Physico-chemical characterization of products of unfolding of cytochrome c by calcium chloride

Multispectroscopic and Molecular Docking Insight into Elucidating the Interaction of Irisin with Rivastigmine Tartrate: A Combinational Therapy Approach to Fight Alzheimer's Disease

This study was aimed to study the interaction between purified irisin and rivastigmine tartrate (RT), a cholinesterase inhibitor used in Alzheimer's therapy. Irisin mainly promotes brown fat-like features in white adipose tissues; however, it has some important role in the nervous system also, i.e., capable of opposing synapse and memory failure in Alzheimer's disease (AD). The recombinant protein was purified by Ni-NTA chromatography and characterized using spectroscopic and in silico techniques. Further, the mechanism of interaction between irisin and RT was investigated using various biophysical techniques. Fluorescence quenching studies suggested that there exists a moderate binding between irisin and RT with a binding constant (K) of 10⁴ M^-1 and the irisin-RT complex is guided by a combination of both static and dynamic modes of quenching. Thermodynamic parameters suggested the reaction to be driven by hydrogen bonding, making it specific. FTIR and CD spectroscopy suggested no secondary structural alterations in irisin in the presence of RT. Molecular docking investigation provided an insight into the important residues that play a key role in irisin-RT interactions. This study delineates an important finding in AD therapy and can provide a platform further to explore the potential of irisin in AD treatment.

The role of key residues in structure, function, and stability of cytochrome-c

Cytochrome-c (cyt-c), a multi-functional protein, plays a significant role in the electron transport chain, and thus is indispensable in the energy-production process. Besides being an important component in apoptosis, it detoxifies reactive oxygen species. Two hundred and eighty-five complete amino acid sequences of cyt-c from different species are known. Sequence analysis suggests that the number of amino acid residues in most mitochondrial cyts-c is in the range 104 ± 10, and amino acid residues at only few positions are highly conserved throughout evolution. These highly conserved residues are Cys14, Cys17, His18, Gly29, Pro30, Gly41, Asn52, Trp59, Tyr67, Leu68, Pro71, Pro76, Thr78, Met80, and Phe82. These are also known as "key residues", which contribute significantly to the structure, function, folding, and stability of cyt-c. The three-dimensional structure of cyt-c from ten eukaryotic species have been determined using X-ray diffraction studies. Structure analysis suggests that the tertiary structure of cyt-c is almost preserved along the evolutionary scale. Furthermore, residues of N/C-terminal helices Gly6, Phe10, Leu94, and Tyr97 interact with each other in a specific manner, forming an evolutionary conserved interface. To understand the role of evolutionary conserved residues on structure, stability, and function, numerous studies have been performed in which these residues were substituted with different amino acids. In these studies, structure deals with the effect of mutation on secondary and tertiary structure measured by spectroscopic techniques; stability deals with the effect of mutation on T m (midpoint of heat denaturation), ∆G D (Gibbs free energy change on denaturation) and folding; and function deals with the effect of mutation on electron transport, apoptosis, cell growth, and protein expression. In this review, we have compiled all these studies at one place. This compilation will be useful to biochemists and biophysicists interested in understanding the importance of conservation of certain residues throughout the evolution in preserving the structure, function, and stability in proteins.

Entrapping intermediates of thermal aggregation in alpha-helical proteins with low concentration of guanidine hydrochloride

Aggregation of proteins is a problem with serious medical implications and economic importance. To develop strategies for preventing aggregation, the mechanism(s) and pathways by which proteins aggregate must be characterized. In this study, the thermally induced aggregation processes of three alpha-helix proteins (myoglobin, cytochrome c, and lysozyme) in the presence and absence of 1.0 m guanidine hydrochloride (GdnHCl) were investigated by means of infrared spectroscopy. In the absence of GdnHCl, intensities of the alpha-helix bands (approximately 1656 cm(-1)) decrease as a function of temperature at above 50 degrees C. With myoglobin and cytochrome c, the loss of helix bands was accompanied by the appearance of two new bands at 1694 and 1623 cm(-1), indicative of the formation of intermolecular beta-sheet aggregates. For lysozyme, bands indicative of intermolecular beta-sheet aggregates did not appear in any significant intensity. In the presence of 1.0 m GdnHCl, two major intermediate states rich in 3(10)-helix (represented by the band at 1663 cm(-1)) and beta-turn structure (represented by the band at 1667 cm(-1)), respectively, were observed. These findings demonstrated that IR spectroscopic studies of protein aggregation using a combination of thermal and chemical denaturing factors could provide a means to populate and characterize aggregation intermediates.

