Exploring structural homology of proteins.

A method for systematically comparing the folding of the three-dimensional structures of proteins has been developed. A search function, plotted in terms of three Eulerian angles, represents the number of sequentially equivalenced amino acids. For each orientation one protein structure is rotated about its center of mass with respect to the other and probabilities are calculated which estimate the degree of structural parallelism. The structurally equivalent residues with highest probabilities are then selected for the best common topology. It was observed that, when structures containing about 150 residues were compared, the random background had a mean value of around 14 residues and the standard deviation was approximately nine residues. The method has been shown to be successful in determining the similarity of the NAD binding domains of lactate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase, and in comparing the heme binding fold of cytochrome b₅ with the globins.Application of the method to compare hen egg white lysozyme and T4 phage lysozyme led to a single significant peak of 62 residues. The structural homology indicated by this peak showed that the substrate, as bound to hen egg white lysozyme, has a corresponding binding site in the large cleft of the phage lysozyme. The predicted binding site of N-acetyl glucosamine at position C compares well with an N-acetyl glucosamine center observed to bind to crystalline phage lysozyme (B. W. Matthews, personal communication).Some results for the comparison of the two Fe-S cage binding domains of ferredoxin are also presented.