Figure 3. Coordination of divalent zinc ions in orthorhombic crystals of hu3S193 Fab.

Two Zn²⁺ ions (A and C) are involved in tetravalent coordination with residues from the two independent crystallographic Fabs. At one site (A) a water molecule interacts with the Zn²⁺ ion, but this interaction does not occur at the second site (C). An additional two Zn²⁺ ions were observed (B and D) and were coordinated by Asp 190L and His 194L (Fab1) or only by His 194A (Fab2) due to small differences in the polypeptide conformation between Fab1 and Fab2.