IN-VIVO FORMATION OF C-S BONDS IN BIOTIN - AN EXAMPLE OF RADICAL CHEMISTRY UNDER REDUCING CONDITIONS

The last step in the biosynthesis of biotin involves the formation of carbon-sulfur bonds at nonactivated carbons catalyzed by biotin syntha se. S-Adenosylmethionine (AdoMet) and an electron source are essential for activity in cell-free systems. This important finding connects bi otin synthase to a family of enzymes, namely pyruvate-formate lyase, a naerobic ribonucleotide reductase and lysine 2,3-aminomutase, which us e the same cofactors. Additional experimental data led to the proposit ion of the following general mechanism. The carbons to be functionaliz ed are first activated by homolytic cleavage of the C-H bonds, initiat ed by the deoxyadenosyl radical produced by a monoelectronic reductive cleavage of AdoMet with NADPH as electron source. The electron transf er system involves flavoproteins and very likely the [Fe-S] center of biotin synthase. NADPH and the flavoproteins can be replaced by photor educed deazaflavin. By using a deuterated substrate, a deuterium trans fer into deoxyadenosine has been observed, indicating that biotin synt hase should be closely related to lysine 2,3-aminomutase, which uses A doMet as a surrogate of vitamin B12. The source of sulfur, the nature of the immediate sulfur donor and hence the mechanism of trapping of t he intermediate radicals are still unknown. (C) 1998 John Wiley & Sons , Ltd.