CRYSTALLOGRAPHIC EVALUATION OF ANISOTROPIC THERMAL MOTION OF a-LACTALBUMIN REFINED BY FULL-MATRIX LEAST-SQUARES METHOD

Kazuaki Harata, Yutaka Abe, and Michiro Muraki

National Institute of Bioscience and Human-Technology, 1-1 Higashi, Tsukuba, Ibaraki 305-0046 Japan. E-mail: harata@nibh.go.jp

Keywords: a-lactalbumin, crystal structure, X-ray structure, thermal motion, rigid-body,motion, TLS model

Crystal structure of the low temperature form of human a-lactalbumin was refined at atomic resolution and the thermal motion of the protein was evaluated by using anisotropic temperature factors. X-ray diffraction data were collected to 1.15 Ä resolution on a Nonius FAST diffractometer equipped with a FR571 rotating anode generator and 37696 unique reflections (98% completeness) were obtained with the R-merge value of 5.5%. The structure was refined by full-matrix least-squares method (SHELXL-93) to the R-value of 0.116 (R-free 0.161) for all the reflections with I>0.

The TLS method (1) was applied for the evaluation of external motion and tensors of translation (T), libration (L), and screw motion (S) were determined by least-squares fit to the observed anisotropic temperature factors. The center of rotation was ca. 7 Ä far from the center of gravity. The contribution of the external motion to the atomic temperature factor was estimated to be about 47%. In the external motion, the translational portion was predominant while the screw motion was almost negligible. The internal motion was characterized by relatively low thermal motion in the region around the calcium ion and large thermal motion of the helix region of residues 4-14. The results were compared with the thermal motion of lysozymes (2).

1. V. Schomaker, K.N. Trueblood, Acta Crystallogr. Sect. B, 34 (1968) 63-76.
2. K. Harata, Y. Abe, M. Muraki, Proteins: Struc. Func. Gen., 30 (1998) 232-243.