Figure 5.

(A) 2D representation of the hydrogen bonding pattern of the β sheet in αTS. The arrows (→) depict the hydrogen bonds, pointing from the backbone amide (NH) to the backbone carbonyl oxygen (C=O), with the exception of F19 in β1 and I97 in β3 where their respective backbone amides are hydrogen bonded to the side chain carboxyl of D46 and D124, respectively. The circles with a cross indicate proline residues, and filled circles indicate that the amide hydrogen is not involved in hydrogen bonding in the crystal structure. The empty circles indicate either the absence of the NMR assignment or the inability to obtain accurate fits due to spectral overlap. The color scheme is indicative of the decreasing ΔG^o_HX (from violet to yellow) in the absence of urea. (B) The measured hydrogen exchange free energies, 6.5 to 11.7 kcal mol^-1, in 0 M urea, mapped on the crystal structure of αTS. The figure is generated using Pymol⁴⁵ from the protein data bank file 1bks for αTS.¹⁴ The coordinates for αTS were extracted from the tetrameric tryptophan synthase, α₂β₂, to generate the figure.