Radical iron sulfur dehydratases

The steadily increasing field of radical biochemistry is dominated by the large family of S-adenosylmethionine dependent enzymes, the so-called radical SAM enzymes, of which several new members are discovered every year. After one-electron reduction, SAM bound to an iron-sulfur cluster fragments into methionine and a 5'-deoxyadensine radical that induces the subsequent chemistry [1]. Here I report on 2- and 4-hydroxyacyl-CoA dehydratases which apply a very different method of radical generation. By one-electron reduction or oxidation the dehydratases form substrate-derived ketyl radicals that are recycled after each turnover without further energy input. The (R)-2-hydroxyacyl-CoA dehydratases that are involved in the fermentation of amino acids by anaerobic bacteria, especially clostridia, require activation by one-electron transfer from an iron-sulfur protein driven by hydrolysis of ATP. The proposed mechanism is highlighted by the identification of an allylic ketyl radical intermediate [2]. The crystal structure of 2-hydroxyisocapryloyl-CoA dehydratase from Clostridium difficile revealed a heterodimeric protein, which contains one [4Fe-4S] cluster in each subunit [3]. Each cluster is coordinated by three conserved cysteines. In the β-subunit the 4^th iron contains a sulfide ligand and a hydroxyl ligand in the α-subunit. The activator, also called ‘Archerase’, transfers or ‘shoots’ one electron into the β-cluster which serves as electron reservoir. As soon the substrate binds to the α-cluster by replacing the hydroxyl, the electron is further transferred to the α-cluster and reduces the thioester carbonyl to a ketyl radical that eliminates the hydroxyl group. Removal of the now acidic proton at C-3 and returning the electron back to the β-cluster yields the product isocaprenoyl-CoA. Reduction and CoA-transfer leads to isocaproate that is excreted by the organism. Biotechnologically significant are the similar conversions of alanine via lactyl-CoA and acryloyl-CoA to propionate and of glutamate via (R)-2-hydroxyglutaryl-CoA and glutaconyl-CoA to glutarate.