The ribosome inactivating protein PD-L4 from Phytolacca dioica is a N-β-glycosidase, probably involved in plant defence. The crystal structures of wild type PD-L4 and of the S211A PD-LA mutant with significantly decreased catalytic activity were determined at atomic resolution. To determine the structural determinants for the reduced activity of S211A PD-L4, both forms have also been co-crystallized with adenine, the major product of PD-L4 catalytic reaction. In the structure of the S211A mutant, the cavity formed by the lack of the Ser hydroxyl group is filled by a water molecule; the insertion of this non-isosteric group leads to small albeit concerted changes in the tightly packed active site of the enzyme. These changes have been correlated to the different activity of the mutant enzyme. This work highlights the importance of atomic resolution studies for the deep understanding of enzymatic properties. © 2007 Wiley-Liss, Inc.
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