Figure 9. Electrostatic surface potential representations of (a) apo S100A4 and (b) Ca²⁺-bound S100A4.

Red, blue and white areas indicate negatively charged, positively charged and hydrophobic regions, respectively. Helices H4 and H4’ and loop regions L2 and L2’ are labeled. The electrostatic surface representation of the Ca²⁺-bound S100A4 structure highlights the exposure of two symmetrically hydrophobic target binding sites formed by helix H4 and loop L2, and by helix H4’ and loop L2’, which are buried in the apo S100A4 structure. The figure was prepared in the CCP4 molecular graphics program.⁷⁰