Direct NMR detection of bifurcated hydrogen bonding in the ?-helix N-caps of ankyrin repeat proteins.
 

Direct NMR detection of bifurcated hydrogen bonding in the ?-helix N-caps of ankyrin repeat proteins.

Direct NMR detection of bifurcated hydrogen bonding in the ?-helix N-caps of ankyrin repeat proteins.

J Am Chem Soc. 2015 Jan 12;

Authors: Preimesberger MR, Majumdar A, Aksel T, Sforza K, Lectka T, Barrick D, Lecomte JT

Abstract
In biomolecules, bifurcated hydrogen bonds typically involve the interaction of two donor protons with the two lone pairs of oxygen. Here, we present direct NMR evidence for a bifurcated H-bonding arrangement involving nitrogen as the acceptor atom. Specifically, the H-bond network comprises the N?1 atom of histidine and both the backbone N-H and side-chain O?-H of threonine within the conserved TXXH motif of ankyrin repeat (AR) proteins. Identification of the H-bonding partners is achieved via solution NMR H-bond scalar coupling (HBC) and H/D isotope shift experiments. Quantitative determination of (2h)JNN HBCs supports that Thr N-H...N?1 His H-bonds within internal repeats are shorter and stronger (~4 Hz) than in the solvent exposed C-terminal AR (~2 Hz). In agreement, pKa values for the buried histidines bridging internal ARs are several units lower than those of the C-terminus. Quantum mechanical calculations show that the relevant (2h)J and (1h)J couplings are dominated by the Fermi contact interaction. Finally, a Thr-to-Val replacement, which eliminates the Thr O?-H...N?1 His H-bond and decreases protein stability, results in a 25% increase in (2h)JNN , attributed to optimization of the Val N-H...N?1 His H-bond. Overall, the results provide new insights into the H-bonding properties of histidine, a refined structural rationalization for the folding cooperativity of AR proteins, and a challenging benchmark for the calculation of HBCs.


PMID: 25578373 [PubMed - as supplied by publisher]



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