Hydrophobic interactions of Val75 are critical for oligomeric thermostability of inorganic pyrophosphatase from Bacillus stearothermophilus

To determine the role of Va175 in the oligomeric structure of trimeric inor ganic pyrophosphatase (PPase) [EC 3.6.1.1] from Bacillus stearothermophilus (Bst.), we used site-directed mutagenesis to prepare variants in which Val 75 was replaced by Ala, Phe, Leu, lie, Lys, Gin, and Asp. As a result, the variants in which valine is replaced by hydrophobic residues such as Ala, P he, Leu, and Ile (V75A, F, L, and I) show almost the same level of enzyme a ctivity and thermostability as the wild type enzyme, whereas variants with hydrophilic residue replacements such as Lys, Gin, and Asp (V75K, Q, and D) showed gross reductions in enzyme activity and thermostability. The dissoc iation of V75K and V75D from trimer to monomers occurred rapidly as the tem perature rose, while V75F, V75L, and V75I dissociated more slowly than the wild type. There was no particular effect of heat treatment on the dissocia tion of V75A or V75Q, but these variants were slightly dissociated even in the native state. Thus, we conclude that Va175 may locate at the interface between the monomers and its hydrophobic interactions with its surroundings may play a key role in the thermostability and oligomeric subunit interact ions of the enzyme.