Almost all the enzymes that catalyze the formation of a-1,4 linkages between glucose molecules require an oligosaccharide as primer. Although on several occasions, it has been reported that starch synthesis can be catalyzed by either starch synthase or phosphorylase without the requirement of a primer (94), but evidence of the presence of primer attached to enzyme has not been ruled out experimentally. The conclusion of a primer independent reaction in starch synthesis is therefore questionable. The most likely candidates for supplying primer molecule in storage tissues have been found in potato tubers. Lavintman and Cardini (95) reported that a particular fraction isolated from potato tuber juice could transfer glucose from UDPGlc to a glucoprotein, which then could act as an acceptor for glucose from either ADPGlc or Glc-l-P using the same fraction. Subsequent investigations (96, 97) suggested that a two step reaction is involved. In the first step, an acceptor protein accepts Glc from micromolar concentrations of UDPGlc, but this reaction plateaus after 15 min with only one Glc molecule attached to the protein. The second step transfers Glc from micromolar concentration of ADPGlc, UDPGlc, or Glc-l-P to the glucoprotein acceptor, resulting in the formation of a-1,4 glucans. The two step nature of the reaction was confirmed by showing that 1,5-D-gluconolactone, a potent inhibitor of starch synthase and phosphorylase, does not affect the glucosylation of the acceptor. The acceptor protein catalyzes addition of the single glucose residue to itself to form the primer utilized in the second step (98). The acceptor protein is designated as a UDPGlc-protein transglucosylase (UPTG: E.C. 184.108.40.206). The reactions involving UDPGlc-protein transglucosylase are similar to the reactions proposed for initiation of glycogen synthesis. The results in the plaint system aire potentiality exciting but some important questions remain such as starch synthesis occurs in the plastids; but where is the transglucosylase/acceptor protein located in the cell ? However, it is important to point out that the lack of genetic evidence to substantiate these reactions, that are correlated to starch synthesis, draws back the physiological importance of these reactions in vivo., Thus, the problem of primer required for the starch synthesizing enzymes is still an important question remained to be answered. However, Pan and Nelson (38) reported that developing maize endosperms contain a phospho oligosaccharide synthase as part of a multi-enzyme complex that may be responsible for synthesizing primer molecules to which chain-elongating enzymes add glucose unit. The enzyme utilizes glucose- 1-P as substrate, but Glc-1,6-bisP, which can be synthesized slowly from Glc-l-P by the complex, markedly accelerates the incorporation of Glc-l-P into oligosaccharide products. The phosphate groups in the products are largely esterified to C6 and are derived from Glc-l,6-bisP. Glc-l,6-bisP is also incorporated into the products. The function of this enzyme likely can have a dual role for the synthesis of primer and the formation of phosphorylated starch through C6 bonding.
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