Streptavidin is a binding protein, from the bacteria Streptomyces avidinii, with remarkable affinity for the vitamin biotin. The lipocalins and the fatty acid-binding proteins (FABPs), are two other protein families which also act by binding small hydrophobic molecules. Within a similar overall folding pattern (a β-barrel with a repeated +1 topology), large parts of the lipocalin, FABP, and streptavidin molecules can be structurally equivalenced. The first structurally conserved region within the three-dimensional alignment, or common core, characteristic of the three groups corresponds to an unusual structural feature (a short 310 helix leading into a β-strand, the first of the barrel), conserved in both its conformation and its location within their folds, which also displays characteristic sequence conservation. These similarities of structure and sequence suggest that all three families form part of a larger group: the calycin structural superfamily.
