archaeal riboflavin synthase (RS); involved in the biosynthesis pathway of riboflavin (vitamin B2)
Archaeal RSs are homopentamers catalyzing the formation of riboflavin from 6,7-dimethyl-8-ribityllumazine in riboflavin biosynthesis. Divalent metal ions, preferably manganese or magnesium, are needed for maximum activity. Riboflavin serves as the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), essential cofactors for several oxidoreductases that are indispensable in most living cells. In the final steps of the riboflavin biosynthetic pathway, lumazine synthase (6,7-dimethyl-8-ribityllumazine synthase, LS) catalyzes the condensation of the 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione with 3,4-dihydroxy- 2-butanone-4-phosphate to release water, inorganic phosphate and 6,7-dimethyl-8-ribityllumazine (DMRL), followed by RS which catalyzes a dismutation of DMRL yielding riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione. In the latter reaction, a four-carbon moiety is transferred between two DMRL molecules serving as donor and acceptor, respectively. Both the LS and RS catalyzed reactions are thermodynamically irreversible and can proceed in the absence of a catalyst. Archaeal RSs share sequence similarity with LSs, both appear to have diverged early in the evolution of archaea from a common ancestor.
Comment:Archaeal riboflavin synthases form homopentamers, each homopentamer contains 5 active sites which are located at the interfaces between adjacent monomers.
Structure:2B99_A/B/C/D/E: Methanocaldococcus jannaschii riboflavin synthase bound with substrate analog inhibitor, 6,7-dioxo-8-ribityllumazine; contacts at 4A.
Comment:Two substrate analogue molecules are bound in each active site with a topology in line with the stereochemistry of an intermediate.
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