Glycogen starch synthase

Reaction 4.2 is catalyzed by starch synthase (E.C. 2.4.1.21 ADP-glucose 1,4-a-D-glucan 4-a-glucosyltransferase). Similar reactions are noted for glycogen synthesis in cyanobacteria and other bacteria,24 26 but the enzyme is referred to as glycogen synthase (also E.C. 2.4.1.21). 

Starch synthase or glycogen synthase activity can be measured by transfer of [14C]glucose from ADPGlc into an appropriate primer, such as amylopectin or rabbit glycogen, followed by precipitation of the labeled polymer.171172... 

The first enzymic studies done on the dul mutant were carried out in 1981, and both SSII and starch-branching enzyme Ha (SBEIIa) were found to have reduced activity in the endosperm compared to normal maize endosperm.204 SSII was shown to be different from SSI.173,205 SSII requires a primer for activity, and could not catalyze an unprimed reaction even in the presence of 0.5 M citrate, it also has less affinity for amylopectin than does SSI. However, 0.5 M citrate lowered the Km for amylopectin 17-fold. The activity with glycogen as a primer is one-half that observed with amylopectin. Therefore, glycogen is not as effective as a primer as is amylopectin, which differs from what was observed for starch synthase I. Both maize endosperm SSI and SSII had a Km for ADPGlc of 0.1 mM.196,205... 

Table 4.10 Alignment of amino acid sequences of bacterial glycogen synthases and plant starch synthases in the region of the conserved putative loop. Residues with gray background are 100% conserved. The number before the source indicates the number of amino acids of the protein, while the number before the sequence is the first aligned amino acid of each protein. The secondary structure prediction of the E. coli sequence is on top of the alignment E -sheet C loop H helix...

The starch synthase isozymes in maize endosperm have different molecular masses. The GBSS isozyme I has a molecular mass of 60,000 GBSSII is 95,000. The SSSI has a molecular mass of 72,000, and SSSII is 95,000. Mu et al. (1994) have reported the molecular mass of maize endosperm SSSI as 76,000, which is similar to the value reported previously for SSSI (reviewed in Sivak and Preiss, 1994 Preiss and Sivak, 1996). These molecular mass values for the starch synthases are all higher than that of the E. coli glycogen synthase with a molecular weight of 52,000 (Kumar et al., 1986). 

Is the reaction independent of added primer relevant This question cannot be answered, but mention can be made of some matters that must be resolved before it can be answered. The first is whether in vivo glycogen and starch synthases are able to use ADPGlc in the absence of glucan (i.e., whether the conditions are favorable for the reaction). The second is whether the site of starch synthesis, the amyloplast, is ever completely devoid of glucans that could act as primers for the starch synthase. 

Subcellular Localization of ADP-GIc PPase in Plants Crystal Structure of Potato Tuber ADP-GIc PPase Bacterial Glycogen Synthase Enzyme structure and Properties Substrate and Catalytic Sites Reversibility of the Glycogen Synthase Reaction Starch Synthases... 

Reaction (7) is catalyzed by starch synthases (EC 2.4.1.21 ADP-Glc 1,4-a-D-glucan 4-a-glucosyltransferase), the same reaction as the one catalyzed by the bacterial glycogen synthase (Reaction (2)). Here a different reaction number is given to stress that the final products of starch, amylose, and amylopectin, are different in structure. Isozymic forms of plant starch synthases z.i i-is,65-69 of branching enzymes 4-i 6,65,66,70-76 reported. 

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