Crystallographic evaluation of internal motion of human α-lactalbumin refined by full-matrix least-squares method

The low temperature form of human alpha-lactalbumin (HAL) was crystallized from a (H2O)-H-2 solution and its structure was refined to the R value of 0.119 at 1.15 Angstrom. resolution by the full-matrix least-squares method. Average estimated standard deviations of atomic parameters for non-hydrogen atoms were 0.038 Angstrom for coordinates and 0.044 Angstrom(2) for anisotropic temperature factors (U-ij) The magnitude of equivalent isotropic temperature factors (U-eqv) was highly correlated with the distance from the molecular centroid and fitted to a quadratic equation as a function of atomic coordinates. The atomic thermal motion was rather isotropic in the core region and the anisotropy increased towards the molecular surface. The statistical analysis revealed the out-of-plane motion of main-chain oxygen atoms, indicating that peptide groups are in rotational vibration around a C-alpha... C-alpha axis. The TLS model, which describes the rigid-body motion in terms of translation, libration, and screw motions, was adopted for the evaluation of the molecular motion and the TLS parameters were determined by the least-squares fit to U-ij. The reproduced U-eqv(cal) from the TLS parameters was in fair agreement with observed U-eqv, but differences were found in regions of residues, 5-22, 44-48, 70-75, and 121-123, where U-eqv was larger than U-eqv(cal) because of large local motions. To evaluate the internal motion of HAL, the contribution of the rigid-body motion was determined to be 42.4% of U-eqv in magnitude, which was the highest estimation to satisfy the condition that the U-ij(int) tensors of the internal motion have positive eigen values. The internal motion represented with atomic thermal ellipsoids clearly showed local motions different from those observed in chicken-type lysozymes which have a backbone structure very similar to HAL. The result indicates that the internal motion is closely related to biological function of proteins. (C) 1999 Academic Press.