1PX4: E. Coli (Lacz) Beta-Galactosidase (G794a) With Iptg Bound

The open-closed conformational switch in the active site of Escherichia coli beta-galactosidase was studied by X-ray crystallography and enzyme kinetics. Replacement of Gly794 by alanine causes the apoenzyme to adopt the closed rather than the open conformation. Binding of the competitive inhibitor isopropyl thio-beta-D-galactoside (IPTG) requires the mutant enzyme to adopt its less favored open conformation, weakening affinity relative to wild type. In contrast, transition-state inhibitors bind to the enzyme in the closed conformation, which is favored for the mutant, and display increased affinity relative to wild type. Changes in affinity suggest that the free energy difference between the closed and open forms is 1-2 kcal/mol. By favoring the closed conformation, the substitution moves the resting state of the enzyme along the reaction coordinate relative to the native enzyme and destabilizes the ground state relative to the first transition state. The result is that the rate constant for galactosylation is increased but degalactosylation is slower. The covalent intermediate may be better stabilized than the second transition state. The substitution also results in better binding of glucose to both the free and the galactosylated enzyme. However, transgalactosylation with glucose to produce allolactose (the inducer of the lac operon) is slower with the mutant than with the native enzyme. This suggests either that the glucose is misaligned for the reaction or that the galactosylated enzyme with glucose bound is stabilized relative to the transition state for transgalactosylation.
PDB ID: 1PX4Download
MMDB ID: 28158
PDB Deposition Date: 2003/7/2
Updated in MMDB: 2012/12
Experimental Method:
x-ray diffraction
Resolution: 1.6  Å
Source Organism:
Similar Structures:
Biological Unit for 1PX4: tetrameric; determined by author and by software (PISA)
Molecular Components in 1PX4
Label Count Molecule
Proteins (4 molecules)
Beta-galactosidase(Gene symbol: lacZ)
Molecule annotation
Chemicals (130 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB