pyridoxal biosynthesis lyase ..
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Pyridoxal biosynthesis lyase ..
The knowledge pertaining to the vitamin B6 pathway in pathogenic bacteria is very limited. Recently, it has been shown that the vitamin B6 biosynthesis is essential for virulence of M. tuberculosis () and H. pylori (). However, in our mouse infection experiment, the ΔpdxS mutant only showed a moderate attenuation and is not statistically significant. It is likely that the levels of vitamin B6 in the lungs are high enough to compensate the vitamin B6 defect of ST2676, as an extracellular pathogen. In contrast, the infection niche of M. tuberculosis differs from S. pneumoniae. M. tuberculosis encounters a relatively low vitamin B6 environment during intracellular replication. This hypothesis was supported by the coinfection experiment that vitamin B6 synthase gene affects the infection of S. pneumoniae in an OM model. Thus, the contribution of vitamin B6 in bacterial pathogenesis largely depends upon the particular infection niche. Understanding how bacterial metabolism pathways impact on infection will provide a molecular basis antimicrobial therapy.
PLP-dependent enzymes are primarily involved in the biosynthesis of amino acids and amino acid-derived metabolites, but they are also found in the biosynthetic pathways of amino sugars and in the synthesis or catabolism of neurotransmitters; pyridoxal phosphate can also inhibit DNA polymerases and several steroid receptors .
Pyridoxal biosynthesis lyase PdxS ..
In addition to a de novo vitamin B6 biosynthesis pathway, bacteria also have a salvage pathway that converts the B6 vitamers taken from the environment into PLP. It is interesting that various bacteria differ in usage of these vitamers. Our results showed that S. pneumoniae could utilize all of the vitamers we tested including PM, PN, PL, and PLP. However, 100 μM PLP is toxic to this bacterium, whereas E. coli grew well with up to 0.5 mM PLP in our study (not shown). When low concentration of a vitamer was supplemented for the growth of the S. pneumoniae ΔpdxS mutant, bacteria grew preferentially with the vitamers other than PN. This result coincides with what was observed in B. subtilis (, ). Mutants of the pdxS orthologs in M. tuberculosis complex bacteria grew well with PN or PL but very poorly with PM or PLP (; G. Bai, unpublished data). It is likely that these bacteria differ in the uptake of B6 vitamers or that they are unable to convert certain vitamers to PLP, which warrants further investigation.
A paradigm of DXP-independent vitamin B6 biosynthesis pathway includes two enzymes: PdxS and PdxT. This pathway has been well described in B. subtilis. In this bacterium, PdxS and PdxT physically form a complex as PLP synthase (–, , ), which is also structurally stabilized by glutamine (–). The PdxS and PdxT complex is required for glutaminase activity, which converts glutamine into ammonium (, ). In addition, a ΔpdxT mutant of this bacterium grows well in the presence of sufficient ammonium in the medium (), supporting that PdxT is not required for PLP synthase if ammonium is available.x
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The vitamin B6 biosynthesis pathways have been studied in several bacteria, but the knowledge regarding regulation of the PLP levels remains very limited. We grew S. pneumoniae WT in complete CDM and then incubated the bacteria in either depleted CDM or supplemented CDM with 10 μM PLP. The expression levels of PdxS were then determined by Western blotting with antibody against pneumococcal PdxS. Our result showed that PdxS was weakly expressed in CDM supplemented with PLP, but it was abundant after incubation for 2 to 4 h in the absence of PLP (), suggesting a feedback inhibition of vitamin B6 biosynthesis by PLP.
It has been reported that Cercospora nicotianae pdx1 complements an E. coli pdxJ mutation (). We also investigated whether heterogeneous expression of S. pneumoniae pdxS or pdxT could complement E. coli mutants defective in vitamin B6 biosynthesis. The ORFs of both pdxS and pdxT were cloned into plasmid pGB104 to express these genes under the control of the E. coli crp promoter. These constructs and the control plasmid pGB104 were then individually transformed into E. coli ΔpdxA (ec048) or ΔpdxJ (ec053). These mutants and their derivatives were compared to the WT strain (ec022) for growth either in M9 broth ( and ) or on M9 plates (see Fig. S2 in the supplemental material), in the presence or absence of 0.1 mM PLP or PN. The results showed that neither ec048 nor ec053 grew with the minimal media in the absence of B6 vitamers, but both the mutants grew similarly to their parental strains by the addition of exogenous PN or PLP. In the absence of B6 vitamers, the growth defect of both the mutants could be restored by the expression of pneumococcal pdxS, but not by the control plasmid ( and ) or expression of pdxT (data not shown). This result also suggests that pneumococcal PdxS is capable of producing PLP independent of PdxT in the presence of ammonium ().
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Vitamin B6 is an essential cofactor for various enzymes and is required for maintenance of the nervous and immune systems in animals and humans (Bender, 1989). One of the biologically active forms of vitamin B6 is pyridoxal 5′-phosphate (PLP), which is essential for many enzyme reactions such as transamination, decarboxylation, racemization and elimination in amino-acid metabolism, DNA biosynthesis and biosynthesis of antibiotic compounds (Eliot & Kirsch, 2004; Mehta & Christen, 2000; Percudani & Peracchi, 2003). There are two pathways for de novo PLP biosynthesis and both pathways exist in bacteria, fungi, protozoa and plants, whereas they are missing in mammals (Belitsky, 2004; Dong et al., 2004; Osmani et al., 1999). The first pathway is employed by many eubacteria to produce PLP from erythrose 4-phosphate and 1-deoxy--xylulose 5-phosphate (Cane et al., 1999; Laber et al., 1999). In the second pathway, PLP is synthesized from ribose 5-phosphate (R5P) or its isomer ribulose 5-phosphate (Ru5P), dihydroxyacetone phosphate (DHAP) or its isomer glyceraldehyde 3-phosphate (G3P), and ammonia. Pyridoxal biosynthesis lyase (PdxS) and glutamine amidotransferase (PdxT) form a complex, PLP synthase, and are responsible for the second de novo PLP-biosynthetic pathway (Zein et al., 2006).
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"Structural dynamics of a methionine γ-lyase for calicheamicin biosynthesis: Rotation of the conserved tyrosine stacking with pyridoxal phosphate." 3, no.
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Bacteria synthesize PLP via two major pathways: a de novo pathway and a salvage pathway (–). There are two distinct de novo pathways, either deoxyxylulose 5-phosphate (DXP)-dependent or DXP-independent, in different organisms (–). The genes of the DXP-dependent pathway have been identified mostly in the γ subdivision of proteobacteria, which form a tight cluster around pdxA and pdxJ of Escherichia coli (, ). In this pathway, the biosynthetic product PNP is oxidized by PdxH to produce PLP (). The DXP-independent pathway (or alternative pathway) requires two enzymes, PdxS (also named as Pdx1, SnzP, or YaaD) and PdxT (also referred to Pdx2, SnoP, or YaaE), which are found in all archaea, fungi, plants, and most bacteria (). PdxS and PdxT directly produce PLP from a pent(μl)ose (ribose 5-phosphate or ribulose 5-phosphate) and a triose (glyceraldehyde 3-phosphate or dihydroxyacetone phosphate) in the presence of glutamine. Biochemical and structural studies revealed that PdxS and PdxT of Bacillus subtilis form a hetero-oligomeric complex (–).
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