The term vitamin B6 is used to refer collectively to the compound pyridoxine and its vitameric forms, pyridoxal, pyridoxamine, and their phosphorylated derivatives. Vitamin B6 is required by all organisms and plays an essential role as a co-factor for enzymatic reactions. Plants, fungi, bacteria, archaebacteria, and protists synthetize vitamin B6. Animals and some highly specialized obligate pathogens obtain it nutritionally. Vitamin B6 has two distinct biosynthetic pathways, which do not coexist in any organism. The pdxA/pdxJ pathway, that has been extensively characterized in Escherichia coli, is found in the γ subdivision of the proteobacteria. A second pathway of vitamin B6 synthesis involving the pdxS/SNZ (see <PDOC00949>) and pdxT/SNO protein families, which are completely unrelated in sequence to the pdxA/pdxJ proteins, is found in plants, fungi, protists, archaebacteria, and most bacteria.

PdxS/SNZ and pdxT/SNO proteins form a complex which serves as a glutamine amidotransferase to supply ammonia as a source of the ring nitrogen of vitamin B6 [1]. PdxT/SNO and pdxS/SNZ appear to encode respectively the glutaminase subunit, which produces ammonia from glutamine, and the synthase subunit, which combines ammonia with five- and three-carbon phosphosugars to form vitamin B6 [2].

The pdxT/SNO family belongs to the triad glutamine aminotransferase fold, characterized by a conserved Cys-His-Glu active site [3]. Two regions are highly conserved across all taxa, the PGGEST motif and the FHPE(LT) motif [4]. PdxT/SNO proteins are an α/β three-layer sandwich containing a seven-stranded twisted mixed parallel β-sheet flanked by a six α-helices on the N-terminal stretch of the sheet, four on one side and two on the other (see <PDB:1R9G>) [3].

Some proteins belonging to the pdxT/SNO family are listed below:

• Bacillus subtilis glutamine amidotransferase subunit pdxT (EC 2.6.-.-).
• Haemophilus influenzae glutamine amidotransferase subunit pdxT (EC 2.6.-.-).
• Methanococcus jannaschii glutamine amidotransferase subunit pdxT (EC 2.6.-.-).
• Yeast probable glutamine amidotransferase SNO1 (EC 2.6.-.-).
• Yeast probable glutamine amidotransferase SNO2 (EC 2.6.-.-).
• Yeast probable glutamine amidotransferase SNO3 (EC 2.6.-.-).

These are hydrophilic proteins of about 19 to 25 Kd.

The pattern we developed for the pdxT/SNO family covers the PGGEST motif. We also developed a profile for the pdxT/SNO family.