Antibodies can be designed so their binding regions mimic enzyme active sites.  The paper “Immune Versus Natural Selection: Antibody Aldolases with Enzymic Rates but Broader Scope” investigates one such antibody.    The antibody 33F12 (Ab 33F12) is a catalytic Ab with aldolase activity.  Aldolases typically catalyze an aldo condensation reaction which is the joining of two carbonyl containing molecules (i.e. aldehyde or ketone) to form an aldol (beta-hydroxy-carbonlyl).
      The basic aldolase mechanism involves a reactive lysine on the enzyme which serves as a nucleophile and attacks the carbonyl carbon of the first molecule (aldehyde or keytone) to form a Schiff base.   The Schiff base collapses through an enamine intermediate as a second carbonyl is attacked.   The lysine forms a second Schiff base, which allows the aldol to be hydrated and the free enzyme is restored.  (see the mechanism below).