Reconstructive denaturation of immunoglobulins by sodium dodecyl sulfate. Circular dichroism studies

Inhibition of chymotrypsin through surface binding using nanoparticle-based receptors

Efficient binding of biomacromolecular surfaces by synthetic systems requires the effective presentation of complementary elements over large surface areas. We demonstrate here the use of mixed monolayer protected gold clusters (MMPCs) as scaffolds for the binding and inhibition of chymotrypsin. In these studies anionically functionalized amphiphilic MMPCs were shown to inhibit chymotrypsin through a two-stage mechanism featuring fast reversible inhibition followed by a slower irreversible process. This interaction is very efficient, with a K(i)(app) = 10.4 +/- 1.3 nM. The MMPC-protein complex was characterized by CD, demonstrating an almost complete denaturation of the enzyme over time. Dynamic light scattering studies confirm that inhibition proceeds without substantial MMPC aggregation. The electrostatic nature of the engineered interactions provides a level of selectivity: little or no inhibition of function was observed with elastase, beta-galactosidase, or cellular retinoic acid binding protein.

Studies of the secondary structures of amelogenin from bovine tooth enamel

Circular dichroism and Fourier transform infrared spectroscopic studies of the major amelogenin protein of developing bovine tooth enamel in solution and in the solid state suggest a unique secondary structure containing beta-sheet and repetitive beta-turn structures. The repetitive beta-turn structure at the C-terminal end results from the unique primary structure of amelogenin.

Circular dichroism tests on the effect of alkali on conformation of lectins

The structural properties of eight lectins (agglutinins) were studied by the circular dichroism (CD) probe. The lectin from Maclura pomifera (osage orange seed) exhibited in the far-ultraviolet spectral zone a positive CD band centered at 201 nm, whereas the other lectins had positive bands at 195--198 nm. Structural transitions were effected by alkali, pH 9.6--11.9. Native M. pomifera lectin had a positive CD band also at 236 nm which shifted to the red and enhanced by alkali. Significant individual differences in resistance to the denaturing effect of alkali were observed even between the lectins the CD spectra of which were similar in neutral solutions. The CD spectrum of the lectin from Wistaria floribunda was similar to the CD spectrum of the Dolichos biflorus lectin, but the former was much less sensitive to alkali than the latter. The CD spectra of the native lectins from soybean (Glycine max) and the erythroagglutinin from kidney bean (Phaseolus vulgaris) also were similar, yet the former was denatured at pH 10.6--11.6, whereas the CD of the latter was affected very little. The tertiary structures of the lectins from D. biflorus, the Sophora japonica and the fucose binding lectin from Tetragonolobus purpureas were more sensitive to alkali than those of the other lectins. While the CD bands at 270--300 nm, related to the tyrosine and tryptophan chromophores are generally diminished on denaturation, in the D. biflorus and S. japonica lectins these bands were enhanced at pH 9.6--10.6. In more strongly alkaline solutions, a new CD band developed at 248--252 nm. The main polypeptide chain was more or less completely disorganized at the very high pH values.

Interconversion of conformational isomers of light chains in the Mcg immunoglobulins

Previous crystallographic studies in this laboratory demonstrated that immunoglobulin light chains with the same amino acid sequence can have at least two and probably three or more conformations, depending on whether the second member of an interacting pair is a light or heavy chain. If a heavy chain is not available in the assembly medium, a second light chain plays the structural role of the heavy chain in the formation of a dimer. In the present work, the lambda-type light chains were dissociated from the heavy chains of a serum IgG1 immunoglobulin from the patient Mcg and reassembled noncovalently into a dimer. The reassembly process was completed by allowing the penultimate half-cystine residues to form an interchain disulfide bond. The covalently linked dimer was compared with the Mcg urinary Bence-Jones dimer, for which an atomic model has been fitted to a 2.3-A electron density map. The assembled dimer and the native Bence-Jones protein were indistinguishable in their chromatographic and electrophoretic properties, as well as in their activity in the binding of bis(dinitrophenyl)lysine. These results indicate that the light chains can be converted into the two types of Bence-Jones conformational isomers. The procedure was also reversed: the two Bence-Jones isomers were dissociated and reassembled as the single type of isomer associating with each of two heavy chains in the IgG1 protein. The change in activity occurring when a light chain associates with a heavy chain instead of a second light chain is illustrated by the fact that the Mcg IgG1 immunoglobulin does not bind dis(dinitrophenyl)lysine in measurable amounts.

Conformational transitions of monellin, an intensely sweet protein

Conformational transitions of monellin, an intensely sweet protein from the berries of Dioscoreophyllum cumminsii, were studied by the circular dichroism (CD) probe. According to the CD spectra, monellin has a low content of the helical structure and a significant amount of the pleated sheet (beta) conformation. The native conformation was found to be sensitive to alkali, sodium dodecyl sulfate, and guanidine-HC1, but it was stable in acid (e.g. pH 2.4) as shown by CD and persistence or the disappearance of sweet taste. The main chain conformation of the alkali-denatured monellin (pH 10.9) was restored upon acidification (pH 3.3) of the alkaline solutions. The tertiary structure, however, was not completely restroed, as indicated by CD in the 230-300 nm spectral zone, although the sweet taste reappeared. If the pH of a neutral solution was raised to 9.6, the CD in the near ultraviolet was significantly altered, though the sweet taste persisted. This indicates that a slight conformational change did not interfere with the effects on the taste buds. While sodium dodecyl sulfate readily disorganized the tertiary structure, the main chain was reconstructed by this reagent into a new form of higher helix content than in the native macromolecule. Reconstruction into a modified conformation of higher helix content was achieved also with 50% ethanol. The main chain conformation was not affected by 25% ethanol which produced slight changes in the CD at 230-260 nm zone and did not abolish the sweet taste.

Conformational transitions of non-helical proteins effected by dodecyl sulfate. Circular dichroism of alpha1-acid glycoprotein, Bence Jones protein, carbonic anhydrase B, deoxyribonuclease, pepsinogen, and plasminogen

Circular dichroism (CD) of serum alpha1-acid glycoprotein, urinary Bence Jones protein, human carbonic anhydrase B, deoxyribonuclease from bovine pancreas, porcine pepsinogen, and plasminogen from human serum was tested in the absence and presence of 0.005-0.05 M sodium dodecyl sulfate. It was found that in all cases the CD spectra of these proteins were modified by the dodecyl sulfate into spectra indicating the presence of a moderate content of alpha-helix. The transitions were enhanced by addition of acid (pH 2.1-4.4) in all cases tested. Comparison of the various proteins with respect to the amount of reconstruction of the main chain conformation showed that the amount of helix formed depended on the amino acid composition of the protein. Rigidity due to cross-linking by disulfide bridges is the strongest deterrant to the conformational change of the main chain. The CD bands of the native proteins in the 250-350 nm spectral zone were extinguished by sodium dodecyl sulfate, and new weak bands were observed the positions of which corresponded approximately to those of the native proteins. In all cases, except the carbonic anhydrase B, the bands of thus denatured proteins were negative.