Role of hydrogen bonds in protein-DNA recognition: effect of nonplanar amino groups.

Amino groups are one of the various types of hydrogen bond donors, abundantly found in protein main chains, protein side chains, and DNA bases. The polar hydrogen atoms of these groups exhibit short ranged, specific, and directional hydrogen bonds, which play a decisive role in the specificity and stability of protein-DNA complexes. To date, planar amino groups are only considered for the analysis of protein-DNA interfacial hydrogen bonds. This assumption regarding hydrogen atom positions possibly failed to establish the expected role of hydrogen bonds in protein-DNA recognition. We have performed ab initio quantum chemical studies on amino acid side chains and DNA bases containing amino groups as well as on specific hydrogen bonded residue pairs selected from high-resolution protein-DNA complex crystal structures. Our results suggest that occurrences of pyramidal amino groups are more probable in comparison with the usually adopted planar geometry. This increases the quality of the existing hydrogen bonds in almost all cases. Further, detailed analysis of protein-DNA interfacial hydrogen bonds in 107 crystal structures using the in-house program "pyrHBfind" indicates that consideration of energetically more preferred nonplanar amino groups improves the geometry of hydrogen bonds and also gives rise to new contacts amounting to nearly 14.5% of the existing interactions. Large improvements have been observed specifically for the amino groups of guanine, which faces the DNA minor groove and thus helps to resolve the problem of insufficient directional contacts observed in many minor groove binding complexes. Apart from guanine, improvement observed for asparagine, glutamine, adenine, or cytosine also indicates that the consideration of nonplanar amino groups leads to a more realistic scenario of hydrogen bonds occurring between protein and DNA residues.