Selenomethionine substitution of orotidine-5 '-monophosphate decarboxylasecauses a change in crystal contacts and space group

Orotidine 5'-monophosphate decarboxylase (ODCase) catalyses the decarboxyla tion of orotidine 5'-monophosphate to uridine 5'-monophosphate, the last st ep in the de novo biosynthesis of uridine 5'-monophosphate. In order to det ermine the structure of ODCase from Escherichia coli by the multi-wavelengt h anomalous dispersion technique, both native and SeMet-substituted protein s have been produced and purified. During the production of SeMet ODCase, i t was observed that SeMet was the only amino acid that it was necessary to add to the defined medium during expression. SeMet-substituted ODCase in co mplex with the inhibitor 1-(5'-phospho-beta -D-ribofuranosyl)barbituric aci d crystallizes under similar conditions as the native enzyme. In contrast t o the native enzyme, where the crystals belong to the orthorhombic space gr oup P2(1)2(1)2(1), the SeMet-substituted enzyme crystallizes in the monocli nic space group P2(1), with a quadrupling of the volume of the asymmetric u nit. Despite the drastic difference in symmetry, the overall crystal packin g is effectively identical in the two crystal forms. The change in space gr oup appears to originate in differences in the crystal contacts near the Se Met and Met residues. These differences can be rationalized in terms of SeM et's larger size and hydrophobicity.