Threonine 138 is crucial for the Quaternary Structure and the Thermostability of Thermus thermophilus Inorganic Pyrophosphatase.

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Inorganic pyrophosphatase (EC. 3.6.1.1) from Thermus thermophilus (Tth PPase) forms the thermostable hexamer，and it was suggested from X-ray studies that its intersubunit interactions stabilize the whole molecule. However，the contribution of Thr138 at the intertrimer interface to quatemary structure and thermostability was unknown functionally. Therefore，we prepared four Thr138-substituted variants (T138A，V ，N ，and H) by site-directed mutagenesis. Then，thermostabilities of the enzyme activity and the quatemary structure for T138V and A were decreased relative to those of the wild type Tth PPase，whereas T138H and N variants remained much hexamer contents and the enzyme activity than T138V and A. Therefore，we suggest that the polar group in Thr138 of Tth PPase is more crucial than the methyl group for thermostability and quatemary structure，and it may contribute to the formation of stable trimer-trimer interface.

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