Crystal structure of hen egg-white lysozyme at a hydrostatic pressure of 1000 atmospheres

The crystal structure of tetragonal hen egg-white lysozyme at a hydrostatic pressure of 1000 atmospheres has been determined by X-ray diffraction to a nominal resolution of 2 A. The crystals, originally grown in 0.83 M-NaCl, had to be transferred to 1.4 M-NaCl to prevent crystal cracking at 300 to 400 atm. The a and b axes of the unit cell contracted by 0.6%, whilst the c axis increased by 0.1%. The unit cell volume contracted by 1.1%. Both the 1 atm and the 1000 atm structures were refined by restrained least-squares to yield final R factors of 14.9% in each case. Since the data were collected by an accurate difference protocol, the change in structure is considered to be more accurate than the absolute structure. The probable accuracy of the atomic shifts is shown to be +/- 0.06 A. The estimated volume decrease of the whole molecule corresponded to an isothermal compressibility of 4.7 X 10(-3) kbar-1. The contraction was non-uniformly distributed. Domain 2 (residues 40 to 88) was essentially incompressible, whilst domain 1 (residues 1 to 39, 89 to 129) had a compressibility of 5.7 X 10(-3) kbar-1. The interdomain region was also compressible. The average B factor decreased about 1 A2 at 1000 atm, but there was a wide range of decreases and increases in individual values. The pressure-induced deformation was analyzed with difference distance matrices. The beta-sheet (residues 42 to 60) and helix 2 (residues 24 to 36) were deformed the least under pressure. The other helices were more deformed and one loop region (residues 61 to 87) actually appeared to expand. The main-chain atoms of the beta-sheet and helix 2 were used to perform a least-squares superposition of the 1 atm and 1000 atm models. The root-mean-square pressure-induced shift for all atoms was 0.2 A, with a few atoms moving more than 1 A. There was no evidence for co-ordinated movement about the hinge axis defined by alpha carbon atoms 38 and 97. The 1 atm and 1000 atm refined structures included 151 and 163 ordered water molecules, respectively. The changes in these ordered water molecules and the mean compressibility of all of the solvent in the crystal will be described elsewhere.