Diffraction crambin

The 46-residue protein crambin has been solved at 0.83 A resolution at 130 K.6 Some proteins have been crystallized in microgravity in space rockets where the convection-free conditions can produce larger and better crystals. RNase A crystals grown in microgravity diffracted x-rays to 1.06 A resolution, approximately 0.2 A higher resolution than previously observed in terrestrially grown RNase A crystals.7 At these very high resolutions, alternate conformations of some side chains may even be seen. 

Zn-insulin is like crambin and rubredoxin in that the crystal has a low proportion of water—for insulin 30.6% by weight. These proteins are similar also in having much of the crystal water ordered sufficiently for detection by X-ray diffraction analysis. 

Anomalous scattering can also be used directly if the protein is small and a suitable anomalous scatterer can be used. The three-dimensional structure of the small protein, crambin, was determined by W ayne A. Hendrickson and Martha Teeter by the use of anomalous dispersion measurements. This protein contains 45 amino acid residues and diffracts to 0.88 A resolution. It crystallizes with 72 water and four ethanol molecules per protein molecule. Since there is a sulfur atom in the protein molecule, the use of its anomalous scattering was made. The nearest absorption edge of sulfur lies at 5.02 A, but for Cu Ka radiation, wavelength 1.5418 A, values of A/ and A/" for sulfur are 0.3 and 0.557, respectively. Friedel-related pairs of reflections were measured to 1.5 A resolution, and sulfur atom positions were computed from difference Patterson maps. The structure is now fully refined and a portion of an a helix was shown in Figure 12.27 (Chapter 12). 

MPP-CRYSTAL has been used recently for the calculation of the FIF total energy of a small structural protein that has been characterized by X-ray diffraction studies (0.52 A) to a very high precision crambin, which has P2j symmetry with two chains per unit cell, 46 aminoacidic residues per chain, and 1284 atoms per cell. 

A strikingly similar yet different peptide has been isolated from seeds of the crucifer Crambe abyssinica, and is called crambin (11). This peptide contains only three disulfide bonds, whose positions correspond to those of viscotoxin (12). It is very hydrophobic, is not positively charged as are the other thionins and consequently is not toxic. Because of its hydrophobic nature it is easy to crystallize, and the three dimensional structure has been determined by X-ray and neutron diffraction (13). Its three dimensional structure is very similar to that determined for the related thionin a 1 -purothionin, as discussed below (14,15). 

---