1Q6F: Crystal Structure of Soybean Beta-amylase Mutant (E178y) With Increased PH Optimum at PH 7.1

Comparison of the architecture around the active site of soybean beta-amylase and Bacillus cereus beta-amylase showed that the hydrogen bond networks (Glu380-(Lys295-Met51) and Glu380-Asn340-Glu178) in soybean beta-amylase around the base catalytic residue, Glu380, seem to contribute to the lower pH optimum of soybean beta-amylase. To convert the pH optimum of soybean beta-amylase (pH 5.4) to that of the bacterial type enzyme (pH 6.7), three mutants of soybean beta-amylase, M51T, E178Y, and N340T, were constructed such that the hydrogen bond networks were removed by site-directed mutagenesis. The kinetic analysis showed that the pH optimum of all mutants shifted dramatically to a neutral pH (range, from 5.4 to 6.0-6.6). The Km values of the mutants were almost the same as that of soybean beta-amylase except in the case of M51T, while the Vmax values of all mutants were low compared with that of soybean beta-amylase. The crystal structure analysis of the wild type-maltose and mutant-maltose complexes showed that the direct hydrogen bond between Glu380 and Asn340 was completely disrupted in the mutants M51T, E178Y, and N340T. In the case of M51T, the hydrogen bond between Glu380 and Lys295 was also disrupted. These results indicated that the reduced pKa value of Glu380 is stabilized by the hydrogen bond network and is responsible for the lower pH optimum of soybean beta-amylase compared with that of the bacterial beta-amylase.
PDB ID: 1Q6FDownload
MMDB ID: 26474
PDB Deposition Date: 2003/8/13
Updated in MMDB: 2012/12 
Experimental Method:
x-ray diffraction
Resolution: 2.1  Å
Source Organism:
Similar Structures:
Biological Unit for 1Q6F: monomeric; determined by author
Molecular Components in 1Q6F
Label Count Molecule
Protein (1 molecule)
Molecule annotation
Chemicals (7 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB