Pullulanases (pullulan 6-glucanohydrolase, EC 3.2.1.41) are debranching enzymes that are able to hydrolyze the alpha-1,6-glycosidic linkage in pullulan, starch, amylopectin, and related oligosaccharides. Type I pullulanases specifically cleave the alpha-1,6-glycosidic linkages in pullulan and branched oligosaccharides to produce maltotriose and linear oligosaccharides, respectively [1]. Structural analysis of Klebsiella lipoprotein pullulanase (PulA) illustrates that the catalytic core is composed of two major regions: the TIM-barrel domain A and beta-sandwich fold domain C. PulA contains an extra domain, a highly mobile Ins subdomain of unknown function which is inserted into the catalytic TIM-barrel domain A of Klebsiella pullulanases. The Ins subdomain is rich in helical and loop secondary structure. A disulfide bond between Cys491 and Cys506 and two Ca2+ ions presumably stabilizes this domain.This insertion is also found in pullulanases from other Gram-negative genera that have a functional T2SS, such as Vibrio, Aeromonas, and Photorhabdus. Functional analysis indicate that this domain is required for PulA secretion via the T2SS [2].

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

2: Structural Basis of Pullulanase Membrane Binding and Secretion Revealed by X-Ray\tCrystallography, Molecular Dynamics and Biochemical Analysis. East A, Mechaly AE, Huysmans GHM, Bernarde C, Tello-Manigne D, Nadeau N, Pugsley AP, Buschiazzo A, Alzari PM, Bond PJ, Francetic O; Structure. 2016;24:92-104. PMID:26688215