Characterization of a Trifluoroethanol-induced Partially Folded State of α-Lactalbumin

Abstract

The protein α-lactalbumin exists in a partially folded molten globule state at pH 2·0, the A state. This state is believed to be compact, possessing a similar amount of secondary structure to the native state but having a flexible tertiary structure comprised mainly of non-specific hydrophobic clustering of residues. Addition of trifluoroethanol (TFE) to bovine, human and guinea pig α-lactalbumin at pH 2·0 has been found in each case to induce a conformational transition in the A state as monitored by circular dichroism, nuclear magnetic resonance chemical shifts, and 1-anilinonaphthalene-8-sulphonate binding. The mid-point of this transition is near 15% (v/v) TFE and is effectively complete by 50% (v/v) TFE at 315 K. Far ultraviolet circular dichroism ellipticities at 208 nm and 220 nm, usually taken as a measure of the degree of helical character, are substantially more negative in the TFE state than in the A state. Furthermore, backbone amide protons protected from solvent exchange in the A state are generally at least as strongly protected in the TFE state; patterns of protection appear similar in the two states and include at least part of both the B and C α-helices. One major difference from the A state is nevertheless evident: the ability to bind the fluorescent probe 1-anilinonaphthalene-8-sulphonate, characteristic of molten globule states, is lost in the TFE state. Like the A state, the TFE state of α-lactalbumin shows little chemical shift dispersion of side-chain resonances. Extensive line broadening in the nuclear magnetic resonance spectra, characteristic of slow conformational averaging in the A state, is, however, much reduced in the TFE state. The line narrowing observed in the TFE state has made it possible to obtain directly sequence-specific assignments for about 25% of the 123 residues of bovine α-lactalbumin in 50% (v/v) TFE. Two helices are amongst regions of structure so far identified from short-range backbone nuclear Overhauser enhancement (NOE) connectivities in two-dimensional spectra of the TFE state. One of the helices (residues 86 to 96) corresponds to the C-helix in the native structure. The other (residues 35 to 41) corresponds, however, to a region of the sequence that is not helical in the native state. The partially folded state of α-lactalbumin formed in TFE, therefore, supports both native and non-native secondary structure in the absence of persistent long-range tertiary structure.