Starch biosynthesis studies

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... 

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. 

In all studies thus far made on starch synthetase, the incorporation of D-glucose from a D-glucosyl ester of a nucleotide into an acceptor molecule has been made by using a radioactively labeled D-glucosyl group in the nucleotide ester, and so the results are unambiguous. However, the extent of the incorporation of D-glucose into the acceptor was very low in the early experiments, and the view has been expressed that starch synthetase is not the major pathway for metabolism of starch. This conclusion seems very reasonable starch biosynthesis is probably a multi-pathway process. Of interest in this connection is a comparison that has been made of starch synthetase activity in non-waxy and waxy maize and rice. ... 

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

CGTases (EC 2.4.1.19) are bacterial enzymes that facilitate the biosynthesis of cyclodextrins from starch through intramolecular transglucosylation. The primary structures of most of these enzymes have been published, and the three-dimensional structure of Bacillus circulans CGTase has been established. Studies of transglucosylation molecular mechanism have indicated that amino acids such as histidine and tryptophan are implicated in such mechanisms. Nitration of CGTase with TNM induces a loss of enzyme activity, a decrease in enzyme affinity towards the (i-CD copolymer, and a loss of tryptophan fluorescence (Villette etal. 1993). 

Geddes, R., Greenwood, C. T., and Mackenzie, S. 1965. Studies on the biosynthesis of starch granules. Part III. The properties of the components of starches from the growing potato tubers. Carbohyd. Res. 1, 71-82. 

Lindeboom, N. (2005). Studies on the characterization, biosynthesis and isolation of starch and protein from quinoa (Chenopodium quinoa Willd.), University of Saskatchewan Degree of Doctor. 

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