ALTERED SPECIFICITY MUTATIONS DEFINE RESIDUES ESSENTIAL FOR SUBSTRATEPOSITIONING IN XANTHINE DEHYDROGENASE

We describe the sequence changes of a number of mutations of the Asper gillus nidulans xanthine dehydrogenase (XDH). We have located the amin o acids affected by these changes in the three-dimensional(3D) structu re of aldehyde oxido-reductase (MOP) from Desulfovibrio gigas, related to eukaryotic XDHs. Of these, two are loss of function mutations, map ping, respectively, in the molybdenum-pterin co-factor (MoCo) domain a nd in the domain involved in substrate recognition. Changes in two ami no acids result in resistance to the irreversible inhibitor allopurino l. in Arg911 two different changes, conserved among all XDHs and MOP b ut not in other aldehyde oxidases (AO), change the position of hydroxy lation of the analogue 2-hydroxypurine from C-8 to C-6. A number of ch anges affect residues adjacent to the molybdenum or its ligands. Arg91 1 is positioned in the substrate pocket in a way that it can account f or the positioning of purine substrates in relation to the MoCo reacti ve center, together with a glutamate residue, universally conserved am ong the XDHs (Glu833). (C) 1998 Academic Press Limited.