Discovery of a Biotin Synthase That Utilizes an Auxiliary 4Fe–5S Cluster for Sulfur Insertion

Abstract

Biotin synthase (BioB) is a member of the Radical SAM superfamily of enzymes that catalyzes the terminal step of biotin (vitamin B7) biosynthesis, in which it inserts a sulfur atom in desthiobiotin to form a thiolane ring. How BioB accomplishes this difficult reaction has been the subject of much controversy, mainly around the source of the sulfur atom. However, it is now widely accepted that the sulfur atom inserted to form biotin stems from the sacrifice of the auxiliary 2Fe−2S cluster of BioB. Here, we bioinformatically explore the diversity of BioBs available in sequence databases and find an unexpected variation in the coordination of the auxiliary iron−sulfur cluster. After in vitro characterization, including the determination of biotin formation and representative crystal structures, we report a new type of BioB utilized by virtually all obligate anaerobic organisms. Instead of a 2Fe−2S cluster, this novel type of BioB utilizes an auxiliary 4Fe−5S cluster. Interestingly, this auxiliary 4Fe−5S cluster contains a ligated sulfide that we propose is used for biotin formation. We have termed this novel type of BioB, Type II BioB, with the E. coli 2Fe−2S cluster sacrificial BioB representing Type I. This surprisingly ubiquitous Type II BioB has implications for our understanding of the function and evolution of Fe−S clusters in enzyme catalysis, highlighting the difference in strategies between the anaerobic and aerobic world.

Iron-sulfur protein odyssey: exploring their cluster functional versatility and challenging identification

“…The biotin synthases that catalyze the conversion of dethiobiotin to biotin possess generally a [2Fe-2S] sacrificial auxiliary cluster (although a [4Fe-5S] cluster containing a ligated sulfide that is proposed to be used for biotin formation has been recently identified in biotin synthase of all obligate anaerobic organisms 105 ) that is the source of sulfur atoms required for biotin formation. 19 This synthesis starts with reductive cleavage of a SAM molecule, which is ligated to the [4Fe-4S] cluster, in L-methionine and 5ʹ-dA• (Fig.…”

Topological structures and adsorption properties of the [Fe4S4] clusters

“…For example, the nitrogenase protein can activate small molecules including N 2 , CO 2 and CO under ambient conditions, which mainly rely on the versatility of their Fe 4 S 4 centers [7]. Enzymes involved in nucleotide metabolism are increasingly reported to contain Fe-S clusters [8][9][10]. Nowadays Fe-S clusters are the widely used inorganic iron cofactors in enzymes, with functions such as reducing disulfide bonds, initiating or stabilizing radical chain reactions, or acting as Lewis acids [11,12].…”