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In the July 2000 issue of Trends in Plant Science, Tom Okita and his associates in the Institute of Biological Chemistry, Washington University, Pullman, review the progress on our understanding of starch synthesis and its regulation.
The authors begin their review by pointing out that starch in its unmodified form has limited industrial uses. Therefore, there is an urgent need to produce modified starches in order to enhance its multipurpose uses in the industry where it serves as a raw material in the food, paper, adhesive and livestock industry. The limited reactivity of glucose as a building block makes it difficult to modify starch molecules. This objective could be achieved by introducing glucose residues with reactive side-chains or charged groups. Some modifications desired are: addition of crosslinkers to introduce molecules such as phosphates or addition of charges to make super-absorbent polymers. A variety of strategies are discussed concerning the creation of novel starch molecules. Some enzymes that can be used for generating novel starches are: starch branching enzyme (SBE), debranching enzyme (DBE) , granular-bound starch synthase (GBSSI) and starch synthase (SS). For example, there is an increased amount of amylose production when SBE and SS are activated; similarly the activation of AGPase and GBSS lead to production of an increased amount of amylopectin. Starch yield is enhanced with the increase of activity of both SBE and DBE in conjunction with ATP/ADP transporter and AGPase.
Because starch formation is reduced by antisense AGPase in transgenic potatoes, it seems reasonable to the authors that the increased activity of this enzyme will increase starch production. Some experimental data supports this view; some does not . When the potato variety Russett Burbank was transformed with a mutant E. coli AGPase gene, there was a 30% increase in starch content. However, the effect on yield was negative in the potato variety Prairie, despite a four-fold increase of AGPase.
The authors propose that bioengineering of AGPase could be achieved by combining the two subunits of this enzyme, large and small in different proportions. The larger subunit plays a regulatory role while the smaller one, a catalytic one. The different levels of AGPase have profound effects also on starch structure; for instance, reduced levels of AGPase lower the amylose content of starch and higher levels enhance short-chain amylopectin.
Studies on starch synthase isoforms reveal that by manipulating this enzyme, it may be possible to generate novel starch molecules by extending glucan chains, or changing their thermosensitivity or by producing chimeric proteins through fusion of an SBE isoform with a GBSS isoform. The authors also suggest that a DNA shuffling approach, in which different species and isoforms of SBE are used, may be applied to bioengineer new enzymes for different catalytic turnover and chain lengths.