title: Folding, stability, and secondary structure of a new dimeric cysteine proteinase inhibitor.
abstract: Clitocypin, a new type of cysteine proteinase inhibitor from the mushroom Clitocybe nebularis, is a 34-kDa homodimer lacking disulphide bonds, reported to have unusual stability properties. Sequence similarity is limited solely to certain proteins from mushrooms. Infrared spectroscopy shows that clitocypin is a high beta-structure protein which was lost at high temperatures. The far UV circular dichroism spectrum is not that of classical beta-structure, but similar to those of a group of small beta-strand proteins, with a peak at 189nm and a trough at 202nm. An aromatic peak at 232nm and infrared bands at 1633 and 1515cm(-1) associated with the peptide backbone and the tyrosine microenvironment, respectively, were used to characterize the thermal unfolding. The reversible transition has a midpoint at 67 degrees C, with DeltaG=34kJ/mol and DeltaH=300kJ/mol, and is, unusually, independent of protein concentration. The kinetics of thermal unfolding and refolding are slow, with activation energies of 167 and 44kJ/mol, respectively. A model for folding and assembly is discussed.
mesh_terms: D000363:Agaricales / Q000737:chemistry*; D015853:Cysteine Proteinase Inhibitors / Q000737:chemistry / Q000378:metabolism*; D019281:Dimerization; D004355:Drug Stability; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: C411656:Clitocypin protein, Clitocybe nebularis; D015853:Cysteine Proteinase Inhibitors; D005656:Fungal Proteins