X-ray diffraction study of the complexes of SAICAR synthase with adenosinetriphosphate

The three-dimensional structures of two enzyme-substrate complexes of SAICAR synthase from the yeast Saccharomyces cerevisiae with adenosinetriphosphate (ATP) prepared under different conditions were studied by X-ray diffraction analysis and then refined. An enzyme molecule was shown to contain two binding sites of ATP. One of these sites is located in the central cavity of the enzyme molecule and apparently binds the ATP molecule directly involved in the enzymatic reaction. In the complexes, the phosphate groups of ATP occupying this site adopt different conformations depending on the Mg2+ concentration. The functional role of the second binding site located at a distance of approximately 15 Å from the first site away from the central enzyme cavity has not been understood as yet. It might be that the second site perform the regulatory role in enzyme functioning.