Charges, hydrogen bonds, and correlated motions in the 1 A resolution refined structure of the mating pheromone Er-1 from Euplotes raikovi.

A detailed description is given of the structure of the small protein mating pheromone Er-1 at atomic resolution. Emphasis is placed on the locations of charges and hydrogen bonds. The model includes all the protein atoms, anisotropic displacement parameters, four disordered side chains, 22 water molecules, a disordered ethanol, and "riding" hydrogen atoms. Analysis of the model revealed that this dense crystal is perfused by hydrogen-bonding networks of solvent and protein atoms. The termini of helices are capped by hydrogen bonding to solvent and protein atoms, and to symmetry-related molecules. An examination of the valencies and charges of the hydrogen-bonding groups suggests that three of the "water" molecules capping the C termini of two helices, and one other, may instead be NH4 ions. Water molecules mediate all but one of the interhelical hydrogen bonds, and many of the lattice interactions. Regions of the molecule where the atomic vibrations deviate from isotropy are identified. There is almost no overall libration of the molecule allowed by the packing, but the side-chains vibrate relative to the backbone. Four side-chains display alternate conformations. Indirect evidence is presented that the switches between their conformations are correlated and driven by protonation of acidic side-chains. These structural features are discussed in the context of function and stability. Equipped with the analysis of the model, we review the course and results of the refinement of the model against 1 A X-ray diffraction data to a crystallographic R-factor of 12.92%.