NMR solution structure of the 205-316 C-terminal fragment of thermolysin. An example of dimerization coupled to partial unfolding.

The solution structure of the C-terminal fragment 205-316 of thermolysin has been determined by 1H-NMR methods. The fragment forms a dimer in which each subunit has two different regions: the largely disordered N-terminal segment 205-260 and the structurally well-defined segment 261-316. The structured part of each subunit is composed of three helices and is largely coincident with the corresponding region in the solution structure of the dimer formed by the shorter fragment 255-316, which in turn coincides with the crystallographic structure of intact thermolysin. As with the fragment 255-316, the subunit interface is highly hydrophobic and coincides topologically with the one between the segment 255-316 and the rest of the protein in the intact enzyme. A fourth helix (residues 235-246), present in the segment 205-316 of native thermolysin, is mostly disordered in the dimer formed by the fragment 205-316. The location of the fourth helix in the native structure of intact thermolysin does not allow the formation of the dimer interface observed in the solution structure of the fragment 255-316. Under the NMR conditions, dimer formation is energetically more favorable than the dissociated monomers. The latter, based on calorimetric data, was proposed to have partial structure in the region 205-254 as in native thermolysin. Thus, it appears that the assembly of the dimer would require an initial unfolding in the region 205-254 of the monomer.