Iron-sulfur clusters of biotin synthase in vivo: a Mössbauer study.

Biotin synthase, the enzyme that catalyzes the last step of the biosynthesis of biotin, contains only 2Fe-2S clusters when isolated under aerobic conditions. Previous results showed that reconstitution with an excess of FeCl(3) and Na(2)S under reducing and anaerobic conditions leads to either 4Fe-4S, 4Fe-4S, or a mixture of 4Fe-4S and 2Fe-2S clusters. To determine whether any of these possibilities or other different cluster configuration could correspond to the physiological in vivo state, we have used (57)Fe Mössbauer spectroscopy to investigate the clusters of biotin synthase in whole cells. The results show that, in aerobically grown cells, biotin synthase contains a mixture of 4Fe-4S and 2Fe-2S clusters. A mixed 4Fe-4S:2Fe-2S cluster form has already been observed under certain in vitro conditions, and it has been proposed that both clusters might each play a significant role in the mechanism of biotin synthase. Their presence in vivo is now another argument in favor of this mixed cluster form.