Structure of Starch Binding Domains of Halophilic Alpha-Amylase at Low pH

The solubility and structural properties of halophilic proteins are ascribed to their abundant acidic residues, resulting in large net negative charges at neutral pH. This study examined the effects of low pH, i.e., reduction of net negative charges on the structural properties of starch binding domain (SBD) of halophilic Kocuria varians α-amylase. Titration to pH 2.1 caused loss of 233 nm peak characteristic of aromatic interactions present in the native SBD at neutral pH and resulted in the spectrum with a 216 nm valley characteristic of β-sheet. The low pH β-sheet structure was stable against heat treatment. The addition of NaCl and trifluoroethanol resulted in decrease and increase of the 216 nm signal, without altering the spectral shape. These structural properties were significantly different from those of the native protein.