Bacillus subtilis strain 168 YteR has been identified as a novel enzyme "unsaturated rhamnogalacturonyl hydrolase" classified in glycoside hydrolase family 105. This enzyme acts specifically on unsaturated rhamnogalacturonan (RG) produced from plant cell wall RG type-I treated with RG lyases, releasing unsaturated galacturonic acid (Delta GalA) from the substrate. The most likely candidate catalytic residue is Asp-143. Here, we show the structure of D143N in complex with unsaturated RG disaccharide (substrate) determined at 1.9 angstrom resolution by X-ray crystallography. This structural feature directly contributes to the postulation of the enzyme reaction mechanism. YteR triggers the hydration of vinyl ether group in Delta GalA, but not of glycoside bond, by using Asp-143 as a general acid and base catalyst. Asp-143 donates proton to the double bond of Delta GalA as an acid catalyst and also deprotonates a water molecule as a base catalyst. Deprotonated water molecule attacks the C5 atom of Delta GalA. (c) 2006 Elsevier Inc. All rights reserved.