Deciphering the molecular structure of cryptolepain in organic solvents.

Solvent composition plays a major role in stabilizing/destabilizing the forces that are responsible for the native structure of a protein. Often, the solvent composition drives the protein into non-native conformations. Elucidation of such non-native structures provides valuable information about the molecular structure of the protein, which is unavailable otherwise. Inclusion of methanol (non-fluorinated alcohol) or TFE (fluorinated alcohol) in the solvent composition drove cryptolepain, a serine protease and an all-ß-protein, into a non-native structure with an enhanced ß-sheet or induction of α-helix. These solvents did not much affect cryptolepain under neutral conditions, even at higher concentrations, but the effects were predominant at lower pH, when the protein molecule is under stress. The organic solvent-induced state is partially unfolded with similar characteristics to the molten globule state seen with protein under a variety of conditions. Chemical- or temperature-induced unfolding of cryptolepain in the presence of organic solvent is distinctly different from that in the absence of organic solvent. Such different unfolding provided evidence of two structural variants in the molecular structure of the protein as well as the differential stabilization/destabilization of such structural variants and their sequential unfolding.

SDS induced molten globule state of heynein; a new thiol protease: Evidence of domains and their sequential unfolding.

The molten globule state can be an intermediate in the protein-folding pathway and its detailed description can help understand the protein folding and an insight into the molecular structure of a protein. Sodium dodecyl sulfate (SDS), an anionic surfactant is known to induce molten globule sate in some proteins. SDS-induced changes in heynein were monitored by CD, fluorescence, 8-anilino-1-napthalenesulfonic acid (ANS) binding and proteolytic activity measurements. An enhancement in the α-helicity of protein with increasing concentration of SDS along with exposure of tryptophans is seen. At a concentration of SDS (∼2mM) heynein loses activity and rigid tertiary structure but possesses considerable amount of secondary structure along with strong ANS binding, indicating the presence of an intermediate state, which is like molten globule state seen in the case heynein. Chemical and temperature induced unfolding of SDS-induced state of heynein is non-cooperative contrary to the protein in the absence of detergent. Further, the cooperative unfolding transition curve of heynein in the absence of SDS intersects at a point where the second transition of SDS-induced state starts suggesting that the molecule of heynein consist of at least two structural domains which are stabilized differentially and unfolds sequentially. Enhancement of α-helicity of heynein in the presence of SDS suggests the α-rich domain of the protein was stabilized and unfold later as compared with ß-rich domain during temperature and chemical induced denaturation. Equilibrium unfolding pathway of heynein in SDS-induced state provides knowledge of the structure and stability of this plant cysteine protease at domain level.

Deglycosylated milin unfolds via inactive monomeric intermediates.

The effect of deglycosylation on the physiological and functional organization of milin was studied under different denaturizing conditions. Trifluoromethanesulfonic acid mediated deglycosylation resulted in insoluble milin, which was found to be soluble only in 1.5 M GuHCl with native-like folded structure. Kinetic stability, proteolytic activity, and dimeric association were lost in deglycosylated milin. Urea-induced unfolding revealed two inactive, highly stable equilibrium intermediates at pH 7.0 and pH 2.0. These intermediates were stable between 5.5-6.5 and 5.0-6.0 M total chaotropes (urea + 1.5 M GuHCl) at pH 7.0 and pH 2.0, respectively. GuHCl-induced unfolding was cooperative and noncoincidental with a broad transition range (2.0-5.0 M) at pH 7.0 and pH 2.0. Equilibrium unfolding of deglycosylated milin by urea and GuHCl substantiates the involvement of various inactive monomeric intermediates. This study provides a way to understand the role of glycosylation in the unfolding mechanism, stability, and functional activity of the serine protease milin.