Biomedical subjects

Huan-Xiang Zhou

Publications and source records attributed to Huan-Xiang Zhou.

At least 19 recordsLinked to original sources

PubMed2007

Calculation of protein-ligand binding affinities.

Accurate methods of computing the affinity of a small molecule with a protein are needed to speed the discovery of new medications and biological probes. This paper reviews physics-based models of binding, beginning with a summary of the changes in potential energy, solvation energy, and configurational entropy that influence affinity, and a theoretical overview to frame the discussion of specific computational approaches. Important advances are reported in modeling protein-ligand energetics, such as the incorporation of electronic polarization and the use of quantum mechanical methods. Recent calculations suggest that changes in configurational entropy strongly oppose binding and must be included if accurate affinities are to be obtained. The linear interaction energy (LIE) and molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) methods are analyzed, as are free energy pathway methods, which show promise and may be ready for more extensive testing. Ultimately, major improvements in modeling accuracy will likely require advances on multiple fronts, as well as continued validation against experiment.

Biophysics↗

PubMed2006

Energy landscape and transition state of protein-protein association.

Formation of a stereospecific protein complex is favored by specific interactions between two proteins but disfavored by the loss of translational and rotational freedom. Echoing the protein folding process, we have previously proposed a transition state for protein-protein association. Here we clarify the specification of the transition state by working with two types of toy models for protein association. A "hemisphere" model consists of two matching hemispheres as associating proteins, and a "crater" model consists of a spherical protein with a crater to which another spherical protein fits snugly. Short-range pairwise interactions between sites across the interface hold together the bound complex. Small relative translation and rotation between the subunits quickly destroy the interactions, leading to a sharp transition between the bound state with numerous short-range interactions but restricted translation and rotational freedom and the unbound state with, at most, a small number of interactions but expanded configurational freedom. This transition sets the outer boundary of the bound state as well as the transition state for association. The energy landscape is funnel-like, with the deep well of the bound state surrounded by a broad shallow basin. Calculations with the toy models suggest that mutational change in the interaction energy in the x-ray structure of a protein-protein complex, commonly used to approximate the effect on the association constant, overestimates the effect of mutation by 10-20%. With an eye toward specifying the transition states of actual protein complexes, we find that the total number of contacts between the subunits serves as a good surrogate of the interaction energy. This formalism of protein-protein association is applied to the barnase-barstar complex, reproducing the experimental results for the association rate over a wide range of ionic strength.

Models, Molecular↗

PubMed2006

Electrostatic contribution to the binding stability of protein-protein complexes.

To investigate roles of electrostatic interactions in protein binding stability, electrostatic calculations were carried out on a set of 64 mutations over six protein-protein complexes. These mutations alter polar interactions across the interface and were selected for putative dominance of electrostatic contributions to the binding stability. Three protocols of implementing the Poisson-Boltzmann model were tested. In vdW4 the dielectric boundary between the protein low dielectric and the solvent high dielectric is defined as the protein van der Waals surface and the protein dielectric constant is set to 4. In SE4 and SE20, the dielectric boundary is defined as the surface of the protein interior inaccessible to a 1.4-A solvent probe, and the protein dielectric constant is set to 4 and 20, respectively. In line with earlier studies on the barnase-barstar complex, the vdW4 results on the large set of mutations showed the closest agreement with experimental data. The agreement between vdW4 and experiment supports the contention of dominant electrostatic contributions for the mutations, but their differences also suggest van der Waals and hydrophobic contributions. The results presented here will serve as a guide for future refinement in electrostatic calculation and inclusion of nonelectrostatic effects.

Acetylcholinesterase↗

PubMed2006

Quantitative relation between intermolecular and intramolecular binding of pro-rich peptides to SH3 domains.

Flexible linkers are often found to tether binding sequence motifs or connect protein domains. Here we analyze three usages of flexible linkers: 1), intramolecular binding of proline-rich peptides (PRPs) to SH3 domains for kinase regulation; 2), intramolecular binding of PRP for increasing the folding stability of SH3 domains; and 3), covalent linking of PRPs and other ligands for high-affinity bivalent binding. The basis of these analyses is a quantitative relation between intermolecular and intramolecular binding constants. This relation has the form K(i) = K(e0)p for intramolecular binding and K(e) = K(e01)K(e02)p for bivalent binding. The effective concentration p depends on the length of the linker and the distance between the linker attachment points in the bound state. Several applications illustrate the usefulness of the quantitative relation. These include intramolecular binding to the Itk SH3 domain by an internal PRP and to a circular permutant of the alpha-spectrin SH3 domain by a designed PRP, and bivalent binding to the two SH3 domains of Grb2 by two linked PRPs. These and other examples suggest that flexible linkers and sequence motifs tethered to them, like folded protein domains, are also subject to tight control during evolution.

Binding Sites↗

PubMed2006

Similarity and difference in the unfolding of thermophilic and mesophilic cold shock proteins studied by molecular dynamics simulations.