Anion-induced stabilization of human serum albumin prevents the formation of intermediate during urea denaturation

The unfolding of human serum albumin (HSA), a multidomain protein, by urea was followed by far-UV circular dichroism (CD), intrinsic fluorescence, and ANS fluorescence measurements. The urea-induced transition, which otherwise was a two-step process with a stable intermediate at around 4.8 M urea concentration as monitored by far-UV CD and intrinsic fluorescence, underwent a single-step cooperative transition in the presence of 1.0 M KCl. The free energy of stabilization (DeltaDelta G(H2O)D) in the presence of 1 M KCl was found to be 1,090 and 1,200 cal/mol as determined by CD and fluorescence, respectively. The salt stabilization occurred in the first transition (0-5.0 M urea), which corresponded to the formation of intermediate (I) state from the native (N) state, whereas the second transition, corresponding to the unfolding of I state to denatured (D) state, remained unaffected. Urea denaturation of HSA as monitored by tryptophan fluorescence of the lone tryptophan residue (Trp(214)) residing in domain II of the protein, followed a single-step transition suggesting that domain(s) I and/or III is (are) involved in the intermediate formation. This was also confirmed by the acrylamide quenching of tryptophan fluorescence at 5 M urea, which exhibited little change in the value of Stern-Volmer constant. ANS fluorescence data also showed single-step transition reflecting the absence of accumulation of hydrophobic patches. The stabilizing potential of various salts studied by far-UV CD and intrinsic fluorescence was found to follow the order: NaClO(4) > NaSCN >Na(2)SO(4) >KBr >KCl >KF. A comparison of the effects of various potassium salts revealed that anions were chiefly responsible in stabilizing HSA. The above series was found similar to the electroselectivity series of anions towards the anion-exchange resins and reverse of the Hofmeister series, suggesting that preferential binding of anions to HSA rather than hydration, was primarily responsible for stabilization. Further, single-step transition observed with GdnHCl can be ascribed to its ionic character as the free energy change associated with urea denaturation in the presence of 1.0 M KCl (5,980 cal/mol) was similar to that obtained with GdnHCl (5,870 cal/mol).

Kinetic studies on the interaction of ferricytochrome c with anionic surfactants

The kinetics of absorbance and fluorescence changes of cytochrome c as induced by an aqueous solution of the anionic surfactant sodium dodecyl sulfate (SDS) or sodium bis(2-ethylhexyl)sulfosuccinate (AOT) are studied. The results are compared with far-UV circular dichroism (CD) spectra. Both surfactants cause similar alterations in the secondary structure of cytochrome c, while their influence on the heme environment of cytochrome c is different. In the presence of AOT below and above critical micellar concentration a conversion of the low-spin native cytochrome c to a denatured low-spin protein not having methionine ligand takes place. In the presence of SDS micelles conversion of the native protein to a denatured mixed-spin form occurs. The changes in the heme group induced by both surfactants occur independently of the alterations in tertiary structure.

Direct electrochemical evidence for an equilibrium intermediate in the guanidine-induced unfolding of cytochrome c

This paper reports a voltammetric and spectroscopic investigation of the guanidine-induced unfolding of cytochrome c at neutral pH and 25 degrees C. Electrochemical data provide direct evidence for the presence of an equilibrium intermediate (form I) strictly dependent on the denaturant concentration. The midpoint potential of form I has been determined (E1/2 = +0.010 V vs. NHE) and its structural features defined from analysis of the circular dichroism and absorbance spectroscopy data obtained under the same experimental conditions. From the correlation of electrochemical and spectroscopic data, we propose that the features detected by the intermediate conform to the molten globule state.

Effects of salts of alkali earth metals and calcium chloride on the stability of cytochrome c and myoglobin

This study suggests a procedure by which binding of denaturants could be detected without any additional information other than that provided in the denaturation profiles of proteins. Two predominantly alpha-proteins, namely ferricytochrome c and metmyoglobin were denatured by guanidine hydrochloride (GdnHCl) in the presence of low fixed concentrations of salts at 25 degrees C and transition between native and denatured states was followed by absorbance measurements in the visible region (500-350 nm). The raw data were converted into transition curves from which Cm, the midpoint of GdnHCl-induced transition, and deltaGapp, the free energy changes on denaturation, were calculated assuming a two-state mechanism, and values of deltaGapp at zero concentration of the denaturant were estimated. It has been observed (1) that chlorides of Na, K, Cs, and Rb do not affect the native conformation of proteins, (2) that GdnHCl-induced denaturations of proteins in presence and absence of sodium bromide, sodium perchlorate and salts of lithium and calcium are reversible, (3) that optical properties of the GdnHCl-denatured state of proteins remain unchanged in presence of the second denaturant, (4) Cm decreases with an increase in the denaturant concentration, and (5) that except for GdnHCl there exist one or more binding sites on the native proteins for the denaturants.