THE POLYETHYLENE GLYCOL (PEG) POSITIVE EFFECT IN PROTEIN CRYSTALLIZATION. Qing Han and Sheng-Xiang Lin, MRC Group in Molecular Endocrinology, CHUL Research Center and Laval University, Québec, G1V 4G2, Canada

The polyethylene glycols (PEG) are one of most frequently utilized group of precitating agents for macromolecular crystallization. The structure-stabilizing effects of PEG for macromolecule have been described in many biochemical literatures. However, the mechanism of PEG in macromolecular crystallization is relatively ignored. The PEG action in crystallization was studied using PEG 4K, hen egg lysozyme and human 17ß-hydroxysteroid dehydrogenase (17ß-HSD) here. The lysozyme crystallization was established at 14 different pH values from 4.0 to 10.5 with 0.5 interval. The protein solution was prepared in four different compositions: lysozyme 10 mg/ml, lysozyme 10 mg/ml with PEG 4K 15%, lysozyme 10 mg/ml with 1.5 % NaCl, lysozyme 10 mg/ml with 1.5% NaCl and PEG 4K 15 %. The quality of crystallization was evaluated with score from 0 to 5. When the protein solution contained 1.5% NaCl, lysozyme crystals could be found with pH values from 4.0 through 10.5. If PEG was added to the protein solution, the quality of almost all the crystals was greatly improved. If the protein solution with PEG did not contained NaCl, crystallization occurred only when the pH is from 4.0 to 7.5; at pH from 8.0 to 10.5, which is around the pI of lysozyme, no crystals of lysozyme were found. These results were compared with each other. The electrical conductivity under different crystallization solutions was determined and analyzed too. Besides, the effects of PEG for stabilizing protein structure were studied by 17ß-HSD in different concentrations and temperatures. The result showed that the PEG can keep 17ß-HSD activity very well. This is necessary for crystal growth of 17ß-HSD.