The pH-dependence of amide chemical shift of Asp/Glu reflects its pKa in intrinsically disordered proteins with only local interactions

Detailed knowledge of the pH-dependence of ionizable residues in both folded and unfolded states of proteins is essential to understand the role of electrostatics in protein folding and stability. The reassembly of E. coli Thioredoxin (Trx) by complementation of its two disordered fragments (1–37/38–108) provides a folded heterodimer in equilibrium with its unfolded state which, based on circular dichroism and NMR spectroscopy, consists of two unfolded monomers. To gain insight into the role of electrostatics in protein folding and stability, we compared the pH-dependence of the carboxylate sidechain chemical shift of each Asp/Glu against that of its backbone amide chemical shift in the unfolded heterodimer. We monitored via C(CO)NH experiments four Asp and four Glu in fragments 38 to 108 (C37) of Trx in the pH range from 2.0 to 7.0 and compared them with results from 1H15N HSQC experiments [Pujato et al., Biophys. J., 89 (2005) 3293–3302]. The 1H15N HSQC analysis indicates two segments with quite distinct behavior: (A) a segment from Ala57 to Ala108 in which ionizable residues have up to three contiguous neighbors with pH-dependent backbone amide shifts, and (B) a segment of fifteen contiguous pH-dependent backbone amide shifts (Leu42 to Val56) in which two Asp and two Glu are implicated in medium range interactions. In all cases, the titration curves are simple modified sigmoidals from which a pH-midpoint (pHm) can be obtained by fitting. In segment A, the pHm of a given backbone amide of Asp/Glu mirrors within 0.15 pH-units that of its carboxylate sidechain (i.e., the pKa). In contrast, segment B shows significant differences with absolute values of 0.46 and 0.74 pH-units for Asp and Glu, respectively. The dispersion in the pHm of the backbone amide of Asp/Glu is also different in the two segments. Segment A shows a dispersion of 0.31 and 0.17 pH-units for Asp and Glu, respectively. Segment B shows a substantially larger dispersion (0.50 and 1.08 pH-units for Asp and Glu, respectively). In both segments, the dispersion in the pHm of its backbone amide is larger than in the pKa of the carboxylate sidechain (the latter is only 0.17 and 0.52 pH-units for Asp and Glu, respectively). Our results indicate that the pHm of the backbone amide chemical shift of Asp/Glu in a disordered polypeptide segment is a good predictor of its pKa whenever there are none or few neighboring backbone amides with similar pH-dependence.