title: Biosynthesis of vitamin B2: a unique way to assemble a xylene ring.
abstract: The biosynthesis of one riboflavin (vitamin B(2)) molecule requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates. In the final step, the tricyclic isoalloxazine chromophore, which is the hallmark of flavocoenzymes, arises from a highly unusual dismutation of bicyclic 6,7-dimethyl-8-ribityllumazine that is catalyzed by riboflavin synthase but can also proceed without catalysis. The reaction proceeds via a pentacyclic adduct of two 6,7-dimethyl-8-ribityllumazine molecules, whose cleavage into riboflavin and a pyrimidine derivative (by a sequence of two elimination steps) is mechanistically straightforward. Recently, the formation of the pentacyclic adduct has been proposed to involve a hydride transfer step followed by a [4+2] cycloaddition. Surprisingly, two different classes of riboflavin synthases utilize different diastereomers of the pentacyclic adduct, but the newly generated chiral centers are lost upon the intermediates' subsequent fragmentation.
authors.affiliation: Institute of Food Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany. markus.fischer@chemie.uni-hamburg.de
authors.forename: Markus
authors.initials: M
authors.lastname: Fischer
authors.forename: Adelbert
authors.initials: A
authors.lastname: Bacher
mesh_terms: D000595:Amino Acid Sequence; D001105:Archaea / Q000201:enzymology; D001419:Bacteria / Q000201:enzymology; D005658:Fungi / Q000201:enzymology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D012256:Riboflavin / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism; D012258:Riboflavin Synthase / Q000378:metabolism; D016415:Sequence Alignment; D014992:Xylenes / Q000737:chemistry / Q000378:metabolism
chemical_list: D014992:Xylenes; D012258:Riboflavin Synthase; D012256:Riboflavin