Biosynthesis of Starch and Sucrose:- ADP-Glucose Is the Substrate for Starch Synthesis in Plant Plastids and for Glycogen Synthesis in Bacteria
Starch, like glycogen, is a high molecular weight poly mer of D-glucose in (α1→4) linkage. It is synthesized in chloroplasts for temporary storage as one of the stable end products of photosynthesis, and for long-term storage it is synthesized in the amyloplasts of the nonphotosynthetic parts of plants—seeds, roots, and tubers (underground stems).
The mechanism of glucose activation in starch syn thesis is similar to that in glycogen synthesis. An activated nucleotide sugar, in this case ADP-glucose, is formed by condensation of glucose 1-phosphate with ATP in a reaction made essentially irreversible by the presence in plastids of inorganic pyrophosphatase (p. 502). Starch synthase then transfers glucose residues from ADP-glucose to preexisting starch molecules. Although it has generally been assumed that glucose is added to the nonreducing end of starch, as in glycogen synthesis (see Fig. 15–8), evidence now suggests that starch synthase has two equivalent active sites that alternate in inserting a glucosyl residue onto the reducing end of the growing chain. This end remains covalently attached to the enzyme, first at one active site, then at the other (Fig. 20–24). Attachment to one active site effectively activates the reducing end of the growing chain for nucleophilic displacement of the enzyme by the attacking C-4 hydroxyl of a glucosyl moiety bound to the other active site, forming the (α1→4) linkage characteristic of starch.
The amylose of starch is unbranched, but amylopectin has numerous (α1→6)-linked branches (see Fig. 7–15). Chloroplasts contain a branching enzyme, similar to glycogen-branching enzyme (see Fig. 15–9), that introduces the (α1→6) branches of amylopectin. Taking into account the hydrolysis by inorganic pyrophosphatase of the PPi produced during ADP-glucose synthesis, the overall reaction for starch formation from glucose 1-phosphate is Starchn + glucose 1-phosphate+ATP→ starchn+1+ADP+2Pi , ΔG0=-50 kJ/mol , Starch synthesis is regulated at the level of ADP-glucose formation, as discussed below. Many types of bacteria store carbohydrate in the form of glycogen (essentially highly branched starch), which they synthesize in a reaction analogous to that catalyzed by glycogen synthase in animals. Bacteria, like plant plastids, use ADP-glucose as the activated form of glucose, whereas animal cells use UDP-glucose. Again, the similarity between plastid and bacterial metabolism is consistent with the endosymbiont hypothesis for the origin of organelles.
