Abstract
Mononuclear non-heme iron-dependent (NHI) enzymes catalyze an array of chemical transformations including hydroxylation, halogenation, and epimerization as well as both cyclization and ring cleavage reactions. These transformations occur in primary and secondary metabolism. A subset of this group of enzymes activates O2 without requiring cosubstrates, cofactors, or reductants. These enzymes extract all four electrons from their organic substrates for complete reduction of oxygen. This article focuses on two enzymes involved in phosphonate biosynthesis that utilize a ferric-superoxo intermediate to initiate catalysis and that involve a subsequent ferryl intermediate for a second step of oxidation. Both enzymes catalyze carbon-carbon bond scission in the same substrate 2-hydroxyethyl phosphonate leading to two products, but the extents of oxidation of the two products is different. One enzyme, 2-hydroxyethylphosphonate dioxygenase, is part of the biosynthesis of the commercially used herbicide phosphinothricin, whereas the second enzyme, methyl phosphonate synthase, catalyzes an important step in the carbon and phosphorus cycles in the oceans. Studies have shown that these two enzymes have a common mechanism up to a branch point and that branching is controlled by a single second sphere amino acid. The article discusses the evolution of the mechanistic and structural understanding of these enzymes.