lumazine synthase (6,7-dimethyl-8-ribityllumazine synthase, LS); catalyzes the penultimate step in the biosynthesis of riboflavin (vitamin B2)
Archaeal LS is an important enzyme in the riboflavin biosynthetic pathway. Riboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) which are essential cofactors for the catalysis of a wide range of redox reactions. These cofactors are also involved in many other processes involving DNA repair, circadian time-keeping, light sensing, and bioluminescence. In the final steps of the riboflavin biosynthetic pathway 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), and riboflavin synthase (RS) catalyzes a dismutation of DMRL which yields 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. LS from Methanococcus jannaschii forms capsids with icosahedral 532 symmetry consisting of 60 subunits. Archaeal LSs share sequence similarity with archaeal RSs, both appear to have diverged early in the evolution of archaea from a common ancestor.
Comment:based on other superfamily members with structure
Comment: The basic building block of enzymes belonging to this superfamily is a pentamer; members exhibit different quaternary assemblies between species.
Comment:Each pentamer contains 5 active sites which are located at the interfaces between adjacent monomers.
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