Interpretation of Seemingly Contradictory Data: Low NMR S Order Parameters Observed in Helices and High NMR S Order Parameters in Disordered Loops of the Protein hGH at Low pH.

At low pH, human growth hormone (hGH) adopts a partially folded state, in which the native helices are maintained, but the long loop regions and side-chain packing become disordered. Some of the S order parameters for backbone N-H vectors derived from NMR relaxation measurements on hGH at low pH initially seem contradictory. Three isolated residues (15, 20, and 171) in helices A and D exhibit low order parameter values (<0.5) indicating flexibility, whereas residue 143 in the centre of a long flexible loop region has a high order parameter (0.82). Using S order parameter restraining MD simulations, this paradox has been resolved. Low S values in helices are due to the presence of a mixture of 3 -helical and α-helical hydrogen bonds. High S values in relatively disordered parts of a protein may be due to fluctuating networks of hydrogen bonds between the backbone and the side chains, which restrict the motion of N-H bond vectors.