Abstract
To reveal the characteristics of three rice BE isozymes and their contribution to amylopectin biosynthesis, in vitro analysis was performed using purified recombinant BEI, BEIIa and BEIIb from rice. To our knowledge the present study shows for the first time the kinetic parameters of all the three BE isozymes towards both amylopectin and synthetic amylose as glucan substrate. Three BE isozymes had different chain-length preferences for substrate glucans. It is noted that BEIIb almost exclusively transferred chains of degree of polymerization (DP) 7 and 6 whereas BEIIa formed short chains of DP6-13 although a chain of DP6 was most effectively produced. On the other hand BEI preferentially formed chains of DP10-12. Although three BE could act on the chains of DP12 or slightly larger for amylopectin, BEI could only attack amylose of DP≥60, but BEIIs were unable to react to amylose of DP≤100. Based on these results, the functional distinction and interaction of BE isozymes during amylopectin biosynthesis in rice endosperm will be discussed.
