Wheat starch synthesis

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... 

Other evidence, however, supports the presence of a different transport system in the amyloplast envelope. Keeling et al. (1988) studied starch synthesis in isolated wheat endosperm tissue or in the intact plant by incubation with [l- - C]- and [6-13C]-glucose and by looking at the extent of the... 

Hawker, J. S., and Jenner, C. F. 1993. High temperature affects the activity of enzymes in the committed pathway of starch synthesis in developing wheat endosperm. Austr. J. Plant Physiol. 20,197-209. 

Jenner, C. F. 1994. Starch synthesis in the kernel of wheat under high temperature conditions. 

Tyson, R. H., and ap Rees, T. 1988. Starch synthesis by isolated amyloplasts from wheat endosperm. Planta 175,33-38. 

In green algae and in leaf cells of higher plants, ADP-Glc PPase has been demonstrated to reside in the chloroplast (82). More recently, using plastids isolated from maize and barley endosperm (83-85), the existence of two ADP-Glc PPases, a plastidial form, and a major cytosolic form were found. Subsequently, cytosolic forms of ADP-glucose pyrophosphorylase have been found in wheat (86, 87) and rice (88). Because starch synthesis occurs in plastids, it was proposed that in cereal endosperms, synthesis of ADP-Glc in the cytosol requires the involvement of an ADP-Glc carrier in the amyloplast envelope (85). Subsequently, characterization of the ADP-Glc transporter has been reported for maize endosperm (89, 90), barley endosperm (91), and wheat endosperm (92). 

Reactions (1) and (2) both occur in wheat germ however, isotope data make it probable that reaction (1) is the major source of sucrose in photosynthesizing cells. The rapid interconversion of sucrose and starch suggests a role for UDPG in starch synthesis also. Sucrose can be broken down in plants by invertase, which occurs in leaves and other tissues (383). 

The Effects of Starch and Wheat Bran on Synthesis of Cellulase and Xylanase... 

Polymers derived from renewable resources (biopolymers) are broadly classified according to the method of production (1) Polymers directly extracted/ removed from natural materials (mainly plants) (e.g. polysaccharides such as starch and cellulose and proteins such as casein and wheat gluten), (2) polymers produced by "classical" chemical synthesis from renewable bio-derived monomers [e.g. poly(lactic acid), poly(glycolic acid) and their biopolyesters polymerized from lactic/glycolic acid monomers, which are produced by fermentation of carbohydrate feedstock] and (3) polymers produced by microorganisms or genetically transformed bacteria [e.g. the polyhydroxyalkanoates, mainly poly(hydroxybutyrates) and copolymers of hydroxybutyrate (HB) and hydroxyvalerate (HV)] [4]. 

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