THE CRYSTAL-STRUCTURE OF INDOLE-3-GLYCEROL PHOSPHATE SYNTHASE FROM THE HYPERTHERMOPHILIC ARCHAEON SULFOLOBUS-SOLFATARICUS IN 3 DIFFERENT CRYSTAL FORMS - EFFECTS OF IONIC-STRENGTH

Indole3-glycerol phosphate synthase from the hyperthermophilic archaeo n Sulfolobus solfataricus is a monomeric enzyme with the common (beta/ alpha)(8)-fold. Recently, its three-dimensional structure was solved i n an orthorhombic crystal form, grown by using 1.3 M ammonium sulfate as precipitating agent. Here we describe the X-ray structure analysis of two new crystal forms of this enzyme that were obtained at medium a nd low ionic strength, respectively. Hexagonal crystals with space gro up P3(1)21 and cell dimensions a = 62.4 Angstrom, b = 62.4 Angstrom, c = 122.9 Angstrom, gamma = 120 degrees grew in 0.1 M Mes buffer at pH 6.0 with 30% polyethylene glycol monomethylether as precipitant and 0. 2 M ammonium sulfate as co-precipitant. A second crystal form with spa ce group P2(1)2(1)2(1) and cell constants a = 62.6 Angstrom, b = 74.0 Angstrom, c = 74.2 Angstrom was obtained using polyethylene glycol and ethylene glycol as precipitants in 0.1 M Mes buffer at pH 6.5. Both s tructures were solved by molecular replacement and refined at 2.5 Angs trom and 2.0 Angstrom resolution, respectively. Although the global fo lds are almost identical, alternative conformations are observed in fl exible loop regions, mostly stabilized by crystal contacts. In none of the three crystal forms is the so-called phosphate binding site empty , suggesting that this position has high affinity for anions with tetr ahedrally arranged oxygen atoms. Differences in ionic strength of the crystallization buffer have only minor effects on number and specifici ty of intramolecular salt bridges. The crystal packing, on the other h and, seems to be influenced by the ionic strength of the solvent, sinc e the number of intermolecular salt bridges in the low ionic strength crystal forms is significantly higher. (C) 1996 Academic Press Limited