SOLUTION STRUCTURE OF A PEPTIDE FRAGMENT OF HUMAN ALPHA-LACTALBUMIN IN TRIFLUOROETHANOL - A MODEL FOR LOCAL-STRUCTURE IN THE MOLTEN GLOBULE

Background: At low pH, human a-lactalbumin forms a partly folded molte n globule state that contains a non-native clustering of the side chai ns of Tyr103, Trp104 and His107. In order to understand the conformati on of this region of the protein in the molten globule state, we inves tigated the structure of a peptide corresponding to residues 101-110 o f human ol-lactalbumin in tifluoroethanol. Results: We determined the structure of the 101-110 peptide from an NMR data set of 145 nuclear O verhauser effects and nine (3)J(HN alpha) coupling constants, using an ensemble calculation approach to take into account the possibilities of conformational averaging of the data. The backbone of residues 3-10 in the peptide adopts a series of turns, that involving residues 5-8 being the best defined, while the side chains of residues 1, 3, 4, 5, 6 and 7 form a hydrophobic cluster. Conclusions: The peptide conformat ion differs from that previously determined for residues 101-110 in cr ystal structures of native ol-lactalbumin determined at both high and low pH, particularly in the relative orientations of the side chains. The series of turns seen in the peptide could, however, be related to the cu-helical structure seen for residues 104-111 in crystals at high pH, and may be important in the molten globule state for bringing the peptide chain into a compact conformation where favourable interactio ns between the side chains can occur.