A preliminary neutron crystallographic study of thaumatin

Abstract

A preliminary neutron crystallographic study of the sweet protein thaumatin is presented. Large hydrogenated crystals were prepared in deuterated crystallization buffer using the gel-acupuncture method. Data were collected to a resolution of 2 A on the LADI-III diffractometer at the Institut Laue Langevin (ILL). The results demonstrate the feasibility of a full neutron crystallographic analysis of this structure aimed at providing relevant information on the location of H atoms, the distribution of charge on the protein surface and localized water in the structure. This information will be of interest for understanding the specificity of thaumatin-receptor interactions and will contribute to further understanding of the molecular mechanisms underlying the perception of taste.

Figure 1

The thaumatin crystal used for this neutron diffraction study on LADI-III. The crystal volume was 1.4 mm³.

Figure 2

Neutron Laue diffraction pattern recorded from the hydrogenated thaumatin crystal shown in Fig. 1 ▶ using the LADI-III instrument at the ILL. The exposure time was 12 h.

Figure 3

Four separate regions of the (2F _o − F _c) neutron Fourier map calculated for the thaumatin LADI-III data. The density map is contoured at +1.5 r.m.s. (blue) and at −1.5 r.m.s. (green). (a) Arg53, with labile H atoms replaced by deuterium. (b) Thr54, showing the H atoms attached to C atoms as negative peaks (green). (c) A D₂O molecule located between Asp113 and Thr202. (d) Tyr169, showing the ring H atoms as well as the position and orientation of an O—D group.