Fluorescence quenching, structural and unfolding studies of a purified cysteine protease, ZCPG from Zingiber montanum rhizome

A first attempt was made to study the fluorescence quenching, structure and unfolding nature of the purified Zingiber montanum (J.Koenig) Link ex A.Dietr. cysteine protease glycoprotein (ZCPG). ATR-IR spectra showed the presences of amide groups along with carbohydrate stretch indicating the glycoprotein nature. UV-vis spectra determined the presences of peptide groups and aromatic sidechains of tyrosine, tryptophan and phenylalanine. Far UV-Circular Dichroism spectrum revealed that the secondary structure consists of 47.6% α-helix, 14.1% β-sheet, 16.1% β-turn, and 22.2% random coil. CD signals revealed pronounced structural stability until 70 °C followed by a significant variation in the secondary structure content in the transition temperature between 80-90 °C. ZCPG retained most of its secondary structure in the pH range of 3.0-10.0. The extrinsic study shows that at pH 2.0, ZCPG revealed characteristics of a molten globule-like state exhibiting strong ANS binding. The effect of GdnHCl on ZCPG evaluated by far-CD emission maximum and fluorescence emission revealed that the unfolding was incomplete determining the stability of the protein. The microenvironment of the tryptophan residues indicated the presence of relatively exposed single tryptophan residue (per monomer) with positively charged side chains.