Comparative fluorescence properties of bovine, goat, human and guinea pig alpha lactalbumin. Characterization of the environments of individual tryptophan residues in partially folded conformers.
alpha Lactalbumin exists as a partially folded conformer (U form) at acid pH. A second partially folded conformer (H form) is formed above 60 degrees. Comparison of the changes in tryptophan fluorescence which occur on forming U and H for the bovine, goat, human and guinea pig proteins, as well as analysis of fluorescence properties for the bovine protein and an N bromo succinimide derivative of this protein, have made it possible to determine which tryptophan residues give rise to such changes in fluorescence, and to draw a distinction between the molecular structure of the U and H forms of the protein. Trp 28 and 109 in the native state transfer their excitation energy to trp 63 whose fluorescence is quenched by a pair of vicinal disulfide bridges. This process persists in the U form of the protein, but is absent in the H conformer. Most of the change in fluorescence seen in the N in equilibrium with U conversion is due to increase in yield of trp 28, while the changes in fluorescence occurring on formation of the H form are due to exposure of trp 63 and elimination of its quenching and/or excited state transfer from 28 to 109.
D000818:Animals; D002417:Cattle; D006041:Goats; D006168:Guinea Pigs; D006801:Humans; D007768:Lactalbumin; D011487:Protein Conformation; D013045:Species Specificity; D013050:Spectrometry, Fluorescence; D013696:Temperature; D014364:Tryptophan / Q000032:analysis*
D014364:Tryptophan; D007768:Lactalbumin