Figure 2. Structural and Functional Differences in Distantly Related Protein Superfamilies.

The three proteins shown above are all members of the Structural Classification of Proteins (SCOP) scorpion toxin–related superfamily. All retain the same basic fold, but have significantly divergent functions. They function as part of the innate immune arsenal in plants and insects, but form part of the offense in scorpions. Evolution has conserved the basic structure, but many residues within the sequences are not structurally superposable. Such positions, often in the loop regions, can be significant in determining function.