THE STRUCTURE OF SAICAR SYNTHASE - AN ENZYME IN THE DE-NOVO PATHWAY OF PURINE NUCLEOTIDE BIOSYNTHESIS

Background: The biosynthesis of key metabolic components is of major i nterest to biologists. Studies of de novo purine synthesis are aimed a t obtaining a deeper understanding of this central pathway and the dev elopment of effective chemotherapeutic agents, Phosphoribosylaminoimid azolesuccinocarboxamide (SAICAR) synthase catalyses the seventh step o ut of ten in the biosynthesis of purine nucleotides. To date, only one structure of an enzyme involved in purine biosynthesis has been repor ted: adenylosuccinate synthetase, which catalyses the first committed step in the synthesis of AMP from IMP. Results: We report the first th ree-dimensional structure of a SAICAR synthase, from Saccharomyces cer evisiae. It is a monomer with three domains. The first two domains con sist of antiparallel beta sheets and the third is composed of two a he lices. There is a long deep cleft made up of residues from all three d omains. Comparison of SAICAR synthases by alignment of their sequences reveals a number of conserved residues, mostly located in the cleft. The presence of two sulphate ions bound in the cleft, the structure of SAICAR synthase in complex with ATP and a comparison of this structur e with that of other ATP-dependent proteins point to the interdomain c left as the location of the active site, Conclusions: The topology of the first domain of SAICAR synthase resembles that of the N-terminal d omain of proteins belonging to the cyclic AMP-dependent protein kinase family. The fold of the second domain is similar to that of members o f the D-alanine:D-alanine ligase family. Together these enzymes form a new superfamily of mononucleotide-binding domains. There appears to b e no other enzyme, however, which is composed of the same combination of three domains, with the individual topologies found in SAICAR synth ase.