Human carbonic anhydrase. The Zn²⁺ ion (cyan sphere) is shown tetrahedrally liganded to three invariant His sidechains (N blue, C green). Note that the bicarbonate ion product, which is shown in ball-and-stick form, forms the fourth ligand with the Zn²⁺. The bicarbonate ion is in a hydrogen-bonded network involving Thr 199 and Glu 106, as can be highlighted by clicking alternately on the "Hide H-bonds" and "Show H-bonds" buttons. [Based on the X-ray structure by K. K. Kannan, Bhabha Atomic Research Center, Bombay, India, PDBid 1HCB]