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Figure 5.
FIGURE 5. a, states along the reaction pathway based on
determined structures. I, the initial state with site 1 occupied
by Mg^2+; II, HisB-N with histidinol phosphate substrate modeled
on the structure of histidinol complex; III, phosphoaspartate
intermediate with Mg^2+ occupying sites 1 and 2; IV, release of
phosphate and Mg^2+ from site 2; b, superposition of the active
site residues of HisB-N (gray) with E. coli AphA (orange). Red
spheres represent water molecules; violet spheres show metal
binding sites. The orientations of Asp^12 and its equivalent in
AphA are different. Asp^12 is stabilized by hydrogen bonds
(blue) to Arg^11, site 2, and bridging water W4. Asp^46 of AphA
is hydrogen-bonded (orange) to Arg^114, approaching from the
opposite direction to Arg^11 of HisB-N, and to the of phosphate
oxygen in the position of water W1. Only HisB residues are
labeled.
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