title: Significance of local electrostatic interactions in staphylococcal nuclease studied by site-directed mutagenesis.
abstract: In this paper, we show that amino acids Glu(73) and Asp(77) of staphylococcal nuclease cooperate unequally with Glu(75) to stabilize its structure located between the C-terminal helix and beta-barrel of the protein. Amino acid substitutions E73G and D77G cause losses of the catalytic efficiency of 24 and 16% and cause thermal stability losses of 22 and 26%, respectively, in comparison with the wild type (WT) protein. However, these changes do not significantly change global and local secondary structures, based on measurements of fluorescence and CD(222 nm). Furthermore, x-ray diffraction analysis of the E75G protein shows that the overall structure of mutant and WT proteins is similar. However, this mutation does cause a loss of essential hydrogen bonding and charge interactions between Glu(75) and Lys(9), Tyr(93), and His(121). In experiments using double point mutations, E73G/D77G, E73G/E75G, and E75G/D77G, significant changes are seen in all mutants in comparison with WT protein as measured by fluorescence and CD spectroscopy. The losses of thermal stability are 47, 59, and 58%, for E73G/D77G, E73G/E75G, and E75G/D77G, respectively. The triple mutant, E73G/E75G/D77G, results in fluorescence intensity and CD(222 nm) close to those of the denatured state and in a thermal stability loss of 65% relative to the WT protein. Based on these results, we propose a model in which significant electrostatic interactions result in the formation of a locally stable structure in staphylococcal nuclease.
authors.affiliation: Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: K W
authors.initials: KW
authors.lastname: Leung
authors.forename: Y C
authors.initials: YC
authors.lastname: Liaw
authors.forename: S C
authors.initials: SC
authors.lastname: Chan
authors.forename: H Y
authors.initials: HY
authors.lastname: Lo
authors.forename: F N
authors.initials: FN
authors.lastname: Musayev
authors.forename: J Z
authors.initials: JZ
authors.lastname: Chen
authors.forename: H J
authors.initials: HJ
authors.lastname: Fang
authors.forename: H M
authors.initials: HM
authors.lastname: Chen
mesh_terms: D002152:Calorimetry, Differential Scanning; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D008836:Micrococcal Nuclease / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D017433:Protein Structure, Secondary; D013050:Spectrometry, Fluorescence; D055672:Static Electricity
chemical_list: D008836:Micrococcal Nuclease