The Study Of 4-α-glucanotransferase And Its Modification On Rice Starch

4–α–glucanotransferase (4αGTase,EC 2.4.1.25) in this study, produced by cloned E.coli DH–5α–TA, is a multifunctional enzyme and belongs to theα–amylase family.It is revealed that the smallest reaction substrate of 4αGTase is maltose, and big molecules acted as the substrates are more efficient than smaller ones in the transglycosylation. 4–α–glucanotransferase has the function of intramolecular transglycosylation, generating large-ring cyclodextrin within a certain range of the degree of polymerization from 20 to 50. The coupling and hydrolysis activities of 4αGTase are 55U/mg and 46 U/mg respectively.In this study, the structural properties of the 4αGTase–modified starches were analysed by observing SEM, average molecular weight (Mw), crystal type and iodine absorption spectrum. Via the investigation from SEM, the surface of starch became rough and irregular with more and smaller fraction stacked. It was found that the Mw decreased sharply at the initial stage, however, slightly after 12h modification when it around 3.0×105, and the iodine absorption increased with increase of enzyme treatment time as well as the optimum wavelength was shifted a little bit to right, however, changed slightly after 9h modification. X-ray diffraction patterns showed that the crystal structure of starch was changed from A-type to B-type after treated by 4αGTase.The properties, including retrogradation ability, digestibility and freeze-thaw stability, were also studied. The results indicated that the content of the slowly digestible starch (SDS) in starch increased to the maximum when modified for 3～4 h by 4αGTase (50 U/g dry starch), and then decreased with the modification time increasing. DSC thermograms indicated that the rate of retrogradation was accelerated when the starch was modified for 2～6 h; However, it was gradually decreased when modified above 6 h. The changes of the digestibility could be attributed to the changes of structure and property above. These findings provide a new applicable vision for the starch modified by 4αGTase.