CRYSTAL STRUCTURE OF SAICAR SYNTHASE FROM SACCHAROMYCES CEREVISIAE. V.M. Levdikov⁺, V.V. Barynin⁺, A.I. Grebenko⁺, W.R. Melik-Adamyan⁺, V.S. Lamzin* and K.S. Wilson*, +Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, Moscow 117333, Russia; *European Molecular Biology Laboratory (EMBL), c/o DESY, Notkestrasse 85, 22603 Hamburg, Germany

The three-dimensional crystal structure of SAICAR synthase of Saccharomyces cerevisiae has been solved by multiple isomorphous replacement and refined at 1.9 Å resolution to an R=0.154. SAICAR synthase is a monomeric enzyme comprising a single polypeptide chain of 306 amino acids arranged into three domains. The cores of the first two domains comprise antiparallel [[beta]]-sheets and the third is composed of two long [[alpha]]-helices. There is a long deep cleft in the middle of the molecule which is made up of residues from all three domains. The positions of two sulphate ions bound in the cleft and comparison of SAICAR synthase structure with known structures of other nucleotide binding proteins indicate the most probable binding sites of the phosphate moieties of ATP and phosphoribosylcarboxyaminoimidazole.

Sequence alignment of SAICAR synthases from different organisms reveals 24 fully conserved amino acid residues, 14 of which are charged. Almost all of them are located at the surface of the interdomain cleft. Some of them are presumably implicated in substrata binding.

Comparison of the SAICAR synthase with other nucleotide binding proteins shows that this protein does not belong to the protein families with classical di- and mononucleotide-binding fold. However SAICAR synthase structure have some resamblance with glutathione synthetase, D-alanine:D-alanine ligase and cyclic AMP-dependent protein kinase. The probable ATP phosphate anchor in the structure of SAICAR synthase made up of a b-loop-b motif typical for the actin and heat-shock cognate protein.

Studies of the enzyme-substrata complexes is in progress.