The homologous angiogenin and ribonuclease N-terminal fragments fold into very similar helices when isolated.

The solution structure of the N-terminal hexadecapeptide of human angiogenin, a protein of unknown tertiary structure, has been precisely delineated by the combined use of CD, NOE and secondary shift data. A helix that starts just after Ser 3 and ends at Asp 14 was stabilized in 30% trifluoroethanol. This helix is strikingly similar in origin and length to the one formed by its homologous, the S-peptide of Ribonuclease (conformationally reexamined here), despite their quite different sequences (only four conserved residues). These results support the idea that individual start and stop signals indeed govern the location and size of natural isolated helices.