The crystal structure of the At4g34215 protein of Arabidopsis thaliana was determined by molecular replacement and refined to an R factor of 14.6% (R(free) = 18.3%) at 1.6 Angstroms resolution. The crystal structure confirms that At4g34215 belongs to the SGNH-hydrolase superfamily of enzymes. The catalytic triad of the enzyme comprises residues Ser31, His238 and Asp235. In this structure the catalytic serine residue was found to be covalently modified, possibly by phenylmethylsulfonyl fluoride. The structure also reveals a previously undescribed variation within the active site. The conserved asparagine from block III, which provides a hydrogen bond for an oxyanion hole in the SGNH-hydrolase superfamily enzymes, is missing in At4g34215 and is functionally replaced by Gln30 from block I. This residue is positioned in a catalytically competent conformation by nearby residues, including Gln159, Gly160 and Glu161, which are fully conserved in the carbohydrate esterase family 6 enzymes.