Folding of thermolysin fragments. Correlation between conformational stability and antigenicity of carboxyl-terminal fragments

Presence of a slow dimerization equilibrium on the thermal unfolding of the 205-316 thermolysin fragment at neutral pH

Differential scanning calorimetry and size-exclusion chromatography have been used to characterize the dimerization and unfolding of the 205-316 C-terminal fragment of thermolysin at pH 7.5. We show that the folded fragment dimerizes at low temperature with a moderate affinity and undergoes thermal unfolding according to a N(2) <==> 2N <==> 2U model. This behavior has already been observed at acid pH, where a similar dissociation equilibrium has been found [Azuaga, A., Conejero-Lara, F., Rivas G., De Filippis, V., Fontana A., & Mateo, P. L. (1995) Biochim. Biophys. Acta 1252, 95-102]. Nevertheless, at pH 7.5 the dimerization equilibrium slows down below about 30 degrees C, with virtually no interconversion between the monomeric and the dimeric states of the fragment. We have studied the kinetics of interconversion between monomer and dimer by size-exclusion chromatography experiments and have shown that a very high energy barrier (83.8 kJ/mol at 26.5 degrees C) exists between either state. A mathematical analysis of the DSC thermograms on the basis of the proposed model has allowed us to obtain the thermodynamic characterization of the dimerization and the unfolding processes of the fragment and confirms the kinetic parameters obtained in the chromatographic experiments. The thermodynamic functions for the unfolding of the fragment are compatible with some degree of disorder in the structures of both the monomer and the dimer. According to circular dichroism measurements, the dimerization of the fragment seems to be linked to some conformational change in the subunits, most probably due to a rearrangement of the existing secondary-structure elements. This fragment displays several features already observed in folding intermediates, such as the partial disorder of the polypeptidic chain, association processes, and kinetic barriers between different regions in the conformational space.

The thermodynamics of association and unfolding of the 205-316 C-terminal fragment of thermolysin

The 205-316 C-terminal fragment of thermolysin has been studied by differential scanning calorimetry at pH values 2.5, 3.0, 3.5, 4.0 and 5.0 and at a constant ionic strength of 130 mM. The thermal unfolding of the fragment occurs at thermodynamic equilibrium under our experimental conditions. The effect of sample concentration at the different pH values on the calorimetric traces is consistent with a monomer-dimer equilibrium of the folded fragment, which undergoes thermal unfolding into individual fragments. Equilibrium sedimentation experiments at 10 degrees C and different pH values confirm the presence of the association equilibrium and provide the value of the dimerization constants. The global analysis of the calorimetric, heat capacity curves has been carried out by a multidimensional fitting to the model N2<-->2N<-->2U. The analysis leads to a complete thermodynamic characterization of both the association and unfolding processes of the fragment. The resulting thermodynamic functions suggest a partially unfolded structure for both the monomeric and dimeric fragment, as well as a conformational change linked to the association process. Our results are discussed in terms of the structural information currently available and compared with the energetics of unfolding of the shorter 255-316 dimeric C-terminal fragment of thermolysin (Conejero-Lara, F., De Filippis, V., Fontana, A. and Mateo, P.L. (1994) FEBS Lett. 344, 154-156). The presence of the additional 50 residues increases the relative population of the 205-316 monomeric fragment versus that of the 255-316 fragment.

The thermodynamics of the unfolding of an isolated protein subdomain. The 255-316 C-terminal fragment of thermolysin

Differential scanning calorimetry has been used to study the thermal unfolding of the 255-316 C-terminal fragment of thermolysin. The concentration effect on the calorimetric transitions of the fragment in 0.1 M NaCl and 20 mM phosphate buffer, pH 7.5, shows that it behaves as a highly stable dimer in solution, within the concentration range 0.19-4.55 mg/ml, undergoing a reversible two-state thermal unfolding process. The thermodynamic parameters of unfolding (delta G = 60 +/- 6 kJ/(mol of dimer) at 20 degrees C) are similar to those normally observed for small, compact, globular proteins. This and previous studies [1989, Eur. J. Biochem. 180, 513-518] show that the 255-316 fragment folds into a stable, native-like globular structure.

