Measurement of histidine pK<sub>a</sub>values and tautomer populations in invisible protein states

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  • Alexandar L. Hansen
    Departments of Molecular Genetics, Biochemistry, and Chemistry, University of Toronto, Toronto, ON, Canada M5S 1A8; and
  • Lewis E. Kay
    Departments of Molecular Genetics, Biochemistry, and Chemistry, University of Toronto, Toronto, ON, Canada M5S 1A8; and

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<jats:title>Significance</jats:title><jats:p>Electrostatic interactions in proteins play significant roles in conferring stability and in dictating function. Histidine residues are particularly important because their side chains can serve as both acids and bases over the physiological pH range and as both hydrogen bond donors and acceptors. Solution NMR spectroscopy is a powerful method for studying these residues in highly populated ground-state conformers. Here we develop a strategy for extending such studies to sparsely populated, short-lived protein states that can also play a significant role in defining protein function. An application to an invisible folding intermediate of the Im7 protein is provided, where site-specific pK<jats:sub>a</jats:sub>values of histidine residues have been determined that explain the strong pH-dependent stability differences between native and intermediate states.</jats:p>

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