<p>Biotin synthase <db_xref db="EC" dbkey="2.8.1.6"/> works with flavodoxin, S-adenosylmethionine, and possibly cysteine to catalyze the last step of the biotin biosynthetic pathway. The reaction consists of the introduction of a sulphur atom intodethiobiotin, thus requiring activation of C-H bonds [<cite idref="PUB00002298"/>]. Biotin (vitamin H) is a prosthetic group in enzymes catalysing carboxylation and transcarboxylation reactions [<cite idref="PUB00001863"/>].</p> <p> Biotin synthase from <taxon tax_id="562">Escherichia coli</taxon> is a homodimer of 76 kDa, with each polypeptidechain carrying an oxygen-sensitive (4Fe-4S) cluster, probably ligated by three cysteines of a CXXXCXXC box conserved among all known BioB sequencesand a fourth still not identified ligand. BioB displays a pyridoxal phosphate-dependent cysteine desulphurase activity, which allows mobilisation of thesulphur atom from free cysteine [<cite idref="PUB00007197"/>]. </p>