research papers Acta Cryst. (2010). D66, 881–888 doi:10.1107/S0907444910020081 881 Acta Crystallographica Section D Biological Crystallography ISSN 0907-4449 Structure of Staphylococcus aureus adenylo- succinate lyase (PurB) and assessment of its potential as a target for structure-based inhibitor discovery Paul K. Fyfe, Alice Dawson, Marie-Theres Hutchison, Scott Cameron and William N. Hunter* Division of Biological Chemistry and Drug Discovery, College of Life Sciences, University of Dundee, Dundee DD1 5EH, Scotland Correspondence e-mail: w.n.hunter@dundee.ac.uk The medium-resolution structure of adenylosuccinate lyase (PurB) from the bacterial pathogen Staphylococcus aureus in complex with AMP is presented. Oxalate, which is likely to be an artifact of crystallization, has been modelled in the active site and occupies a position close to that where succinate is observed in orthologous structures. PurB catalyzes reactions that support the provision of purines and the control of AMP/ fumarate levels. As such, the enzyme is predicted to be essential for the survival of S. aureus and to be a potential therapeutic target. Comparisons of this pathogen PurB with the enzyme from Escherichia coli are presented to allow discussion concerning the enzyme mechanism. Comparisons with human PurB suggest that the close similarity of the active sites would make it difficult to identify species-specific inhibitors for this enyme. However, there are differences in the way that the subunits are assembled into dimers. The distinct subunit–subunit interfaces may provide a potential area to target by exploiting the observation that creation of the enzyme active site is dependent on oligomerization. Received 8 March 2010 Accepted 27 May 2010 PDB Reference: adenylo- succinate lyase, 2x75. 1. Introduction The purine-biosynthetic pathway is ultimately responsible for the generation of inosine 5 0 -monophosphate from -d-ribose- 5-phosphate (Zhang et al., 2008) and provides the essential purine nucleotides required for DNA replication and cell division. The pathway consists of ten enzyme-catalyzed steps in vertebrates (Buchanan & Hartman, 1959) and 11 in Escherichia coli (Mueller et al., 1994). In this pathway, adenylosuccinate lyase (PurB; EC 4.3.2.2) catalyzes the breakdown of 5-aminoimidazole-4-(N-succinylcarboxamide) ribotide (SAICAR) to 5-aminoimidazole-4-carboxamide ribotide (AICAR) and fumarate (Fig. 1). PurB demonstrates dual substrate specificity and can also break down adenylo- succinate to adenosine monophosphate (AMP) and fumarate (Fig. 1); therefore, it also determines the levels of AMP and fumarate in the cell via the purine-nucleotide cycle. In humans, mutations in PurB and adenylosuccinate lyase defi- ciency have profound effects on the central nervous system (Spiegel et al., 2006). It has been predicted on the basis of bioinformatics comparisons and metabolic reconstructions that the purB gene encodes an essential enzyme activity in Staphylococcus aureus (Heinemann et al., 2005), although we note that it would be important to genetically and chemically validate this prediction. Selective inhibition of PurB offers the possibility of new therapies targeting a range of microbial infections and structural studies provide useful data to assess the potential of this protein for such early stage drug discovery (Hunter, 2009).