FOLDING OF THE SQUASH TRYPSIN-INHIBITOR EETI-II - EVIDENCE OF NATIVE AND NONNATIVE LOCAL STRUCTURAL PREFERENCES IN A LINEAR ANALOG

A peptide, corresponding to the entire sequence of the squash trypsin inhibitor EETI II (Ecballium elaterium trypsin inhibitor) in which the six cysteines, engaged in three disulphide bridges in native EETI II, have been replaced by six serines, has been synthesised. CD, Fourier- transform infrared spectroscopy (FTIR) and H-1-NMR studies of this pep tide revealed that some secondary structures present in native EETI II are still populated in the absence of disulphide bonds. Native-like s econdary structures were observed for segments 10-15 (helix), 16-19 an d 22-25 (reverse turns) but no native tertiary interaction was detecte d. However, a non-native local interaction between the aromatic ring o f Phe26 and the amide group of Gly28 was observed. It is hypothesised that the 10-15, 16-19 and 22-25 native-like local conformations could play a major role in the early folding of EETI II.