Solvent-induced differentiation of protein backbone hydrogen bonds in calmodulin

In apo and holoCaM, almost half of the hydrogen bonds (H-bonds) at the protein backbone expected from the corresponding NMR or X-ray structures were not detected by (h3)J(NC') couplings. The paucity of the (h3)J(NC') couplings was considered in terms of dynamic features of these structures. We examined a set of seven proteins and found that protein-backbone H-bonds form two groups according to the (h3)J(NC') couplings measured in solution. H-bonds that have (h3)J(NC') couplings above the threshold of 0.2 Hz show distance/angle correlation among the H-bond geometrical parameters, and appear to be supported by the backbone dynamics in solution. The other H-bonds have no such correlation; they populate the water-exposed and flexible regions of proteins, including many of the CaM helices. The observed differentiation in a dynamical behavior of backbone H-bonds in apo and holoCaM appears to be related to protein functions.