<p>Closeup of <a href="https://en.wikipedia.org/wiki/active_site" class="extiw" title="w:active site">active site</a> of human <a href="https://en.wikipedia.org/wiki/carbonic_anhydrase" class="extiw" title="w:carbonic anhydrase">carbonic anhydrase</a> II, showing three <a href="https://en.wikipedia.org/wiki/histidine" class="extiw" title="w:histidine">histidine</a> residues and <a href="https://en.wikipedia.org/wiki/hydroxide" class="extiw" title="w:hydroxide">hydroxide</a> (red and white) coordinating <a href="https://en.wikipedia.org/wiki/zinc" class="extiw" title="w:zinc">zinc</a> (gray).<br>Created using <a href="https://en.wikipedia.org/wiki/PyMOL" class="extiw" title="w:PyMOL">PyMOL</a> (<a rel="nofollow" class="external free" href="http://www.pymol.org">http://www.pymol.org</a>) and <a href="https://en.wikipedia.org/wiki/GIMP" class="extiw" title="w:GIMP">GIMP</a>.
</p>
<dl><dt>Legend</dt></dl><div class="legend" style="direction:ltr;line-height:23px;"><div class="legend-content" style="display:inline-block;vertical-align:top;box-sizing:border-box;width:34px;height:23px;overflow:hidden;font-size:15px;border:thin solid #333;background:#7d80b0;color:#202122;text-align:center;"> </div> Zinc ion</div>
<span class="int-own-work" lang="en">Own work</span>
From <a href="https://en.wikipedia.org/wiki/Protein_Data_Bank" class="extiw" title="w:Protein Data Bank">PDB</a> entry <a rel="nofollow" class="external text" href="http://www.rcsb.org/pdb/cgi/explore.cgi?pdbId=1CA2">1CA2</a>.