Characterization of the Cofactor Composition of Escherichia coli Biotin Synthase

The cofactor content of in vivo, as-isolated, and reconstituted forms of recombinant Escherichia coli biotin synthase (BioB) has been investigated using the combination of UV-visible absorption, resonance Raman, and Mössbauer spectroscopies along with parallel analytical and activity assays. In contrast to the recent report that E. coli BioB is a pyridoxal phosphate (PLP)-dependent enzyme with intrinsic cysteine desulfurase activity (Ollagnier-deChoudens, S., Mulliez, E., Hewitson, K. S., and Fontecave, M. (2002) Biochemistry 41, 9145-9152), no evidence for PLP binding or for PLP-induced cysteine desulfurase or biotin synthase activity was observed with any of the forms of BioB investigated in this work. The results confirm that BioB contains two distinct Fe-S cluster binding sites. One site accommodates a [2Fe-2S]²⁺ cluster with partial noncysteinyl ligation that can only be reconstituted in vitro in the presence of O₂. The other site accommodates a [4Fe-4S]^2+,+ cluster that binds S-adenosylmethionine (SAM) at a unique Fe site of the [4Fe-4S]²⁺ cluster and undergoes O ₂-induced degradation via a distinct type of [2Fe-2S]²⁺ cluster intermediate. In vivo Mössbauer studies show that recombinant BioB in anaerobically grown cells is expressed exclusively in an inactive form containing only the as-isolated [2Fe-2S]²⁺ cluster and demonstrate that the [2Fe-2S]²⁺ cluster is not a consequence of overexpressing the recombinant enzyme under aerobic growth conditions. Overall the results clarify the confusion in the literature concerning the Fe-S cluster composition and the in vitro reconstitution and O₂-induced cluster transformations that are possible for recombinant BioB. In addition, they provide a firm foundation for assessing cluster transformations that occur during turnover and the catalytic competence of the [2Fe-2S]²⁺ cluster as the immediate S-donor for biotin biosynthesis.