Effects of oscillating electric fields on conotoxin peptide conformation: A molecular dynamic simulation study.

Molecular Dynamics (MD) simulation is a computational method frequently used in biological and material sciences to efficiently model atomic and molecular interactions occurring in biological systems and effects of external stimuli on molecules and cells. In this study, M - 1 conotoxin protein was simulated under the oscillating (time-varying) electric fields of strengths 2e-9, 6e-9 and 4.7e-8 V/nm at the frequency of 1800 MHz. Conformational changes in conotoxin were studied using Root Mean Square Deviation (RMSD), Root Mean Square Fluctuation (RMSF), secondary structure analysis and Radial Distribution Functions (RDF). We also performed cluster analysis for Conotoxin by running three simulations at the same parameters to analyse an overall conformation of the peptide. Our findings show that applied oscillating electric field of 4.7e-8 V/nm produced changes in the conformation of conotoxin, whereas, at 6e-9 V/nm, only minor changes were observed, which were then stabilised during the simulation. The results also reveal that the applied field at the lowest strength of 2e-9 did not induce any effects on conotoxin's conformation.

Structural effects of divalent calcium cations on the α7 nicotinic acetylcholine receptor: A molecular dynamics simulation study.

The α7 subtype of neuronal nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel protein that is vital to various neurological functions, including modulation of neurotransmitter release. A relatively high concentration of extracellular Ca2+ in the neuronal environment is likely to exert substantial structural and functional influence on nAChRs, which may affect their interactions with agonists and antagonists. In this work, we employed atomistic molecular dynamics (MD) simulations to examine the effects of elevated Ca2+ on the structure and dynamics of α7 nAChR embedded in a model phospholipid bilayer. Our results suggest that the presence of Ca2+ in the α7 nAChR environment results in closure of loop C-in the extracellular ligand-binding domain, a motion normally associated with agonist binding and receptor activation. Elevated Ca2+ also alters the conformation of key regions of the receptor, including the inter-helical loops, pore-lining helices and the "gate" residues, and causes partial channel opening in the absence of an agonist, leading to an attendant reduction in the free energy of Ca2+ permeation through the pore as elucidated by umbrella sampling simulations. Overall, the structural and permeability changes in α7 nAChR suggest that elevated Ca2+ induces a partially activated receptor state that is distinct from both the resting and the agonist-activated states. These results are consistent with the notion that divalent ions can serve as a potentiator of nAChRs, resulting in a higher rate of receptor activation (and subsequent desensitization) in the presence of agonists, with possible implications for diseases involving calcium dysregulation.

Molecular simulation study of the unbinding of α-conotoxin [ϒ4E]GID at the α7 and α4ß2 neuronal nicotinic acetylcholine receptors.

The α7 and α4ß2 neuronal nicotinic receptors belonging to the family of ligand-gated ion channels are most prevalent in the brain, and are implicated in various neurodegenerative disorders. α-conotoxin GID (and its analogue [ϒ4E]GID) specifically inhibits these subtypes, with more affinity towards the human α7 (hα7) subtype, and is valuable in understanding the physiological roles of these receptors. In this study, we use umbrella-sampling molecular dynamics simulations to understand the mechanism of interaction between [ϒ4E]GID and the agonist binding pockets of the α4ß2 and the hα7 receptors, and to estimate their relative binding affinities (ΔGbind). The obtained ΔGbind values indicate stronger interaction with the hα7 receptor, in agreement with previous experimental studies. Simulations also revealed different unbinding pathways between the two receptor subtypes, enabling identification of a number of interactions at locations far from the orthosteric binding site which may explain the difference in [ϒ4E]GID potency. The pathways identified will help in the design of novel conotoxins with increased potency at α4ß2, for which there is currently no known highly potent conotoxin inhibitor. Computational mutational free energy analyses also revealed a number of possible single-site mutations to GID which might enhance its selective binding to α4ß2 over α7.

Structural features for homodimer folding mechanism.

