Monooxygenase enzymes incorporating an oxygen-bridged diiron cluster in the catalytic site represent a new class of enzymes that perform β-hydroxylation of amino acids, reports a research group led by Thomas M. Makris and John D. Lipscomb of the University of Minnesota, Minneapolis (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1007953107). In natural products used as chemotherapeutic and antimicrobial agents, hydroxylation at the β-carbon of amino acids is common. These hydroxylations were previously known to be catalyzed only by monoiron α-ketoglutarate-dependent dioxygenases or cytochrome P450 monooxygenases. Lipscomb’s group hypothesized that diiron enzymes could catalyze the same reaction, and they found that enzymes in several biosynthetic pathways had sequences that were consistent with the presence of dinuclear metal sites. Detailed studies of CmlA, an enzyme that facilitates the β-hydroxylation step in the biosynthesis of the antibiotic chloramphenicol, revealed that the enzyme indeed contains a diiron center and has a structure previously unseen in monooxygenases. A better understanding of these enzymes and their mechanism will be important in efforts to develop nonnatural products through biosynthetic engineering, says biochemist Jason Micklefield of the University of Manchester, in England.