Defining the core structure of the α-lactalbumin molten globule state

Molten globules are partially folded states of proteins which are generally believed to mimic structures formed during the folding process. In order to determine the minimal requirements for the formation of a molten globule state, we have prepared a set of peptide models of the molten globule state of human alpha-lactalbumin (alpha LA). A peptide consisting of residues 1-38 crosslinked, via the native 28-111 disulfide bond, to a peptide corresponding to residues 95-120 forms a partially folded state at pH 2.8 which has all of the characteristics of the molten globule state of alpha LA as judged by near and far UV CD, fluorescence, ANS binding and urea denaturation experiments. The structure of the peptide construct is the same at pH 7.0. Deletion of residues 95-100 from the construct has little effect. Thus, less than half tl-le sequence is required to form a molten globule. Fm:ther truncation corresponding to the selective deletion of the A (residues 1-19) or D (residues 101-110) helices or the C-terminal 3(10) helix (residues 112-120) leads to a significant loss of structure. The loss of structure which results from the deletion of any of these three regions is much greater than that which would be expected based upon the noncooperative loss of local helical structure. Deletion of residues corresponding to the legion of the D helix or C-terminal 3(10) helix region results in a peptide construct which is largely unfolded and contains no more helical structure than is expected from the sum of the helicity of the two reduced peptides. These experiments have defined the minimum core structure of the alpha LA molten globule state. (C) 1999 Academic Press.