Unusual dehydrations in anaerobic bacteria

In amino acid fermenting anaerobic bacteria a set of unusual dehydratases is found which use 2-hydroxyacyl-CoA, 4-hydroxybutyryl-CoA or 5-hydroxyvaleryl-CoA as substrates. The extremely oxygen-sensitive 2-hydroxyacyl-CoA dehydratases catalysing the elimination of water from (R)-lactyl-CoA to acryloyl-CoA or from (R)-2-hydroxyglutaryl-CoA to glutaconyl-CoA contain iron-sulfur clusters as well as riboflavin and require additional activation by ATP. The dehydration of 4-hydroxybutyryl-CoA to crotonyl-CoA is catalysed by a moderately oxygen-sensitive enzyme also containing an iron-sulfur cluster and FAD. In all these reactions a non-activated C-H-bond at C3 has to be cleaved by mechanisms not yet elucidated. The dehydration of 5-hydroxyvaleryl-CoA to 4-pentenoyl-CoA, however, has been characterised as a redox process mediated by enzyme-bound FAD. Finally, an iron-sulfur cluster-containing but pyridoxal-phosphate-independent l-serine dehydratase is described.