 | Acta Crystallographica Section D Acta Crystallographica Section D BIOLOGICAL CRYSTALLOGRAPHY |
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![[Figure 5]](tm5016fig5mag.jpg)
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Figure 5
The essential features of the catalytic mechanism of IMPase. (a) Modelling of the phosphate moiety of L-Ins(1)P in the pre-reaction state. A slight rotation of the phosphate moiety about O1 superposes the axial phosphate O atoms onto the positions of three active-site waters and orientates the phosphoester bond for a direct inline attack by the putative water nucleophile (W1), which is depicted in orange. Mg-1 and Mg-3 coordinate W1, thus lowering its pKa and facilitating proton removal by the Thr95/Asp49 dyad. (b) Modelling of the trigonal bipyramidal transition state based upon the structure of the pentavalent phosphorus intermediate of phosphorylated β-phosphoglucomutase (Lahiri et al., 2003  ). The O6 hydroxyl of L-Ins(1)P is within hydrogen-bonding distance of O9 of the phosphate moiety. Whether the species represents a classical transition state as implied here or a trappable pentavalent intermediate is not the focus of the present work. (c) Modelling of the post-reaction structure based upon the yeast Hal2p PAPase–3Mg2+–AMP–Pi end-product complex (Patel, Martínez-Ripoll et al., 2000). The collapse of the transition state yields inositolate complexed to Mg-2 and the cleaved phosphate, formed by an inversion of configuration. Inositol is generated through protonation by W2 (depicted in pink) and released as the first product. |
![[Figure 5]](tm5016fig5.jpg)
| BIOLOGICAL CRYSTALLOGRAPHY |
ISSN: 1399-0047
