We have analyzed the tertiary structure and dynamics of the C-terminal domain of the novel 29-kDa lectin from the earthworm Lumbricus terrestris (EW29Ch) in the sugar-free state and the lactose-bound states by NMR methods. Recently, we tailored a novel sialic acid-binding lectin (SRC) from EW29Ch by natural evolution-mimicry. Further, the crystal structures of the sugar-free SRC, the SRC-lactose complex, and the SRC-sialyllactose (6’-sialyllactose) complex, respectively, have been determined. These results showed that a significant conformational change was observed in a large loop region of the γ subdomain of the SRC, even though the overall structure of SRC was identical to that of EW29Ch. Further, in the complex structures of SRC with lactose or 6’-sialyllactose, SRC bears a single-binding site formed by subdomain α and the extended loop of subdomain γ. In the α subdomain of SRC, the binding mechanism for lactose unit was essentially the same as that of EW29Ch. Additional interactions with α2-6Sia are, further, depicted in SRC: they consist of two hydrogen bonds via G148 in subdomain α and S239 in the extended loop of subdomain γ. However, the X-ray crystallographic analysis does not explain the characterization of dynamic structure of the extended loop of subdomain γ. In this study, we performed 15N relaxation experiments for the backbone of SRC in the lactose-bound state, because SRC is unstable in the sugar-free state. These results show that the extended loop of subdomain γ is flexible, because the 15N{1H}-NOE value of the extended loop region is low. By contrast, that of the corresponding region of EW29Ch is similar to the average 15N{1H}-NOE value in the lactose-bound state. Thus, difference in the backbone dynamics between the extended loop region of SRC and the corresponding region of EW29Ch may be associated with the sialic acid-binding mechanism of SRC.