Preliminary identification of the secondary structure of the trp repressor from Escherichia coli

The probable secondary structure content of the trp repressor from Escherichia coli has been inferred from NMR and circular dichroic measurements; the results are compared with those of prediction algorithms. 70% of the amide protons have exchange rate constants orders of magnitude smaller than the intrinsic rate constants, identifying them as participating in hydrogen bonds. The exchange rate constants fall into two distinct classes, one having half-lives of 20 min and the other more than 24 h. The latter class, consisting of 50% of all amide protons, indicates a stable core. The exchange data are consistent with circular dichroism and predictions that suggest that about 55% of the peptides from alpha helix, and 20% form beta sheets and turns. The NMR spectrum further indicates that there is little beta sheet, suggesting that the secondary structure class is alpha.