Starch synthetase

SELENOPHOSPHATE SYNTHETASE STARCH PHOSPHORYLASE SUCCINYL-CoA SYNTHETASE SUCROSE PHOSPHORYLASE TUBULIN.TYROSINE LIGASE ORTHOPHOSPHATE CONTINUOUS ASSAY Orthovanadate,... 

Because of the importance of ADPGlc synthetase for plant starch synthesis, efforts have been made to determine the structure of the enzyme and to relate catalytic and allosteric function to structure. The spinach leaf enzyme is composed of two subunits of 51 and 54 kd mass (5,6). The molecular mass of the native enzyme is 206,000 and presumably is a tetramer composed of two of each subunit. The subunits are antigenically dissimilar, exhibit different peptide patterns on HPLC after trypsin digestion and their N-terminal amino acid sequences are different (7). Thus it is likely that the peptide subunits are products of different genes. 

Hawker, J. S., and Downtown, W. J. S. 1974. Starch synthetases from Vitis vinifera and Zea mays. Phytochem. 13, 893-900. 

Ozbun, J. L., Hawker, J. S., Greenberg, E., Lammel, C, Preiss, J., and Lee, E. Y. C. 1973. Starch synthetase, phosporylase, ADPglucose pyrophosphorylase and UDP-glucose pyrophosphorylase in developing maize kernels. Plant PhysioL 51,1-5. 

Starch synthetase Adenosine/Uridine diphosphate D-glucose -... 

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. ... 

Few kinetic studies on starch synthetase appear to have yet been made. The value of Km for uridine 5 -(a-D-glucopyranosyl pyrophosphate) was found to be 6 x 10 for bean-starch synthetase, whereas for the sweet-corn enzyme Km — 0.75 mM, 1.1 mM, and 1.66 mM for amylopectin, phytoglycogen, and maltotriose, respectively, when expressed in terms of molarity of end groups. 

The various soluble starch synthetases extend ot-(l->4)-glucans, starting from chain lengths of various sizes. SSI, for example, appears to have an extended starch-binding-domain at the C-terminus and transfer glucose residues up to a chain length of about 10. The three other soluble starch synthetases found in cereals (SSIIa, SSIIb and SSIII) appear to make longer chains between clusters there is a possibility that branches may have to be introduced before some of them operate. 

It has been reported that preparations of starch grains display very high synthetase activity if they are isolated in 0.5 M sucrose, presumably because they are well preserved after such treatment. However, the possibility that the high reaction rate may be due to the combined action of nucleotide pyrophosphatase and starch phosphorylase has not been adequately ruled out in that study. 

The formation of a branched polysaccharide, similar to glycogen, which is present in certain plants, especially waxy maize, has not yet been clarified however, it could be explained by interaction with starch phosphorylase or starch synthetase, causing branching. Neither the mechanisms for the separate formation of the linear and branched amylopectin components, nor the proportions of each in various plants, are known at present. These problems are undoubtedly a matter of genetic determinants that are awaiting future elucidation. 

They postulated that UDP-D-glucose may have an important role in the initial step of transformation of sucrose into starch, whereas synthesis of starch is primarily catalyzed by ADP-D-glucose-starch transglycosylase. This concept has been based on the smaller constant of the sucrose synthetase toward UDP as compared with that for ADP, and by the fact that the ADP D-sucrose transglycosylase is specifically inhibited by UDP. From these results, de Fekete and Cardini postulated the following two sequences of reaction for the incorporation of D-glucose into the granule. 

Murata and coworkers, working with a coupling system of sucrose synthetase and starch synthetase, both isolated from ripening rice grains, found that a more efficient transfer of D-glucose- C from sucrose- C to starch occurs in the presence of ADP as compared with UDP, indicating that ADP-D-glucose plays a prominent role in the... 

Since Leloir s group first isolated a particulate starch synthetase in 1961, numerous other workers have obtained preparations of the enzyme from many different sources. The native forms of the enzyme always appear to be particle-bound, but in several instances solubilisation has proved possible, with retention of activity. However, this has often been accompanied by alterations in the specificity of the transferase towards UDPGlc and ADPGlc and somewhat similar variations occur with the source of the enzyme. This poses considerable problems. 

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