Molecular dynamics simulations were performed to unfold a homologous pair of thermophilic and mesophilic cold shock proteins at high temperatures. The two proteins differ in just 11 of 66 residues and have very similar structures with a closed five-stranded antiparallel beta-barrel. A long flexible loop connects the N-terminal side of the barrel, formed by three strands (beta1-beta3), with the C-terminal side, formed by two strands (beta4-beta5). The two proteins were found to follow the same unfolding pathway, but with the thermophilic protein showing much slower unfolding. Unfolding started with the melting of C-terminal strands, leading to exposure of the hydrophobic core. Subsequent melting of beta3 and the beta-hairpin formed by the first two strands then resulted in unfolding of the whole protein. The slower unfolding of the thermophilic protein could be attributed to ion pair formation of Arg-3 with Glu-46, Glu-21, and the C-terminal. These ion pairs were also found to be important for the difference in folding stability between the pair of proteins. Thus electrostatic interactions appear to play similar roles in the difference in folding stability and kinetics between the pair of proteins.

Amino Acid Sequence↗

PubMed2006

Histidines, heart of the hydrogen ion channel from influenza A virus: toward an understanding of conductance and proton selectivity.

The heart of the H+ conductance mechanism in the homotetrameric M2 H+ channel from influenza A is a set of four histidine side chains. Here, we show that protonation of the third of these imidazoles coincides with acid activation of this transmembrane channel and that, at physiological pH, the channel is closed by two imidazole-imidazolium dimers, each sharing a low-barrier hydrogen bond. This unique construct succeeds in distributing a pair of charges over four rings and many atoms in a low dielectric environment to minimize charge repulsion. These dimers form with identical pKas of 8.2 +/- 0.2, suggesting cooperative H+ binding and clearly illustrating high H+ affinity for this channel. The protonation behavior of the histidine side chains has been characterized by using solid-state NMR spectroscopy on the M2 transmembrane domain in fully hydrated lipid bilayers where the tetrameric backbone structure is known. Furthermore, electrophysiological measurements of multichannel and single-channel experiments confirm that these protein constructs are functional.

Dimerization↗

PubMed2005

Interactions of macromolecules with salt ions: an electrostatic theory for the Hofmeister effect.

Salting-out of proteins was discovered in the nineteenth century and is widely used for protein separation and crystallization. It is generally believed that salting-out occurs because at high concentrations salts and the protein compete for solvation water. Debye and Kirkwood suggested ideas for explaining salting-out (Debeye and MacAulay, Physik Z; 1925;131:22-29; Kirkwood, In: Proteins, amino acids and peptides as ions and dipolar ions. New York: Reinhold; 1943. p 586-622). However, a quantitative theory has not been developed, and such a theory is presented here. It is built on Kirkwood's idea that a salt ion has a repulsive interaction with an image charge inside a low dielectric cavity. Explicit treatment is given for the effect of other salt ions on the interaction between a salt ion and its image charge. When combined with the Debye-Hückel effect of salts on the solvation energy of protein charges (i.e., salting-in), the characteristic curve of protein solubility versus salt concentration is obtained. The theory yields a direct link between the salting-out effect and surface tension and is able to provide rationalizations for the effects of salt on the folding stability of several proteins.

Animals↗

PubMed2005

Prediction of interface residues in protein-protein complexes by a consensus neural network method: test against NMR data.

The number of structures of protein-protein complexes deposited to the Protein Data Bank is growing rapidly. These structures embed important information for predicting structures of new protein complexes. This motivated us to develop the PPISP method for predicting interface residues in protein-protein complexes. In PPISP, sequence profiles and solvent accessibility of spatially neighboring surface residues were used as input to a neural network. The network was trained on native interface residues collected from the Protein Data Bank. The prediction accuracy at the time was 70% with 47% coverage of native interface residues. Now we have extensively improved PPISP. The training set now consisted of 1156 nonhomologous protein chains. Test on a set of 100 nonhomologous protein chains showed that the prediction accuracy is now increased to 80% with 51% coverage. To solve the problem of over-prediction and under-prediction associated with individual neural network models, we developed a consensus method that combines predictions from multiple models with different levels of accuracy and coverage. Applied on a benchmark set of 68 proteins for protein-protein docking, the consensus approach outperformed the best individual models by 3-8 percentage points in accuracy. To demonstrate the predictive power of cons-PPISP, eight complex-forming proteins with interfaces characterized by NMR were tested. These proteins are nonhomologous to the training set and have a total of 144 interface residues identified by chemical shift perturbation. cons-PPISP predicted 174 interface residues with 69% accuracy and 47% coverage and promises to complement experimental techniques in characterizing protein-protein interfaces. .

Binding Sites↗

PubMed2005

How do biomolecular systems speed up and regulate rates?

