The structure of the unligated Glu 239 → Gln mutant of Escherichia coli aspartate carbamoyltransferase (EC 18.104.22.168) has been determined to 3.1-Å resolution and refined to a crystallographic residual of 0.22 in the space group P321. The unit-cell dimensions of the unligated enzyme are a = 122.3 Å, c = 147.1 Å. The c axis cell length is intermediate between the c axis lengths of the T (tense) (c = 142.2 Å) and R (relaxed) (c = 156.2 Å) state structures. Furthermore, the quaternary structure of the mutant enzyme is intermediate between the quaternary structures of the T form and the R form. The differences between the quaternary structures of the Glu-239 → Gln and T-form enzymes can be described as follows: the separation between the catalytic trimers increases by ≃ 1.5 Å along the threefold axis, and they each rotate in opposite directions ≃ 0.5° around the threefold axis, whereas the regulatory dimers rotate ≃ 2° around the twofold axes.
|Original language||English (US)|
|Number of pages||5|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Jan 1 1989|
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