Stability of monoclonal antibody-defined epitopes

Epitope instability can limit the applications of monoclonal antibody (mAb) technology in laboratory and clinical research. We exposed a group of representative antigens on human hepatocellular carcinoma (HCC) cells to physiochemical insults to study epitope stability as measured by mAb immunoreactivity. Each epitope was found to have a unique pattern of instability which serves to biophysically characterize the antigen and defines the conditions to which the antigen can be exposed during laboratory and clinical investigations. Individual antigens were found to be unstable within a surprisingly well defined window of solvent polarities while being stable on either side of that window. Several antigens were observed to be unstable when exposed to transient changes in pH. When a critical temperature between 42 degrees C and 65 degrees C was achieved, epitopes which were thermosensitive underwent a sudden loss in immunoreactivity. This critical temperature was found to be pH dependent. The effects of polarity, pH, and temperature on epitope stability are consistent with changes in protein structural conformation. In addition, we found that certain fixatives cause a time and concentration dependent loss of epitope immunoreactivity. This study provides a rapid and easy determination of monoclonal antibody-defined epitope stability; the results of which serve to guide further studies on the antigen and to characterize the antigen on the basis of its unique physiochemical stability.

Folding of thermolysin fragments. Hydrodynamic properties of isolated domains and subdomains

Sedimentation analysis in the analytical ultracentrifuge has been used to characterize the size and shape of thermolysin and a number of its fragments obtained by chemical or enzymatic cleavage of the protein. Four fragments (121-316, 206-316, 225/226-316 and 255-316) originate from the C-terminal domain, and two (1-155 and 1-205) from the N-terminal domain of the intact molecule. In aqueous solution at neutral pH the hydrodynamic properties of the C-terminal fragments, except 255-316, are consistent with compact homogeneous monomers. Fragment 255-316 is a monomeric species below 0.08 mg/ml concentration and forms a dimer above this concentration. Dimerization does not lead to changes in fragment conformation, as determined by far-ultraviolet circular dichroic measurements, but to an increase of 5.6 degrees C (to 68.2 degrees C at 1.0 mg/ml) in the temperature for thermal unfolding and a corresponding increase of 4.6 kJ/mol in the free energy of unfolding. Fragments derived from the N-terminal domain show a strong tendency to form high-molecular-mass aggregates. Previous experiments utilizing circular dichroic measurements and antibody binding data suggested that the C-terminal fragments listed above are able to refold in aqueous solution at neutral pH into a stable conformation of native-like characteristics [Dalzoppo, D., Vita, C. & Fontana, A. (1985) J. Mol. Biol. 182, 331-340] (and references cited therein). Present data establish that all these C-terminal fragments are globular monomeric species in solution (at concentrations approximately 0.1 mg/ml) and thus represent 'isolated' domains (or subdomains) with intrinsic conformational stability typical of small globular proteins.

Structure and stability of thermophilic enzymes. Studies on thermolysin

The molecular mechanisms responsible for the unusual stability of enzymes isolated from thermophilic microorganisms are much more complex and subtle than was originally thought. In particular, a general mechanism cannot be proposed, since individual enzymes can be stabilized by specific molecular interactions and forces. The results of studies on thermophilic enzymes obtained in recent years in our laboratory will be summarized, with particular emphasis being placed on those obtained with thermolysin, a stable metalloendopeptidase isolated from Bacillus thermoproteolyticus. Fragmentation of thermolysin by limited proteolysis by added protease (subtilisin) or autolysis mediated by heat or the ion-chelating agent EDTA leads to quite selective peptide bond fissions, allowing isolation of 'nicked' thermolysin species. Correlation of the sites of proteolytic cleavage with the known three-dimensional structure of thermolysin allowed us to infer some of the key characteristics of the structure, folding, dynamics and stability of the thermolysin molecule. The potential utility of these and other studies on thermophilic enzymes in devising strategies for enhancing the stability of mesophilic enzymes using genetic engineering techniques is discussed.

Independent folding of autolytic fragments of thermolysin and their domain-like properties

High molecular weight autolysis products from thermolysin have been isolated and identified. The primary fragments correspond to residues 1 to 187-204 (21kD) and residues 187-204 to 316 (12kD), respectively. The fragments are both capable of independent refolding upon removal of denaturant. On the basis of these results, we suggest that the first step in the unfolding pathway of thermolysin involves unfolding of an interdomain region and domain separation. Bound calcium ions at sites 1, 2 and 4 play a major role in protecting the protein against both autolysis and unfolding, probably by stabilizing the interdomain region and enhancing domain-domain interactions.