The homodimers have essential role in catalysis and regulation. The homodimer folding mechanism through 2-state without stable intermediate (2S), 3-state with monomer intermediate (3SMI) and 3-state with dimer intermediate (3SDI) is fascinating. 23MI and 3SDI constitute 3-state (3S). Hence, it is important to differentiate 2S, 3SMI and 3SDI homodimers using structural features. We used the dataset of Li et al. [L. Li, K. Gunasekaran, J.G. Gan, C. Zhanhua, P. Shapshak, M.K. Sakharkar, P. Kangueane, Structural features differentiate the mechanisms between 2S and 3S folding of homodimers, Bioinformation 1 (2005) 42-49] consisting of twenty-five 2S, ten 3SMI and six 3SDI homodimer structures for the study. Interface to total (I/T) residues ratio is large for 2S than 3SMI and 3SDI. Interface to total residues ratio is similar for 3SMI (mean monomer length (ML)=208) and 3SDI (mean monomer length (ML)=404) despite difference in mean monomer size. Interface residues correlate with monomer size in 2S (Pearson's correlation coefficient (r); r(2)=0.41) and 3SMI (r(2)=0.52). This is not true for 3SDI with interface residues and monomer length (r(2)=0.17). Interface area (B/2) does not correlate with interface residues (r(2)<0.001) and monomer size (r(2)=0.023) in 2S. This is despite a relationship with interface residues and monomer size (r(2)=0.41) in 2S. However, this is not true for 3SMI (r(2)=0.61 with interface residues and r(2)=0.25 with monomer size). In 3SDI, a different relationship is seen (r(2)=0.28 with interface residues and r(2)=0.09 with size). The mean hydrophobicity factor (H(f)) is 3-fold less in 3S than 2S. H(f) does not correlate with interface area in 2S (r(2)=0.03) and 3SDI (r(2)=0.0). However, a weak causal relation is seen in 3SMI (r(2)=0.23). Hydrophilic amino acid residues (E, R, K, S and Q) are prominent in 2S than 3S. Charged negative amino acid residues (D, E) are more than positive amino acid residues (R, K, H) in 2S and charged positive amino acid residues (R, K, H) are more than negative amino acid residues (D, E) in 3S. These features help to distinguish 2S, 3SMI and 3SDI providing insights to homodimer folding and binding.

Types of interfaces for homodimer folding and binding.

Homodimers have a role in catalysis and regulation through the formation of stable interfaces. These interfaces are formed through different folding mechanisms such as 2-state without stable intermediate (2S), 3-state with monomer intermediate (3SMI) and 3-state with dimer intermediate (3SDI). Therefore, it is of interest to understand folding mechanism using structural features at the interfaces. Several studies have documented the significance of structural features for the understanding of homodimer folding mechanisms. However, the known features provide limited information for understanding homodimer folding mechanisms. Hence, we created an extended dataset of 47 homodimers (twenty eight 2S, twelve 3SMI and seven 3SDI) to examine the types of interfaces in protein homodimers. 2S are usually small sized, 3SMI are often medium sized and 3SDI often exist as large sized proteins. The ratio of interface to total (I/T) residue is large in 2S and small in 3SMI and 3SDI. Hence, we used I/T measure to group 2S, 3SMI and 3SDI into categories with large I/T (>> 50%), moderate I/T (50 - 25%) and small I/T (<< 25%) interfaces. The grouping is further sub-grouped based on the type of physical interaction visualized at the interface using representations in two dimensions (2D). 2D representation of the interface shows eight different forms of interactions in these homodimers. 2S homodimers frequently have large I/T and thus, utilize the entire protein structure in the formation of the interface where the individual subunits are heavily inter communicated with each other. This is not true in the case of 3SMI and 3SDI. 3SMI subunits usually interact with each other at the interface with a gentle touch-like contact and hence, they have low I/T ratio. 3SDI are often quite different in interaction compared to 3SMI and their subunits do deeply interact at the interface with only one part of the surface and hence also having low I/T ratio.

A decision tree model for the prediction of homodimer folding mechanism.

The formation of protein homodimer complexes for molecular catalysis and regulation is fascinating. The homodimer formation through 2S (2 state), 3SMI (3 state with monomer intermediate) and 3SDI (3 state with dimer intermediate) folding mechanism is known for 47 homodimer structures. Our dataset of forty-seven homodimers consists of twenty-eight 2S, twelve 3SMI and seven 3SDI. The dataset is characterized using monomer length, interface area and interface/total (I/T) residue ratio. It is found that 2S are often small in size with large I/T ratio and 3SDI are frequently large in size with small I/T ratio. Nonetheless, 3SMI have a mixture of these features. Hence, we used these parameters to develop a decision tree model. The decision tree model produced positive predictive values (PPV) of 72% for 2S, 58% for 3SMI and 57% for 3SDI in cross validation. Thus, the method finds application in assigning homodimers with folding mechanism.