Riboflavin biosynthesis protein

On this basis, the proteins of riboflavin biosynthesis may be relevant targets for the development of novel antibacterial agents, notably with selectivity for mycobacteria, Enterobacteriaceae, or pathogenic yeasts. Since these proteins are absent in human and animal hosts, the risks of off-target toxicity should be comparatively low. 

Guanosine triphosphate and ribulose-5-phosphate are recruited in a 1 2 stoichiometric ratio by GTP cyclohydrolase II and DHBP synthase, respectively, for riboflavin biosynthesis. Since at substrate saturation the activity of B. subtilis DHBP is twice the activity of B. suhtilis cyclohydrolase II (DSM, unpublished observations) and since both enzymatic activities are associated with the same bifunctional protein encoded by rihA, the balanced formation of the pyrimidinedione and the dihydroxybutanone intermediates is ensured. However, the ifg.s constant of DHBP synthase ( 1 mmol is about 100-fold higher than the ifg.s constant of GTP cyclohydrolase II imposing the risk of excessive synthesis of the pyrimidinone and pyrimidinedione intermediates in case of reduced intracellular concentrations of pentose phosphate pathway intermediates. This can be expected, for instance, in glucose-limited fed-batch fermentations, which are frequentiy used in industrial applications. The pyrimidinone and pyrimidinedione intermediates are highly reactive, oxidative compounds, which can do serious damage on the bacteria. 

Phylogenetic analysis of the quaternary arrangements and sequences of lumazine synthases and related pentameric riboflavin synthases derived from archaea, bacteria, plants, and fungi suggests a family of proteins composed of four different clades. These include the archaeal lumazine and riboflavin synthases, type I lumazine synthases, and the eubacterial type II lumazine synthases. We could thus construct a picture of the evolution of the latter steps of riboflavin biosynthesis where initially there were two enzymes catalyzing the riboflavin synthase reaction. The archaeal riboflavin synthase evolved into the different lumazine synthases and the other enzyme evolved into the trimeric enzymes found in present day eubacterial and eukaryotic riboflavin synthases. This would explain how the two different enzymes can use two different diastereomeric intermediates in the generation of riboflavin. That the lumazine synthase evolved from the riboflavin synthases is supported by the finding that it still binds riboflavin. Perhaps the fact that two different enzymes have evolved for this last step in riboflavin reflects on the ease at which riboflavin is formed nonenzymatically. ... 

Several strategies to increase the production of electron shuttles have been developed to improve the MFC performance in the model exoeleetrogens. For Shewanella species, flavins (riboflavin and flavin mononucleotide) are the most well-known self-secreted electron shuttles. Using deletion mutants lacking various Mtr-associated proteins, the significance of the Mtr extracellular respiratory pathway for the reduction of flavins has been demonstrated. The decaheme cytochromes found on the outer surface of the cell (MtrC and OmcA) are required for the majority of Mtr-associated proteins activity. Weakly acidic pH resulted in poor performance of the MFC and low riboflavin concentrations in the bacterial cultures, while enhanced electrochemical activity of riboflavin was reported at alkaline pH. The increase of riboflavin biosynthesis by Shewanella at the alkaline condition underlies the improvement in the electricity output in MFCs. ... 

In terms of coenzyme evolution, it is also noteworthy that the biosynthesis of a given coenzyme frequently requires the cooperation of other coenzymes. For example, the biosynthesis of riboflavin (24) requires tetrahydrofolate (33) for the biosynthesis of GTP serving as precursor (Fig. 3). Pyridoxal 5 -phosphate is required for the biosynthesis of the activated pyrosulfide type protein (2) that serves as the common precursor for iron/sulfur clusters and various sulfur-containing organic coenzymes (Fig. 1). 

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