Conformational aspects of the Cu2+ binding to alpha-lactalbumin. Characterization and stability of the Cu-bound state.
By circular dichroism experiments the existence of a typical Cu2(+)-bound state is demonstrated for bovine- and for goat alpha-lactalbumin. As in the near-UV region an important ligand to metal charge-transfer band overlaps with the aromatic band of the protein, a subtraction method is developed in order to determine the net effect of Cu2+ ions on the protein conformation. The Cu2(+)-bound state, characterized by a vanishing tertiary structure and a substantial loss of secondary structure, clearly differs from the well-known Ca2(+)-, apo-, and acid conformers. At room temperature, the Cu2+ binding has already decreased the alpha-helix content of bovine alpha-lactalbumin to the extent that further unfolding by thermal or guanidine hydrochloride denaturation behaves in a non-cooperative way. Since for goat alpha-lactalbumin the Cu2+ binding to His-68 is much less important than for bovine alpha-lactalbumin, we observe a somewhat different conformational behaviour for goat alpha-lactalbumin. The results of this conformational circular dichroism study are confirmed by isothermal calorimetric data.
Van Dael
H
H
Interdisciplinair Research Centrum, K.U. Leuven, Kortrijk, Belgium.
Tieghem
E
E
Haezebrouck
P
P
Van Cauwelaert
F
F
D000818:Animals; D002413:Cations, Divalent; D002417:Cattle; D002942:Circular Dichroism; D003300:Copper / Q000378:metabolism*; D019791:Guanidine; D006146:Guanidines / Q000737:chemistry; D006358:Hot Temperature; D007768:Lactalbumin / Q000378:metabolism*; D011487:Protein Conformation; D011489:Protein Denaturation; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
D002413:Cations, Divalent; D006146:Guanidines; D003300:Copper; D007768:Lactalbumin; D019791:Guanidine