Abstract
The crystal structure of xylose isomerase [E.C. 5.3.1.5] from Streptomyces olivochromogenes has been determined to 3.0 Å resolution. The crystals belong to space group P22 1 2 1 with unit cell parameters a = 98.7, b = 93.9, c = 87.7. The asymmetric unit contains half of a tetrameric molecule of 222 symmetry. The two-fold axis relating the two molecules in the asymmetric unit is close to where a crystallographic two-fold would be if the space group were 1222. This causes the diffraction pattern to have strong 1222 pseudo-symmetry, so all data were collected in this pseudo-space group. Since the sequence of this enzyme has not been reported, a polyalanine backbone has been fitted to the electron density. Xylose isomerase has two domains: the N-terminal domain is an eight-stranded α/β barrel of 299 residues. The C-terminal domain is a large loop of 50 residues which is involved in inter-molecular contacts. Comparison of xylose isomerase with the archetypical α/β barrel protein, triose phosphate isomerase, reveals that the proteins overlap best when the third (αβ) strand of xylose isomerase is superimposed on the first (αβ) strand of triose phosphate isomerase. This same overlap has also been found between the muconate lactonising enzyme and triose phosphate isomerase [Goldman et al . (1987) J. Mol. Biol ., in press].