Abstract
The crystal structure of human erythrocytic ubiquitin has been refined at 1.8Åresolution using a restrained least-squares procedure. The crystallographic R-factor for the final model is 0.176. Bond lengths and bond angles in the molecule have root-mean-square deviations from ideal values of 0.016Åand 1.5 °, respectively. A total of 58 water molecules per molecule of ubiquitin are included in the final model. The last four residues in the molecule appear to have partial occupancy or large thermal motion. The overall structure of ubiquitin is extremely compact and tightly hydrogen-bonded; approximately 87% of the polypeptide chain is involved in hydrogen-bonded secondary structure. Prominent secondary structural features include three and one-half turns of α-helix, a short piece of 3 10-helix, a mixed β-sheet that contains five strands, and seven reverse turns. There is a marked hydrophobic core formed between the β-sheet and α-helix. The molecule features a number of unusual secondary structural features, including a parallel Gl β-bulge, two reverse Asx turns, and a symmetrical hydrogen-bonding region that involves the two helices and two of the reverse turns.
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