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.
Keywords: Halophilic, starch binding domain, low pH, circular dichroism, melting.
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