Figure 1Domain architectures of selected histidine phosphatase superfamily members
All domain combinations that are at least partially characterized are shown, as well as some designed to illustrate the range of sizes of phosphatase domains. Domains are drawn approximately to scale. Catalytic domains are coloured blue (histidine phosphatase domains; dark for branch 1, light for branch 2), red (PFK-2 domains), purple (the 2H phosphoesterase domain seen in Sts-1 and relatives ), pink (the reductase domain of enzymes involved in opine synthesis [50–52]) and brown (the Vip1 kinase domain ). Other domains are coloured mauve (the carbohydrate-binding module of plant PFK-2/F26BPases ), green (the ankyrin repeats seen in some trypanosomatid PFK-2/F26BPases ; darker shades indicating more reliably assigned repeats), and grey [UBA (ubiquitin-associated)] or yellow (SH3), both found in Sts-1 and relatives .
Figure 3Structure-based sequence alignment of selected histidine phosphatase superfamily members
The Figure was generated by post-processing of a MUSTANG  structure alignment with STACCATO . The structures shown are PDB codes 1UJC, E. coli SixA ; 1H2E, G. stearothermophilus PhoE ; 1K6M, human liver F26BPase ; 1E58, E. coli dPGM ; 1NT4, E. coli glucose-1-phosphatase ; 1DKQ, E. coli phytase ; 1QWO, A. fumigatus phytase . The secondary structures of the smallest (1UJC) and largest (1QWO) proteins are shown below the alignment with β-strands of the core β-sheet (six in 1UJC, seven in 1QWO) and selected helices numbered. The alignment is shaded blue according to sequence conservation with certain sets of residues (see text for details) picked out as follows: white on red, conserved catalytic core (see also Figure 6); black on green, proton donors; black on yellow, additional members of the ‘phosphate pocket’; black on purple, substrate binding residues. Boxed blue residues are non-binding, but conserved positions lying near the catalytic histidine (see Figure 6 and text). Note that the alanine mutations present at position 17 of 1DKQ and position 18 of 1NT4 have been replaced by the naturally occurring histidine. The Figure was produced using JALVIEW .
Figure 4Cross-eyed stereo cartoons of structures of representative histidine phosphatase superfamily members
Structures are shown as cartoons and are coloured grey except for orange, N-terminal region; red, C-terminal tail; magenta, insertion in the β3–α3 region; blue, insertion in the β1–α1 region; and turquoise, oligomeric dPGM-specific insertion (d). The structures are: (a) PDB code 1UJC, E. coli SixA ; (b) PDB code 1H2E, G. stearothermophilus PhoE ; (c) PDB code 1K6M, human liver F26BPase ; (d) PDB code 1E58, E. coli dPGM ; (e) PDB code 1NT4, E. coli glucose-1-phosphatase ; (f) PDB code 1DKQ, E. coli phytase ; (g) PDB code 1QWO, A. fumigatus phytase . The phosphorylable histidine residue of SixA is shown as green spheres to localize the catalytic site.
Figure 5Catalytic mechanism of the histidine phosphatase superfamily
The essentially invariant four residues of the catalytic core (see also Figures 3 and 6) are shown numbered as in E. coli SixA. His8 is phosphorylated during the course of the reaction. The other three residues interact electrostatically with the phospho group before, during and after its transfer and form most or all of the ‘phosphate pocket’. Additional neutral or positive residues, represented as PP in the diagram, may also contribute to the ‘phosphate pocket’ by hydrogen-bonding to the phospho group (see also Figure 3). The proton donor, an aspartate or glutamate residue whose position varies in different families (Figure 3), is shown as PD.
Figure 6The near-superimposable conserved catalytic cores of E. coli SixA and A. fumigatus phytase
The cores are shown in green and nearby conserved residues in purple for E. coli SixA (left; 156 residues; PDB code 1UJC ) and A. fumigatus phytase (right; 442 residues; PDB code 1QWO ) which share 12% sequence identity overall. Broken lines represent hydrogen bonds. Selected β-strands are drawn and labelled. The tungstate ion binding to the ‘phosphate pocket’ of SixA is drawn as ball-and-stick in both panels. The Figure was generated using PyMOL (DeLano Scientific; http://www.pymol.org). A three-dimensional interactive version of this Figure can be seen at http://www.BiochemJ.org/bj/409/0333/bj4090333add.htm.