Biotin synthase

Biotin synthase (BioB) (EC 2.8.1.6) is an enzyme that catalyzes the conversion of dethiobiotin (DTB) to biotin; this is the final step in the biotin biosynthetic pathway. Biotin, also known as vitamin B7, is a cofactor used in carboxylation, decarboxylation, and transcarboxylation reactions in many organisms including humans. Biotin synthase is an S-Adenosylmethionine (SAM) dependent enzyme that employs a radical mechanism to thiolate dethiobiotin, thus converting it to biotin.dethiobiotin + sulfur + 2 S-adenosyl-L-methionine ⇌ {displaystyle ightleftharpoons } biotin + 2 L-methionine + 2 5'-deoxyadenosine Biotin synthase (BioB) (EC 2.8.1.6) is an enzyme that catalyzes the conversion of dethiobiotin (DTB) to biotin; this is the final step in the biotin biosynthetic pathway. Biotin, also known as vitamin B7, is a cofactor used in carboxylation, decarboxylation, and transcarboxylation reactions in many organisms including humans. Biotin synthase is an S-Adenosylmethionine (SAM) dependent enzyme that employs a radical mechanism to thiolate dethiobiotin, thus converting it to biotin. This radical SAM enzyme belongs to the family of transferases, specifically the sulfurtransferases, which transfer sulfur-containing groups. The systematic name of this enzyme class is dethiobiotin:sulfur sulfurtransferase. This enzyme participates in biotin metabolism. It employs one cofactor, iron-sulfur. In 2004, the crystal structure of biotin synthase in complex with SAM and dethiobiotin was determined to 3.4 angstrom resolution. The PDB accession code for this structure is 1R30. The protein is a homodimer, meaning it is composed of two identical amino acid chains that fold together to form biotin synthase. Each monomer in the structure shown in figure contains a TIM barrel with an 2+cluster, SAM, and an 2+cluster. The 2+cluster is used as a catalytic cofactor, directly coordinating to SAM.  Orbital overlap between SAM and a unique Fe atom on the 2+cluster has been observed. The predicted role of the 2+cofactor is to transfer an electron onto SAM through an inner sphere mechanism, forcing it into an unstable high energy state that ultimately leads to the formation of the 5’deoxyadenosyl radical. The 2+cluster is thought to provide a source of sulfur from which to thiolate DTB. Isotopic labelling and spectroscopic studies show destruction of the 2+cluster accompanies BioB turnover, indicating that it is likely sulfur from 2+that is being incorporated into DTB to form biotin.