Principal component analysis to detect the similarity of distantly related proteins; its application to cytochromes c, c1 and f

: A new method has been developed for detecting the similarity between distantly related families of proteins. The amino acid sequences of each family of proteins are vertically aligned by a homologous alignment method and the physico-chemical properties of the amino acid residues at the corresponding site are evaluated simultaneously, by method of the principal component analysis. Taking into account the species diversity of each family of proteins, we assign the similar regions between the different families of proteins by the overlapping degree of the standard deviations around the mean values of the first principal component. To investigate the homologous relationship between the electron transport proteins in photosynthetic and O2 respiratory systems, this method has been applied to 70 species of mitochondrial cytochrome c, 4 species of cytochrome c1 and 7 species of cytochrome f. This analysis reveals that both cytochrome f and cytochrome c1 have large regions which are similar to those of cytochrome c. Assuming that these similar regions have the same stereochemical structures as those in cytochrome c, we can predict the outlines of the tertiary structures of cytochrome c1 and cytochrome f, respectively, each able to interact with its electron acceptor, cytochrome c and plastocyanin.