Subsequently, carbohydrates without the methyl group were used to determine the binding affinity of IPO. From preliminary study, 1 mM IPO titrated with 25 mM Man, Glc, and Gal revealed no obvious exothermal reaction. After increasing the concentration with 3 mM IPO titrated with 75 mM Man, Glc, and Gal, the exothermal curves could be observed and calculated. The K A values for IPO binding to Man, Gal and Glc were 1.05610 2 M 21 , 0.57610 2 M 21 , and 0.32610 2 M 21 (Table 2 and Figure 5 ). Thus, the interactions between IPO and carbohydrates were stronger with than without the methyl group. Figure 6 ). Thus, the N-terminus of IPO is involved in tetramerization in regulating the binding affinity to carbohydrates.