The Effect of Salt Bridge in the Bacillus amyloliquefaciens α-amylase (BAA) on the Accessible Surface Area of Residues and Enzyme Thermal Stability due to Local Compactness

In the Bacillusamyloliquefaciens α-amylase (BAA)، the loop (residues from 177-185; region І) is the constructive part of the cage responsible for attachment to calcium. It has two more amino acid residues than the α-amylase from Bacillus licheniformis (BLA). Arg176 in this region makes an ionic interaction with Glu126 from region ІІ (residues 118-131) but this interaction is lost in BLA due to substitution of R176Q and E126V. It is the common feature of α-amylases that calcium ion is required for their thermal stability. The present work quantitatively estimates the effect of ionic interaction on the overall stability of the enzyme. To clarify the functional and structural significance of corresponding salt bridge، first an automated homology model of the mutant enzyme (∆E126) was built by the Swiss-Model Protein Modeling Server. Bacillus amyloliquefaciens α-amylase (3BH4. pdb) was used as the template and examined by GETAREA and WHAT IF programs، then Glu126 was deleted (∆E126) by site-directed mutagenesis and the thermostability was examined for the wild-type and mutant enzymes. Modeling results showed that deletion of salt bridge affected on the hydrophobic and hydrophilic residues orientation of two discussed regions (Ι، ΙΙ). The mutant enzyme also exhibited lower thermostability relative to the wild-type enzyme. Thus، it may be suggested that salt bridge could affect on accessible surface area of the discussed regions، decrease water diffusion، prevent diffusion of cations and improve the thermostability of the whole protein.