Crystal Structure of Chitinase ChiW from Paenibacillus sp. str. FPU 7 Reveals a Novel Type of Bacterial Cell Surface Expressed Multi Modular Enzyme Machinery
The Gram positive bacterium Paenibacillus sp. str. FPU 7 effectively hydrolyzes chitin by using a number of chitinases. A unique chitinase with two catalytic domains, ChiW, is expressed on the cell surface of this bacterium and has high activity towards various chitins, even crystalline chitin. Here, the crystal structure of ChiW at 2.1 A resolution is presented and describes how the enzyme degrades chitin on the bacterial cell surface. The crystal structure revealed a unique multi modular architecture composed of six domains to function efficiently on the cell surface: a right handed b helix domain (carbohydrate binding module family 54, CBM 54) a Gly Ser rich loop, 1st immunoglobulin like (Ig like) fold domain, 1st b/a barrel catalytic domain (glycoside hydrolase family 18, GH 18) 2nd Ig like fold domain and 2nd b/a barrel catalytic domain (GH 18) The structure of the CBM 54, flexibly linked to the catalytic region of ChiW, is described here for the first time. It is similar to those of carbohydrate lyases but displayed no detectable carbohydrate degradation activities. The CBM 54 of ChiW bound to cell wall polysaccharides, such as chin, chitosan, b 1,3 glucan, xylan and cellulose. The structural and biochemical data obtained here also indicated that the enzyme has deep and short active site clefts with endo acting character. The affinity of CBM 54 towards cell wall polysaccharides and the degradation pattern of the catalytic domains may help to efficiently decompose the cell wall chitin through the contact surface. Furthermore, we clarify that other Gram positive bacteria possess similar cell surface expressed multi modular enzymes for cell wall polysaccharide degradation.
PloS one