Quantitative Analysis of Multiplex H-Bonds.

Esther S Brielle,Isaiah T Arkin

doi:10.1021/jacs.0c04357

Abstract

H-bonding is the predominant geometrical determinant of biomolecular structure and interactions. As such, considerable analyses have been undertaken to study its detailed energetics. The focus, however, has been mostly reserved for H-bonds comprising a single donor and a single acceptor. Herein, we measure the prevalence and energetics of multiplex H-bonds that are formed between three or more groups. We show that 92% of all transmembrane helices have at least one non-canonical H-bond formed by a serine or threonine residue whose hydroxyl side chain H-bonds to an over-coordinated carbonyl oxygen at position i–4, i–3, or i in the sequence. Isotope-edited FTIR spectroscopy, coupled with DFT calculations, enables us to determine the bond enthalpies, pointing to values that are up to 127% higher than that of a single canonical H-bond. We propose that these strong H-bonds serve to stabilize serine and threonine residues in hydrophobic environments while concomitantly providing them flexibility between different configurations, which may be necessary for function.

Highlights

H-bonds are relatively weak interactions that are driven by the electrostatic attraction between a positively charged hydrogen and a negatively charged acceptor
Motivated by the abundance of multiplex H-bonds and their importance to membrane proteins, we have previously measured the strength of one such bond: the over-coordination of the carbonyl of residue i−4 to the hydroxyl and amide hydrogens of serine or threonine residues at position i
Serines and threonines are the most common, together representing 11% of all transmembrane helical amino acids leading to the fact that 92% of all transmembrane helices contain one or more serine or threonine residues

Summary

■ INTRODUCTION

H-bonds are relatively weak interactions that are driven by the electrostatic attraction between a positively charged hydrogen and a negatively charged acceptor. Motivated by the abundance of multiplex H-bonds and their importance to membrane proteins, we have previously measured the strength of one such bond: the over-coordination of the carbonyl of residue i−4 to the hydroxyl and amide hydrogens of serine or threonine residues at position i.18. We followed by calculating the same frequencies for structures in which the hydroxyl group, which participated in the multiplex H-bonding, is absent (panels a−c of Figure 5 and Figure S3). To determine the particular contribution of the hydroxyl sidechain interaction with the backbone carbonyl, we manipulated each of the above systems to abolish any non-canonical H-bond This was achieved by rotating the χ1 dihedral such that the hydroxylic side chain is rotated about the Cα−Cβ bond by 180°, thereby breaking the multiplex H-bond (bottom row in Figure 5 and Figure S3).

C O at i

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES