lumazine synthase (6,7-dimethyl-8-ribityllumazine synthase, LS), catalyzes the penultimate step in the biosynthesis of riboflavin (vitamin B2); type-II
Type-II LS also known as RibH2, catalyzes the penultimate step in the biosynthesis of riboflavin in plants and microorganisms. LS catalyses the formation of 6,7-dimethyl-8-ribityllumazine by the condensation of 5-amino-6-ribitylamino- 2,4(1H,3H)-pyrimidinedione with 3,4-dihydroxy- 2-butanone-4-phosphate. Subsequently, the lumazine intermediate dismutates yielding riboflavin and 5-amino-6-ribitylamino- 2,4(1H,3H)-pyrimidinedione, in a reaction catalyzed by riboflavin synthase (RS); RS belongs to a different family of the Lumazine-synthase-like superfamily. 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. Riboflavin is biosynthesized in plants, fungi and certain microorganisms; as animals lack the necessary enzymes to produce this vitamin, they acquire it from dietary sources. Type II LSs are distinct from type-I LS not only in protein sequence, but in that they exhibit different quaternary assemblies; type-II LSs form decamers (dimers of pentamers). The pathogen Brucella spp. have both a type-I LS and a type-II LS called RibH1 and RibH2, respectively. RibH1/type-I LS appears to be a functional LS in Brucella spp., whereas RibH2/type-II LS has much lower catalytic activity as LS and may be regulated by a riboswitch that senses FMN, suggesting that the type-II LSs may have evolved into very poor catalysts or, that they may harbor a new, as-yet-unknown function.
Comment:Type II LSs form homopentamers, each pentamer contains 5 active sites which are located at the interfaces between adjacent monomers.
Structure:1T13_A/B/C/D/E; Brucella abortus RibH2/type-II LS pentamer binds the substrate analogue inhibitor, 5-nitro-6-(D-ribitylamino)-2,4(1h,3h) pyrimidinedione and a phosphate ion; contacts at 4A
Jiyao W et al.(2023). "The conserved domain database in 2023.",