Epidermal Growth Factor from the Mouse. Structural Characterization by Proton Nuclear Magnetic Resonance and Nuclear Overhauser Experiments at 500 MHz

Mouse epidermal growth factor (mEGF), a protein hormone effector molecule that regulates cellular development and division, has been investigated by using proton nuclear magnetic resonance techniques at 500 MHz. Well-resolved downfield aromatic and α-CH proton resonances (5.0-8.0 ppm) and an upfield ring current shifted isoleucine δ-methyl resonance (~0.5 ppm) have been examined by using principally nuclear Overhauser methods. The data are analyzed in terms of model building based on the predictive Chou-Fasman secondary structure algorithm applied to mEGF [Holladay, L. A., Savage, C. R., Cohen, S., & Puett, D. (1976) Biochemistry 15, 2624-2633], which suggests the existence of some β-structure and little or no α-helicity. Proximity relationships derived from nuclear Overhauser data among Tyr-3, -10, and -13, His-22, and Ile-23 allow refinement of some aspects of the predicted secondary structure and render additional information on how the protein backbone in mEGF is folded (i.e., tertiary structure). Nuclear Overhauser effects (NOEs) from irradiation of several α-CH proton resonances give evidence for tiered j8-sheet structure in mEGF. Such proximity relationships derived from NOE data place stringent limitations on possible models for the molecule. pH titration data demonstrate a His-22 pK_a of 7.1, indicating either a salt bridge or hydrogen-bond formation between His-22 and another residue. The His-22 pK_a is also reflected in the chemical shift changes of several other resonances as a function of pH. Nuclear Overhauser methods, used to differentiate direct (protonation) and indirect (conformation) effects on the chemical shift changes in the spectra of mEGF by varying the pH, yield evidence for a pH-induced conformational transition in the protein hormone associated with the breaking of the His-22 salt bridge or hydrogen bond.