3UN2: Phosphopentomutase T85q Variant Enzyme

Citation:
Abstract
Prokaryotic phosphopentomutases (PPMs) are di-Mn(2+) enzymes that catalyze the interconversion of alpha-d-ribose 5-phosphate and alpha-d-ribose 1-phosphate at an active site located between two independently folded domains. These prokaryotic PPMs belong to the alkaline phosphatase superfamily, but previous studies of Bacillus cereus PPM suggested adaptations of the conserved alkaline phosphatase catalytic cycle. Notably, B. cereus PPM engages substrates when the active site nucleophile, Thr-85, is phosphorylated. Further, the phosphoenzyme is stable throughout purification and crystallization. In contrast, alkaline phosphatase engages substrates when the active site nucleophile is dephosphorylated, and the phosphoenzyme reaction intermediate is only stably trapped in a catalytically compromised enzyme. Studies were undertaken to understand the divergence of these mechanisms. Crystallographic and biochemical investigations of the PPM(T85E) phosphomimetic variant and the neutral corollary PPM(T85Q) determined that the side chain of Lys-240 underwent a change in conformation in response to active site charge, which modestly influenced the affinity for the small molecule activator alpha-d-glucose 1,6-bisphosphate. More strikingly, the structure of unphosphorylated B. cereus PPM revealed a dramatic change in the interdomain angle and a new hydrogen bonding interaction between the side chain of Asp-156 and the active site nucleophile, Thr-85. This hydrogen bonding interaction is predicted to align and activate Thr-85 for nucleophilic addition to alpha-d-glucose 1,6-bisphosphate, favoring the observed equilibrium phosphorylated state. Indeed, phosphorylation of Thr-85 is severely impaired in the PPM(D156A) variant even under stringent activation conditions. These results permit a proposal for activation of PPM and explain some of the essential features that distinguish between the catalytic cycles of PPM and alkaline phosphatase.
PDB ID: 3UN2Download
MMDB ID: 97626
PDB Deposition Date: 2011/11/15
Updated in MMDB: 2012/03
Experimental Method:
x-ray diffraction
Resolution: 1.8  Å
Source Organism:
Similar Structures:
Biological Unit for 3UN2: monomeric; determined by author and by software (PISA)
Molecular Components in 3UN2
Label Count Molecule
Protein (1 molecule)
1
Phosphopentomutase(Gene symbol: BC4087)
Molecule annotation
Chemicals (4 molecules)
1
3
2
1
* Click molecule labels to explore molecular sequence information.

Citing MMDB
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