Structural and dynamical comparison of alpha, beta and gamma forms of murine epidermal growth factor.

Reversed-phase high-performance liquid chromatography of mouse epidermal growth factor (mEGF) purified by the method of Savage and Cohen allows resolution of four forms of the protein hormone: alpha, beta, gamma and delta. alpha-mEGF, the major form isolated by HPLC, is the parent mEGF originally sequenced by Savage and Cohen, and beta-mEGF is des-asparaginyl1-alpha-mEGF. Proton nuclear magnetic resonance spectroscopy has been used to investigate structural and dynamical differences among the alpha, beta and gamma forms of the peptide. Based on these data, gamma-mEGF can be tentatively identified as des-Asn1, Ser2-mEGF. Comparative nuclear Overhauser experiments on amide and aromatic proton resonances suggest that there are significant conformational changes in the peptide structure on cleavage of the N-terminal residues. Backbone amide proton/deuteron exchange rates in gamma-mEGF and beta-mEGF are significantly faster than those in alpha-mEGF suggesting that structural dynamics are enhanced in the minor forms; this interpretation is supported by the decrease in Tyr(2,6)-(3,5) intraresidue NOE magnitudes on going from alpha to beta to gamma forms. These data suggest that the average conformations of beta and gamma-mEGF favor a more open or denatured state of the protein and that the N terminus is critical to the structural integrity of the parent protein.