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Combined computational and biochemical study reveals the importance of electrostatic interactions between the “pH sensor” and the cation binding site of the sodium/proton antiporter NhaA of Escherichia coli
Author(s) -
Olkhova Elena,
Kozachkov Lena,
Padan Etana,
Michel Hartmut
Publication year - 2009
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.22368
Subject(s) - antiporter , antiporters , chemistry , membrane , sodium–hydrogen antiporter , active site , sodium , binding site , biophysics , ion , electrostatics , proton , enzyme , biochemistry , biology , organic chemistry , physics , quantum mechanics
Sodium proton antiporters are essential enzymes that catalyze the exchange of sodium ions for protons across biological membranes. The crystal structure of NhaA has provided a basis to explore the mechanism of ion exchange and its unique regulation by pH. Here, the mechanism of the pH activation of the antiporter is investigated through functional and computational studies of several variants with mutations in the ion‐binding site (D163, D164). The most significant difference found computationally between the wild type antiporter and the active site variants, D163E and D164N, are low p K a values of Glu78 making them insensitive to pH. Although in the variant D163N the p K a of Glu78 is comparable to the physiological one, this variant cannot demonstrate the long‐range electrostatic effect of Glu78 on the pH‐dependent structural reorganization of trans‐membrane helix X and, hence, is proposed to be inactive. In marked contrast, variant D164E remains sensitive to pH and can be activated by alkaline pH shift. Remarkably, as expected computationally and discovered here biochemically, D164E is viable and active in Na + /H + exchange albeit with increased apparent K M . Our results unravel the unique electrostatic network of NhaA that connect the coupled clusters of the “pH sensor” with the binding site, which is crucial for pH activation of NhaA. Proteins 2009. © 2009 Wiley‐Liss, Inc.