Structures and reaction mechanisms of riboflavin synthases of eubacterial and archaeal origin

Abstract

The biosynthesis of one riboflavin molecule requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates. GTP is hydrolytically opened, converted into 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione by a sequence of deamination, side chain reduction and dephosphorylation. Condensation with 3,4-dihydroxy-2-butanone 4-phosphate obtained from ribulose 5-phosphate leads to 6,7-dimethyl-8-ribityllumazine. The dismutation of 6,7-dimethyl-8-ribityllumazine catalysed by riboflavin synthase produces riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione. A pentacyclic adduct of two 6,7-dimethyl-8-ribityllumazines has been identified earlier as a catalytically competent reaction intermediate of the Escherichia coli enzyme. Acid quenching of reaction mixtures of riboflavin synthase of Methanococcus jannaschii, devoid of similarity to riboflavin synthases of eubacteria and eukaryotes, afforded a compound whose optical absorption and NMR spectra resemble that of the pentacyclic E. coli riboflavin synthase intermediate, whereas the CD spectra of the two compounds have similar envelopes but opposite signs. Each of the compounds could serve as a catalytically competent intermediate for the enzyme by which it was produced, but not vice versa. All available data indicate that the respective pentacyclic intermediates of the M. jannaschii and E. coli enzymes are diastereomers. Whereas the riboflavin synthase of M. jannaschii is devoid of similarity with those of eubacteria and eukaryotes, it has significant sequence similarity with 6,7-dimethyl-8-ribityllumazine synthases catalysing the penultimate step of riboflavin biosynthesis. 6,7-Dimethyl-8-ribityllumazine synthase and the archaeal riboflavin synthase appear to have diverged early in the evolution of Archaea from a common ancestor.

Transcriptomes of the Extremely Thermoacidophilic Archaeon Metallosphaera sedula Exposed to Metal “Shock” Reveal Generic and Specific Metal Responses

“…Violet, white, and green represent above average, average, and below average transcription levels, respectively. involvement in redox reactions (85,86), with an important connection to scavenging of reactive oxygen species being observed in Ashbya gossypii (87). Zn is known to induce the expression of proteins with protective functions (88) and is essential in redox homeostasis (77).…”

Evolution of Vitamin B2Biosynthesis: 6,7-Dimethyl-8-Ribityllumazine Synthases ofBrucella

“…6). Notably, these archaeal riboflavin synthases are evolutionarily old and have no detectable lumazine synthase activity (13,15,45). Completely sequenced archaeal genomes typically comprise sets of two similar genes, coding for a lumazine synthase-like riboflavin synthase and a regular lumazine synthase.…”

“…From this analysis, depicted in Fig. 5, a completely new picture emerges, showing that in addition to eubacterial type I lumazine synthases and archaeal lumazine synthases, this folding gave origin to at least two new evolutionarily related functions: the archaeal riboflavin synthases, which are lumazine synthaselike riboflavin synthases with no detectable lumazine synthase activity (13,15,45), and the eubacterial type II lumazine synthases.…”

Adaptation of Lactobacillus rhamnosus riboflavin assay to microtiter plates

“…Riboflavin deficiency is also considered a potential risk factor for cancer, although this has not been demonstrated in humans (Powers, 2003). Some eubacteria and archea organisms are able to synthesize riboflavin (Fischer et al, 2005). However, for humans, riboflavin is essential and must be supplemented with their diets (McCormick, 1989).…”