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

The three-dimensional structures of two enzyme-substrate complexes of SAICA R synthase from the yeast Saccharomyces cerevisiae with adenosinetriphospha te (ATP) prepared under different conditions were studied by X-ray diffract ion analysis and then refined. An enzyme molecule was shown to contain two binding sites of ATR One of these sites is located in the central cavity of the enzyme molecule and apparently binds the ATP molecule directly involve d in the enzymatic reaction. In the complexes, the phosphate groups of ATP occupying this site adopt different conformations depending on the Mg2+ con centration. The functional role of the second binding site located at a dis tance of approximately 15 Angstrom from the first site away from the centra l enzyme cavity has not been understood as yet. It might be that the second site perform the regulatory role in enzyme functioning. (C) 2001 MAIK "Nau ka/Interperiodica".