There exists a large super-family of receptors which adopt the same tertiary structure. Various hydrophobicity scales for amino acids based on physicochemical considerations such as the free energy of transfer of the side chain from an inorganic solvent to water have been compiled for the purpose of predicting transmembrane sequences. Structural proteins Important proteins made by higher organisms function to build up the organisms own structural integrity. Usually, these proteins are readily recognized by the typical periodic repeats in their sequences. Folds and topologies Two sequences are homologous, if they are descended from a common ancestor.
Homology is a clear indication of shared structure and frequently related function.
At the simplest level of similarity we can group proteins into families of homologous sequences with a clear evolutionary relationship. Superfamilies can be loosely defined as composed of families with a probable evolutionary relationship, supported mainly by common structural and functional features, in the absence of detectable sequence homology. Folds can be described as representing the architecture of a protein - two proteins will have a common fold if they have comparable elements of secondary structure with the same topology of connections. Sharing a fold requires no evidence for homology. Superfolds are folds taken up by a large number of non-homologous sequences.