Figure 3

The butterfly lipocalin (BBP) crystal structure was used as a negative control. The tertiary structures for the Ra-HBP2 and the butterfly BBP (PDB: 1BBP) with 180° turn (below) are depicted in panel (A) and colored from the N-terminus (blue) to the C-terminus (red). The electrostatic potential below each tertiary structure in 180° turns (blue = positive, red = negative and white = neutral) was calculated according to the Poisson-Boltzmann equation implemented in the Schrodinger’s Maestro package. Panel (B) depicts the same respective Cartesian coordinates as in Figure 1 and Additional file 1 for competitive histamine binding between the human H1R and the BBP.