Starch biosynthesis

Metabolic Engineering of Starch Biosynthesis in Potato Tubers... 65... 

To illustrate the problems that are faced in contemporary metabolic engineering it is worth considering two fields that have recently received much attention starch biosynthesis in potato tubers and the production of bioplastics in Arabidopsis. 

Erlander, S. R. (1998a). Starch biosynthesis. I. The size distributions of amylose and amylo-pectin and their relationships to the biosynthesis of starch. Starch/Stdrke. 50,227-240. 

Tetlow, 1. J. (2006). Understanding storage starch biosynthesis in plants A means to quality improvement. Can. J. Bot. 84,1167-1185. 

Enzymes Involved in Starch Biosynthesis. Much of the eady data dealing with starch biosynthesis in plants are derived from the study of various mutants. The shrunken-2 and britde-2 mutants of maize have gready reduced levels of ADPGPP activity owing to the absence of one of the two subunits of this enzyme, and result in a shrunken seed appearance. Mendel s eady work on inheritance of traits was performed with a pea mutant deficient in branching enzyme activity (61). Mutations in plants affecting starch biosynthesis can have severe results to plant morphology and viability. 

Starch is formed in chloroplasts of moss, fern and green algae.18 Chlorophyceae (green algae) starch is similar to that of higher plants, and several species have been used in studies of starch biosynthesis.19,22,29 In a recent set of studies, Ball et al.22 used Chlamydomonas reinhardtii to study starch biosynthesis. They produced several Chlamydomonas mutants which produce starch with characteristics similar to starches produced by maize endosperm mutants.31-34 The various starch mutations of Chlamydomonas will be discussed in Section 3.7. Other classes of algae which produce starch are Prasinophyceae19,35 and Cryptophyceae.35,36... 

Many of the metabolite uptake studies cited above rely on combined uptake and incorporation into starch. In order to separate uptake from incorporation, Schott et al.226 extracted amyloplast membrane proteins from potato tubers and reconstituted them into liposomes. These reconstituted liposomes transported Pi, triose phosphates and G6P in a counter-exchange mode. The liposomes were ineffective in the transfer of G1P uptake of ADP-Glc was not tested. Mohlmann et al.236 have used a proteoliposomic system to reconstitute plastid envelope proteins. In this system, ADP-Glc is transported in exchange for AMP. Thus the more widely studied plastid ATP/ADP transporter was not responsible for ADP-Glc uptake. More recently, Bowsher et al.237 reported that wheat endosperm amyloplasts membrane proteins reconstituted into proteoliposomes took up ADP-Glc in exchange for AMP and ADP. In addition, they showed that under conditions of ADP-Glc dependent starch biosynthesis, the efflux of ADP from intact amyloplasts was equal to that of ADP-Glc utilization by starch synthesis. The amyloplast membrane ADP-Glc/ADP transporter was a 38 000 molecular weight integral membrane protein.237... 

By using mutants of maize and other species, progress has been made in understanding the pathways and enzymes involved in starch biosynthesis and the fine structure of starch polysaccharides. However, starch biosynthesis (Chapter 4) and granule formation are still not completely understood. Thus, integration of the information on polysaccharide biosynthesis (Section 3.6) with that on mutant effects (Section 3.7), is necessary to fully understand polysaccharide biosynthesis and to delineate the limits of this knowledge. 

Obviously, our understanding of starch biosynthesis is still incomplete, since mutants occur for which the primary metabolic effect has not been determined. Continued evaluation of isozymes and effector compounds, and studies of the in vivo pattern and rate of 14C labeling of intermediates of starch biosynthesis in normal, mutants and mutant combinations should aid in clarifying the nature of the mutations and the pathways of starch biosynthesis. Other aspects of starch formation also remain to be explained. For example, how are starch granules formed as the... 

In spite of these limitations to our complete knowledge of starch biosynthesis, information about the pathway of starch biosynthesis gained from studies of maize endosperm mutants can probably be generalized to other plant species because related mutants have occurred in peas, sorghum, barley, rice and Chlamydomonas, and because the same enzymes are found in starch-synthesizing tissues in other plant species. Variation in the number of isozymes and their developmental expression, and variations in cellular compartmentation, however, could result in a range of pathways with significant differences. 

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