The 2-His-1-carboxylate facial triad--an emerging structural motif in mononuclear non-heme iron(II) enzymes.

A 2-His-1-carboxylate facial triad is a common feature of the active sites in a number of mononuclear non-heme iron(II) enzymes. This structural motif was established crystallographically for five different classes of enzymes and inferred from sequence similarity for two other classes. The 2-His-1-carboxylate facial triad anchors the iron in the active site and at the same time maintains three additional cis-oriented sites. These sites can be used to bind other endogenous ligands or exogenous ligands such as substrate and/or O2, giving the metal center great flexibility to use different mechanistic strategies to perform a variety of chemical transformations.