Consistent with its essential role in starch biosynthesis at low temperatures, the plastidial starch phosphorylase from rice endosperm is highly active at low temperature. Moreover, contrary to results on other higher plant phosphorylases, the L80 peptide, a domain unique to plant phosphorylases and not present in orthologous phosphorylases from other organisms, is not involved in enzyme catalysis. Starch phosphorylase (Pho) is an essential enzyme in starch synthesis in developing rice endosperm as the enzyme plays a critical role in both the early and maturation phases of starch granule formation especially at low temperature. In this study, we demonstrated that the rice Pho1 maintains substantial enzyme activity at low temperature (<20 °C) and its substrate affinities for branched α-glucans and glucose-1-phosphate were significantly increased at the lower reaction temperatures. Under sub-saturating substrate conditions, OsPho1 displayed higher catalytic activities at 18 °C than at optimal 36 °C, supporting the prominent role of the enzyme in starch synthesis at low temperature. Removal of the highly charged 80-amino acid sequence L80 peptide, a region found exclusively in the plastidial Pho1 of higher plants, did not significantly alter the catalytic and regulatory properties of OsPho1 but did affect heat stability. Our kinetic results support the low temperature biosynthetic role of OsPho1 in rice endosperm and indicate that its L80 region is unlikely to have a direct enzymatic role but provides stability of the enzyme under heat stress.
Keywords: Heat stability; Low temperature; Rice endosperm; Starch phosphorylase; Starch synthesis.