A modified amino acid that is the target of diphtheria toxin is assembled through some unusual enzyme reactivity, a multi-institution team has found (Nature 2010, 465, 891). Researchers have known for decades about the molecular target, called diphthamide. But its biosynthesis had remained unclear. Steven E. Ealick, Jack Freed, and Hening Lin of Cornell University in collaboration with Carsten Krebs of Pennsylvania State University and coworkers have revealed the structure and chemistry of a novel iron-sulfur enzyme that they say catalyzes the first step of diphthamide biosynthesis in a microorganism. Diphthamide, which is found on a protein factor used during translation, is a histidine residue modified with help from the cofactor (S)-adenosyl methionine (SAM). The team’s work suggests the new enzyme’s 4Fe-4S cluster transfers an electron to SAM to generate a 3-amino-3-carboxypropyl radical intermediate, which goes toward making diphthamide. Similar iron-sulfur enzymes all generate a 5´-deoxyadenosyl radical from SAM instead, so the new enzyme represents “a remarkable adaptation of hallmark reactivity,” Krebs says. Eventually, the team hopes to understand how many different enzymes generate different reactive species from SAM.