X-ray diffraction study of the complex of the enzyme SAICAR synthase with substrate analogues

The three-dimensional structure of the complex of the enzyme SAICAR synthase with analogues of natural substrates, namely, phosphoribosylaminoimidazolecarboxamide (AICAR) and succinic acid, was determined and refined by methods of protein crystallography. Two AICAR-binding sites were revealed in the protein molecule. One of these sites is located in the active center of the enzyme in the vicinity of the ATP-binding site, which has been found in the complex of SAICAR synthase with ATP that had been studied earlier. The second AICAR-binding site is located at the periphery of the protein molecule and coincides with the additional ATP-binding site present in the complexes studied earlier. The binding site of succinic acid was revealed in the active center of the enzyme in the vicinity of the AICAR molecule. The electron density distribution for the AICAR molecule in the active center is indicative of the possible lability of the atomic group of the adenine base.