A method for determining the positions of polar hydrogens added to a protein structure that maximizes protein hydrogen bonding

An automated method for the optimal placement of polar hydrogens in a protein structure is described. This method treats the polar, side chain hydrogens of lysine, serine, threonine, and tyrosine and the amino terminus of a protein. The program, called NETWORK, divides the potential hydrogen‐bonding pairs of a protein into groups of interacting donors and acceptors. A search is conducted on each of the local groups to find an arrangement which forms the most hydrogen bonds. If two or more arrangements have the same number of hydrogen bonds, the arrangement with the shortest set of hydrogen bonds is selected. The polar hydrogens of the histidyl side chain are specifically treated, and the ionization state of this residue is allowed to change, if this change results in additional hydrogen bonds for the local group. The program will accept Protein Data Bank as well as Biosym‐format coordinate files. Input and output routines can be easily modified to accept other coordinate file formats. The predictions from this method are compared to known hydrogen positions for bovine pancreatic trypsin inhibitor, insulin, RNase‐A, and trypsin for which the neutron diffraction structures have been determined. The usefulness of this program is further demonstrated by a comparison of molecular dynamics simulations for the enzyme cytochrome P‐450cam with and without using NETWORK.