This is the N-terminal domain found in debranching enzyme such as Pullulanase (PulA)from Anoxybacillus sp. LM18-11. The PulA structure comprises four domains (N1, N2, A, and C). This is the N1 domain which has been identified as a carbohydrate-binding motif. Two maltotriose or maltotetraose molecules were found between the N1 domain and a loop of the A domain in the PulA-maltotriose or PulA-maltotetraose structures. These carbohydrates are bound in a parallel binding mode close to each other and form hydrogen bonds. The sugar moieties bound to the N1 domain are not immediately adjacent to the active site, but the enzyme might use N1 binding to attract and grab the substrate. Functional analysis indicate that N1 is important for catalytic activity and thermostability in addition to assisting substrate binding. The structure of the N1 domain reveals a classic distorted beta-jelly roll fold consisting of two anti-parallel beta-sheets, forming a concave and a convex surface. On the concave side of N1 domain there is a cleft to accommodate two molecules of maltotriose or maltotetraose [1].

1: Functional and structural studies of pullulanase from Anoxybacillus sp. LM18-11. Xu J, Ren F, Huang CH, Zheng Y, Zhen J, Sun H, Ko TP, He M, Chen CC, Chan HC, Guo RT, Song H, Ma Y; Proteins. 2014;82:1685-1693. PMID:24375572