Thermal expansion of hen egg-white lysozyme: Comparison of the 1·9 Å resolution structures of the tetragonal form of the enzyme at 100 K and 298 K

Abstract

The average structural and dynamic properties of tetragonal hen egg-white lysozyme have been compared, in structures refined at 1.9 A resolution, using data collected at 100 K and 298 K. The molecule expands by 1.8% over this temperature range with the expansion occurring primarily in its small sub-atomic-sized spaces in an anisotropic manner. Hen egg-white lysozyme consists of two domains: domain 1 (residues 40 to 88) is composed primarily of beta-sheet and is observed to expand by only 0.3%; domain 2 (residues 1 to 39 and 89 to 129) is chiefly alpha-helix and is observed to expand by 2.2%. This is consistent with previous observations that proteins composed primarily of alpha-helix expand more with temperature than do those composed primarily of beta-sheet. The largest movement in the molecule is undergone by the two domains of the structure that move further apart as the temperature is raised. This motion is not a cleft opening but rather consists of a tilt by 2.3 degrees of domain 1 away from domain 2. Within the individual domains the largest movement is undergone by loop T1 of domain 2, consisting of residues 17 to 23. This loop moves in the opposite direction to the rest of the molecule as the temperature is raised. Average temperature factors for the room-temperature and low-temperature structures are 15.2 A2 and 8.1 A2, respectively, when all protein atoms are considered, while these values are 14.0 A2 and 7.8 A2, when only main-chain atoms (N, C alpha, C) are taken into account. An examination of the main-chain averaged B-factor per residue shows that residues involved in intermolecular protein-protein contacts, with symmetry-related molecules, have somewhat lower B-factors than the average and undergo smaller than average changes in B-factor as the temperature is lowered.