An enzyme (<a href="//commons.wikimedia.org/w/index.php?title=TEV_protease&amp;action=edit&amp;redlink=1" class="new" title="TEV protease (page does not exist)">TEV protease</a>) bound to a substrate (black) contains a catalytic triad of residues (red). The triad consists of an acidic residue (<a href="//commons.wikimedia.org/w/index.php?title=Acid&amp;action=edit&amp;redlink=1" class="new" title="Acid (page does not exist)">Acid</a>), histidine (<a href="//commons.wikimedia.org/w/index.php?title=Base_(chemistry)&amp;action=edit&amp;redlink=1" class="new" title="Base (chemistry) (page does not exist)">Base</a>) and the nucleophile (<a href="//commons.wikimedia.org/w/index.php?title=Nucleophile&amp;action=edit&amp;redlink=1" class="new" title="Nucleophile (page does not exist)">Nuc</a>). In the case on TEV protease, the acid is aspartate and the nucleophile is cysteine. (<span class="plainlinks"><a href="https://en.wikipedia.org/wiki/Protein_Data_Bank" class="extiw" title="w:Protein Data Bank">PDB</a>: <a rel="nofollow" class="external text" href="http://www.rcsb.org/pdb/explore/explore.do?structureId=1lvm">1lvm</a></span>​)
<a href="//commons.wikimedia.org/wiki/User:Evolution_and_evolvability" title="User:Evolution and evolvability">Thomas Shafee</a>
CC BY 4.0
<cite style="font-style:normal">Thomas, Shafee, (2014). "<a rel="nofollow" class="external text" href="https://www.repository.cam.ac.uk/handle/1810/245207">Evolvability of a viral protease: experimental evolution of catalysis, robustness and specificity</a>". <i>PhD Thesis</i>. University of Cambridge.</cite>