<a href="//commons.wikimedia.org/w/index.php?title=Serine_protease&amp;action=edit&amp;redlink=1" class="new" title="Serine protease (page does not exist)">Serine proteases</a> operate by a two-step catalytic mechanism. First, the <a href="//commons.wikimedia.org/w/index.php?title=Substrate_(chemistry)&amp;action=edit&amp;redlink=1" class="new" title="Substrate (chemistry) (page does not exist)">substrate</a> (blue) is <a href="//commons.wikimedia.org/w/index.php?title=Nucleophilic_attack&amp;action=edit&amp;redlink=1" class="new" title="Nucleophilic attack (page does not exist)">attacked</a> by the <a href="//commons.wikimedia.org/w/index.php?title=Catalytic_triad&amp;action=edit&amp;redlink=1" class="new" title="Catalytic triad (page does not exist)">catalytic triad</a> (red) to form an <a href="//commons.wikimedia.org/w/index.php?title=Acyl-enzyme_intermediate&amp;action=edit&amp;redlink=1" class="new" title="Acyl-enzyme intermediate (page does not exist)">acyl-enzyme intermediate</a>. The intermediate is typically resolved by <a href="//commons.wikimedia.org/w/index.php?title=Hydrolysis&amp;action=edit&amp;redlink=1" class="new" title="Hydrolysis (page does not exist)">hydrolysis</a> by water. However, when the reactive centre loop (RCL) of a serpin is attacked, the <a href="//commons.wikimedia.org/w/index.php?title=Conformational_change&amp;action=edit&amp;redlink=1" class="new" title="Conformational change (page does not exist)">conformational change</a> (blue arrow) pulls the catalytic triad out of position, preventing it from completing catalysis. (Based on <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=1K9O">1K9O</a>, <a rel="nofollow" class="external text" href="http://www.rcsb.org/pdb/explore/explore.do?structureId=1EZX">1EZX</a></span>​)
<a href="//commons.wikimedia.org/wiki/User:Evolution_and_evolvability" title="User:Evolution and evolvability">Thomas Shafee</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>