FIGURE 2.
Comparison of the GlmS hexamer and the GlmS·Fru6P dimer. A, superposition of the hexameric structure of the synthase domains of wild-type GlmS (PDB code 2VF4) generated using the crystallographic symmetry (in yellow) with the two different hexamers present in the asymmetric unit of the C1A-GlmS structure (in blue and cyan). The two synthase hexamers of C1A-GlmS superpose with a root mean square deviation of 0.42 Å2 over 1837 Cα atoms, and the hexamer of the GlmS structure superposes on the noncrystallographic hexamer of C1A-GlmS with a root mean square deviation of 1.14 Å2 over 1798 Cα atoms and a root mean square deviation of 0.70 Å2 over 1720 Cα atoms, excluding residues 417–429. B, stereoview of the superposition of one synthase domain of the C1A-GlmS (blue) and GlmS·Fru6P (PDB code 2BPL; purple) structures. The two regions (417–429 and 520–540 that correspond to helix CF) that differ significantly between the two structures are indicated in pink for the GlmS·Fru6P structure. The sugar at the synthase sites of the GlmS·Fru6P and C1A-GlmS structures are shown as blue and cyan sticks, respectively, and Glc6P at the secondary sugar-binding site of the C1A-GlmS structure is shown as green sticks. C, comparison of the synthase sites of the GlmS (PDB code 2VF4) and GlmS·Fru6P (PDB code 2BPL) structures. Upon Fru6P binding, ordering of the C-loop and the glutaminase domains leads to the anchoring of Trp-74 on the C-loop. This induces a conformational change of the His-loop, including a huge rearrangement of Lys-503*, and a shift of helix CF. An asterisk denotes that the residue belongs to the adjacent GlmS monomer. D, artificial hexamer assembled by superimposing three dimers of the GlmS·Fru6P structure (colored as in Fig. 1A) onto the hexameric C1A-GlmS structure (not shown). Several clashes indicate that the glutaminase domains cannot adopt the catalytic orientation in the hexameric form.