Abstract

The size of the cavity around Ser68 of Escherichia coli ribonuclease HI was modulated by amino acid substitutions to examine the effects on the stability of the enzyme. Five mutant proteins, Ser68----Gly, Ser68----Ala, Ser68----Thr, Ser68----Val and Ser68----Leu, were constructed. Each of the mutant proteins exhibited at least 40% of the enzyme activity of the wild-type protein. The stabilities of the mutant proteins were determined from urea-denaturation and thermal-denaturation curves. Among the five mutations, only the Ser----Val mutation resulted in an increase in the stability of the enzyme. The melting temperature, tm, at pH 3.0 of the mutant protein Ser68----Val was increased by 1.9 degrees C. Its free-energy change of unfolding in the absence of urea, delta G(H2O), and the midpoint of the denaturation curve, [D]1/2, were also increased by 5.4 kJ/mol and 0.18 M, respectively. The increase in the stability of the enzyme is probably due to the filling of the cavity space around Ser68 by valine. However, the mutation of Ser68 to glycine or leucine residues resulted in a considerable decrease in stability. In these cases, some conformational changes occur, as suggested by the CD and 1H-NMR spectra of these mutant proteins. Study holds ProTherm entries: 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 13323, 13324, 13325, 13326, 13327 Extra Details: Escherichia coli ribonuclease HI; enzyme activity; stability;,conformational changes

Submission Details

ID: 6k3SJLe44

Submitter: Connie Wang

Submission Date: April 24, 2018, 8:15 p.m.

Version: 1

Publication Details

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