Receptor-binding affinities of human epidermal growth factor variants having unnatural amino acid residues in position 23.

Ile-23 of human epidermal growth factor (hEGF) has been indicated, by mutagenesis and NMR studies, to be directly recognized by the receptor. In the present study, an unnatural phenylalanine analog, either 2-azaphenylalanine (2aF), 3-azaphenylalanine (3aF), 4-azaphenylalanine (4aF), or 4-fluorophenylalanine (4fF), was incorporated by an in vivo protein synthesis system into position 23 of [Phe23]hEGF, which retains appreciable receptor-binding affinity (about 20% of the wild type). We compared the receptor-binding affinities of the variants with that of [Phe23]hEGF and found that substitution of Phe-23 with 2aF or 3aF raised the affinity, while substitution with 4aF or 4fF remarkably reduced the affinity. The tertiary structure of [Phe23]hEGF was not significantly affected by the substitution of Phe-23 with 2aF, as shown by the two-dimensional nuclear magnetic resonance analysis. In addition, the substitution of residue 23 with His or Tyr produced an hEGF variant with a slightly higher receptor-binding affinity than that of [Phe23]hEGF. Our results suggest that the receptor has an asymmetric hydrophobic pocket for recognition of the side chain in position 23 of hEGF. Furthermore, on the receptor surface, this pocket seems to be adjacent to a less hydrophobic region with a hydrogen-bond acceptor and donor. Thus, the use of unnatural amino acids in addition to the 20 natural ones allows analyses of the structure-function relationship of a protein at a higher resolution than conventional site-directed substitution by only natural amino acid residues.