FIGURE 6.

Comparison of the active site pockets of 103S_EGCase I and M777_EGCase II. A, Phe¹⁶², Pro¹⁶³, Leu¹⁶⁴, Tyr³⁰², and Leu³⁰³ in 103S_EGCase I form a large cap over the active site. Lys⁶¹, Asp¹³³, Phe¹⁶², Tyr³⁰², Asp³⁴², and Trp³⁶⁵ form a small opening for the active site pocket. B, Arg¹⁷⁷ and Asp³¹¹ in M777_EGCase II form a small cap over the active site. Lys⁶⁶, Asp¹³⁷, Arg¹⁷⁷, Trp¹⁷⁸, Asp³¹¹, Asp³¹⁴, Asp³⁵⁵, Leu³⁵⁸, Trp³⁸², and Trp³⁸⁹ in M777_EGCase II form a large opening for the active site pocket. C, loop 2 and the short loop 4 of 103S_EGCase I define a broad sugar-binding cavity. The model of fucosyl-GM1 in the binding site was created by superimposing its structure onto GM1 and then adjusting the fucosyl unit to a reasonable conformation using Coot. The small pocket was able to accommodate the fucosyl unit. D, loop 2 and the long loop 4 of M777_EGCase II define a crowded sugar-binding cavity.