title: Conformational and thermodynamic characterization of the molten globule state occurring during unfolding of cytochromes-c by weak salt denaturants.
abstract: The denaturation of bovine and horse cytochromes-c by weak salt denaturants (LiCl and CaCl(2)) was measured at 25 degrees C by observing changes in molar absorbance at 400 nm (Delta epsilon(400)) and circular dichroism (CD) at 222 and 409 nm. Measurements of Delta epsilon(400) and mean residue ellipticity at 409 nm ([theta](409)) gave a biphasic transition for both modes of denaturation of cytochromes-c. It has been observed that the first denaturation phase, N (native) conformation <--> X (intermediate) conformation and the second denaturation phase, X conformation <--> D (denatured) conformation are reversible. Conformational characterization of the X state by the far-UV CD, 8-anilino-1-naphthalene sulfonic acid (ANS) binding, and intrinsic viscosity measurements led us to conclude that the X state is a molten globule state. Analysis of denaturation transition curves for the stability of different states in terms of Gibbs energy change at pH 6.0 and 25 degrees C led us to conclude that the N state is more stable than the X state by 9.55 +/- 0.32 kcal mol(-1), whereas the X state is more stable than the D state by only 1.40 +/- 0.25 kcal mol(-1). We have also studied the effect of temperature on the equilibria, N conformation <--> X conformation and X conformation <--> D conformation in the presence of different denaturant concentrations using two different optical probes, namely, [theta](222) and Delta epsilon(400). These measurements yielded T(m), (midpoint of denaturation) and Delta H(m) (enthalpy change) at T(m) as a function of denaturant concentration. A plot of Delta H(m) versus corresponding T(m) was used to determine the constant-pressure heat capacity change, Delta C(p) (= ( partial differential Delta H(m)/ partial differential T(m))(p)). Values of Delta C(p) for N conformation <--> X conformation and X conformation <--> D conformation is 0.92 +/- 0.02 kcal mol(-1) K(-1) and 0.41 +/- 0.01 kcal mol(-1) K(-1), respectively. These measurements suggested that about 30% of the hydrophobic groups in the molten globule state are not accessible to the water.
authors.affiliation: Department of Biosciences, Jamia Millia Islamia, Jamia Nagar, New Delhi - 110 025, India.
authors.forename: Shabir H
authors.initials: SH
authors.lastname: Qureshi
authors.forename: Beenu
authors.initials: B
authors.lastname: Moza
authors.forename: Sushma
authors.initials: S
authors.lastname: Yadav
authors.forename: Faizan
authors.initials: F
authors.lastname: Ahmad
mesh_terms: D000818:Animals; D002122:Calcium Chloride / Q000737:chemistry; D002417:Cattle; D002942:Circular Dichroism; D003574:Cytochrome c Group / Q000737:chemistry*; D004795:Enzyme Stability; D006736:Horses; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D018021:Lithium Chloride / Q000737:chemistry; D008956:Models, Chemical; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D012492:Salts / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D003574:Cytochrome c Group; D012492:Salts; D018021:Lithium Chloride; D002122:Calcium Chloride