Unfolding of a leucine zipper is not a simple two-state transition

Temperature-induced unfolding of the leucine zipper, an alpha-helical, double-stranded, coiled-coil, was studied by circular dichroism spectroscopy, spectrofluorimetry and heat capacity scanning calorimetry. It is shown that this process does not represent a simple two-state transition, as previously believed, but consists of several stages. The first transition starts at the very beginning of heating from 0 degrees C and proceeds with significant heat absorption and decrease of ellipticity. This transition does not depend on the concentration of protein and is sensitive to modification of the N terminus; it is therefore associated with unfolding or fraying of this part of the leucine zipper. The second transition takes place at a considerably higher temperature; it is more pronounced than the first one and does not depend on the concentration of protein, i.e. it is unimolecular. This transition is sensitive to modification of both termini of the leucine zipper and affects the optical properties of a tryptophan residue placed in the central part of the zipper. It therefore involves the whole dimer but does not result in its dissociation, presumably being associated with some repacking of the coiled-coil. This second transition is followed at higher temperatures by the concentration-dependent cooperative unfolding/dissociation of the two strands. The enthalpy and entropy of the temperature-induced structural changes of the leucine zipper that take place before its cooperative unfolding/dissociation comprises almost 40% of the total enthalpy and entropy of unfolding of the completely folded coiled-coil, the state in which it appears to be below 0 degrees C. Comparison of the total enthalpy of leucine zipper unfolding with that of a single-stranded alpha-helix shows that their temperature-dependence correlates with the extent of intramolecular non-polar contacts and allows an assessment of the enthalpy of hydrogen bonding in alpha-helices, which appears to be about 3.3kJmol(-1) at 20 degrees C.