|Gene:||At5G60540||Accession Number: AT5G60540 (MetaCyc)|
Species: Arabidopsis thaliana col
Component of: pyridoxal 5'-phosphate synthase complex (extended summary available)
Subunit of: pyridoxal 5'-phosphate synthase complex
Species: Arabidopsis thaliana col
Subunit composition of
pyridoxal 5'-phosphate synthase complex = [At5G60540][At2g38230]
glutaminase = At5G60540
pyridoxal 5'-phosphate synthase = At2g38230
Pyridoxal 5'-phosphate (PLP) synthase is a protein complex composed of two subunits, PDX1 and PDX2. The PDX2 subunit functions as a glutaminase that deaminates glutamine to producing glutamate and ammonia. The ammonia is channeled to the active site of the PLP synthase subunit PDX1 [TambascoStudart07] (reviewed in [Mooney10, Fitzpatrick07]).
In Arabidopsis, there are three homologous genes encoding PDX1, PDX1.1, 1.2 and 1.3, and one gene encoding PDX2. In a yeast complementation experiment, PDX1.1 and PDX1.3 were able to restore the yeast PDX1 mutant. In addition, the Arabidopsis PDX2 was also able to complement the yeast PDX2 mutant [TambascoStudart05].
Enzyme activities of PDX1.1 and PDX1.3 were proved in an in vitro assay of the recombinant proteins [TambascoStudart05, TambascoStudart07]. The activities observed were very similar with PDX1.1 and PDX1.3. Both proteins were able to accept either ribose 5-phosphate or ribulose 5-phosphate as the pentose sugar, and either glyceraldehyde 3-phosphate or dihydroxyacetone phosphate as the triose sugar, together with ammonia to form PLP. Whereas no preference was found for the triose, both proteins preferred ribose 5-phosphate to ribulose 5-phosphate [TambascoStudart07]. On the other hand, PDX1.2 was not active in the assay. Together with the genetic complementation data, PDX1.2 is not a functional PDX1.
The glutaminase activity of PDX2 was also proven in an in vitro assay [TambascoStudart07]. Comparing to PDX2 of other organisms (i.e. B. subtilis and P. falciparum), the Arabidopsis PDX2 had a substantially lower Kcat whereas its Km for glutamine was in the same range of those reported from the microorganisms. In microorganisms, 12 PDX1 subunits assemble with 12 PDX2 subunits to form a highly conserved cogwheel-like structure. Although it is unknown how precisely the Arabidopsis PDX complex assembles, it was shown that optimum enzyme activity was achieved when PDX1 and PDX2 were in a 1:1 molar ratio [TambascoStudart07].
Enzymatic reaction of: pyridoxal 5'-phosphate synthase complex
EC Number: 220.127.116.11
The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the Enzyme Commission system.
The reaction is favored in the direction shown.
Fitzpatrick07: Fitzpatrick TB, Amrhein N, Kappes B, Macheroux P, Tews I, Raschle T (2007). "Two independent routes of de novo vitamin B6 biosynthesis: not that different after all." Biochem J 407(1);1-13. PMID: 17822383
TambascoStudart05: Tambasco-Studart M, Titiz O, Raschle T, Forster G, Amrhein N, Fitzpatrick TB (2005). "Vitamin B6 biosynthesis in higher plants." Proc Natl Acad Sci U S A 102(38);13687-92. PMID: 16157873
TambascoStudart07: Tambasco-Studart M, Tews I, Amrhein N, Fitzpatrick TB (2007). "Functional analysis of PDX2 from Arabidopsis, a glutaminase involved in vitamin B6 biosynthesis." Plant Physiol 144(2);915-25. PMID: 17468224
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