The enzyme binds a [4Fe-4S] and a [2Fe-2S] cluster. In every reaction cycle, the enzyme consumes two molecules of AdoMet, each producing 5'-deoxyadenosine (dAH) and a putative dethiobiotinyl carbon radical. The first half reaction results in the hydrogen abstraction from C6 in dithiobiotin (DTB) and initial sulfur addition. Reaction with another equivalent of AdoMet results in abstraction of the C6 methylene pro-S hydrogen atom from 9-mercaptodethiobiotin (MTDB), and the resulting carbon radical is quenched via formation of an intramolecular C-S bond, thus closing the biotin thiophane ring. The sulfur donor [S] is believed to be the [2Fe-2S] cluster, which is sacrificed in the process, so that in vitro the reaction is a single turnover. In vivo, the [2Fe-2S] cluster can be reassembled by the Isc or Suf iron-sulfur cluster assembly systems, to allow further catalysis.

It has been demonstrated that biotin formation proceeds in a stepwise manner, with only one monomer within the dimeric enzyme undergoing turnover to form biotin. The second monomer binds substrates, but does not undergo a reaction, i.e. the enzyme is half-site active. Further, the dAH and methionine generated during the first half-reaction (conversion of DTB to MDTB) can competitively compete with AdoMet during the second half-reaction, preventing the conversion of MDTB to biotin.

No notes.

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