3T0D: E.Coli (Lacz) Beta-Galactosidase (S796t) In Complex With Galactonolactone

A loop (residues 794-803) at the active site of beta-galactosidase (Escherichia coli) opens and closes during catalysis. The alpha and beta carbons of Ser-796 form a hydrophobic connection to Phe-601 when the loop is closed while a connection via two H-bonds with the Ser hydroxyl occurs with the loop open. beta-Galactosidases with substitutions for Ser-796 were investigated. Replacement by Ala strongly stabilizes the closed conformation because of greater hydrophobicity and loss of H-bonding ability while replacement with Thr stabilizes the open form through hydrophobic interactions with its methyl group. Upon substitution with Asp much of the defined loop structure is lost. The different open-closed equilibria cause differences in the stabilities of the enzyme.substrate and enzyme.transition state complexes and of the covalent intermediate that affect the activation thermodynamics. With Ala, large changes of both the galactosylation (k(2)) and degalactosylation (k(3)) rates occur. With Thr and Asp, the k(2) and k(3) were not changed as much but large DeltaH(double dagger) and TDeltaS(double dagger) changes showed that the substitutions caused mechanistic changes. Overall, the hydrophobic and H-bonding properties of Ser-796 result in interactions strong enough to stabilize the open or closed conformations of the loop but weak enough to allow loop movement during the reaction.
PDB ID: 3T0DDownload
MMDB ID: 96538
PDB Deposition Date: 2011/7/20
Updated in MMDB: 2012/01
Experimental Method:
x-ray diffraction
Resolution: 1.93  Å
Source Organism:
Similar Structures:
Biological Unit for 3T0D: tetrameric; determined by author and by software (PISA)
Molecular Components in 3T0D
Label Count Molecule
Proteins (4 molecules)
Beta-galactosidase(Gene symbol: lacZ)
Molecule annotation
Chemicals (133 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB