Investigations were performed with regard to the function of the iron—sulfur cluster of l-serine dehydratase from Peptostreptococcus asaccharolyticus, an enzyme which is novel in the class of deaminating hydro-lyases in that it lacks pyridoxal-5′-phosphate. Anaerobically purified l-serine dehydratase from P. asaccharolyticus revealed EPR spectra characteristic of a [3Fe–4S]+ cluster constituting 1% of the total enzyme concentration. Upon incubation of the enzyme under air the intensity of the [3Fe–4S]+ signal increased correlating with the loss of enzymatic activity. Addition of l-serine prevented this. Hence, active l-serine dehydratase probably contains a diamagnetic [4Fe–4S]2+ cluster which is converted by oxidation and loss of one iron ion to a paramagnetic [3Fe–4S]+ cluster, resulting in inactivation of the enzyme. In analogy to the mechanism elucidated for aconitase, it is proposed that l-serine is coordinated via its hydroxyl and carboxyl groups to the labile iron atom of the [4Fe–4S]2+ cluster.
