in chloroplasts (Fig. 9.6). Fructose 6-phosphate is con-verted by hexose phosphate isomerase to glucose 6-phosphate, and a cis-enediol is formed as an intermediate of this reaction. Phosphoglucomutase transfers the phosphate residue from the 6-position of glucose to the 1-position. A crucial step for starch synthesis is the activation of glu-cose 1-phosphate by reaction with ATP to ADP-glucose, accompanied by the release of pyrophosphate. This reaction, catalyzed by the enzyme ADP-glucose pyrophosphorylase (Fig. 9.7), is reversible. The high activityof pyrophosphatase in the chloroplast stroma, however, ensures that the pyrophosphate formed is immediately hydrolyzed to phosphate and thus withdrawn from the equilibrium. Therefore the formation of ADP-glucose is an irreversible process and is very suitable for regulating starch synthesis. The American biochemist Jack Preiss, who has studied the properties of ADP-glucose pyrophosphorylase in detail, found that this enzyme is allos-terically activated by 3-phosphoglycerate and inhibited by phosphate.. The glucose residue is transferred by starch synthases from ADP-glucose to the OH-group in the 4-position of the terminal glucose molecule in the polysaccharide chain of starch (Fig. 9.7). The deposition of glucose resi-dues in a starch grain proceeds by an interplay of several isoenzymes of starch synthase.
Branches are formed by a branching enzyme. At certain chain lengths, the polysaccharide chain is cleaved at the ( α1→4) glycosidic bond (Fig. 9.8) and the polysaccharide fragment thus separated is connected via a newly formed ( α1→6) bond to a neighboring chain. These chains are elongated further by starch synthase until a new branch develops. In the course of starch syn-thesis, branches are also cleaved again by a debranching enzyme, which will be discussed later. It is assumed that the activities of the branching and the debranching enzymes determine the degree of branching in starch. The wrinkled peas with the high amylose content are the result of a decrease in the activity of the branching enzyme in these plants, leading in total to lowered starch content.