The viability of a biological system depends upon careful regulation of the rates of various processes. These rates have limits imposed by intrinsic chemical or physical steps (e.g., diffusion). These limits can be expanded by interactions and dynamics of the biomolecules. For example, (a) a chemical reaction is catalyzed when its transition state is preferentially bound to an enzyme; (b) the folding of a protein molecule is speeded up by specific interactions within the transition-state ensemble and may be assisted by molecular chaperones; (c) the rate of specific binding of a protein molecule to a cellular target can be enhanced by mechanisms such as long-range electrostatic interactions, nonspecific binding and folding upon binding; (d) directional movement of motor proteins is generated by capturing favorable Brownian motion through intermolecular binding energy; and (e) conduction and selectivity of ions through membrane channels are controlled by interactions and the dynamics of channel proteins. Simple physical models are presented here to illustrate these processes and provide a unifying framework for understanding speed attainment and regulation in biomolecular systems.

Enzymes↗

PubMed2005

Effects of pH, salt, and macromolecular crowding on the stability of FK506-binding protein: an integrated experimental and theoretical study.

Environmental variables can exert significant influences on the folding stability of a protein, and elucidating these influences provides insight on the determinants of protein stability. Here, experimental data on the stability of FKBP12 are reported for the effects of three environmental variables: pH, salt, and macromolecular crowding. In the pH range of 5-9, contribution to the pH dependence of the unfolding free energy from residual charge-charge interactions in the unfolded state was found to be negligible. The negligible contribution was attributed to the lack of sequentially nearest neighboring charged residues around groups that titrate in the pH range. KCl lowered the stability of FKBP12 and the E31Q/D32N double mutant at small salt concentrations but raised stability after approximately 0.5 M salt. Such a turnover behavior was accounted for by the balance of two opposing types of protein-salt interactions: the Debye-Hückel type, modeling the response of the ions to protein charges, favors the unfolded state while the Kirkwood type, accounting for the disadvantage of the ions moving toward the low-dielectric protein cavity from the bulk solvent, disfavors the unfolded state. Ficoll 70 as a crowding agent was found to have a modest effect on protein stability, in qualitative agreement with a simple model suggesting that the folded and unfolded states are nearly equally adversely affected by macromolecular crowding. For any environmental variable, it is the balance of its effects on the folded and unfolded states that determines the outcome on the folding stability.

Ficoll↗

PubMed2005

Rugged Metropolis sampling with simultaneous updating of two dynamical variables.

The rugged Metropolis (RM) algorithm is a biased updating scheme which aims at directly hitting the most likely configurations in a rugged free-energy landscape. Details of the one-variable ( RM1 ) implementation of this algorithm are presented. This is followed by an extension to simultaneous updating of two dynamical variables ( RM2 ). In a test with the brain peptide Met-Enkephalin in vacuum RM2 improves conventional Metropolis simulations by a factor of about 4. Correlations between three or more dihedral angles appear to prevent larger improvements at low temperatures. We also investigate a multihit Metropolis scheme, which spends more CPU time on variables with large autocorrelation times.

Algorithms↗

PubMed2005

Prediction of solvent accessibility and sites of deleterious mutations from protein sequence.

Residues that form the hydrophobic core of a protein are critical for its stability. A number of approaches have been developed to classify residues as buried or exposed. In order to optimize the classification, we have refined a suite of five methods over a large dataset and proposed a metamethod based on an ensemble average of the individual methods, leading to a two-state classification accuracy of 80%. Many studies have suggested that hydrophobic core residues are likely sites of deleterious mutations, so we wanted to see to what extent these sites can be predicted from the putative buried residues. Residues that were most confidently classified as buried were proposed as sites of deleterious mutations. This proposition was tested on six proteins for which sites of deleterious mutations have previously been identified by stability measurement or functional assay. Of the total of 130 residues predicted as sites of deleterious mutations, 104 (or 80%) were correct.

Amino Acid Sequence↗

PubMed2005

Electrostatic recognition and induced fit in the kappa-PVIIA toxin binding to Shaker potassium channel.

Brownian dynamics (BD) and molecular dynamics (MD) simulations and electrostatic calculations were performed to study the binding process of kappa-PVIIA to the Shaker potassium channel and the structure of the resulting complex. BD simulations, guided by electrostatic interactions, led to an initial alignment between the toxin and the channel protein. MD simulations were then carried out to allow for rearrangements from this initial structure. After approximately 4 ns, a critical "induced fit" process was observed to last for approximately 2 ns. In this process, the interface was reorganized, and side chains were moved so that favorable atomic contacts were formed or strengthened, while unfavorable contacts were eliminated. The final complex structure was stabilized through electrostatic interactions with the positively charged side chain of Lys7 of kappa-PVIIA deeply inserted into the channel pore and other hydrogen bonds and by hydrophobic interactions involving Phe9 and Phe23 of the toxin. The validity of the predicted structure for the complex was assessed by calculating the effects of mutating charged and polar residues of both the toxin and the channel protein, with the calculated effects correlating reasonably well with experimental data. The present study suggests a general binding mechanism, whereby proteins are pre-aligned in their diffusional encounter by long-range electrostatic attraction, and nanosecond-scale rearrangements within the initial complex then lead to a specifically bound complex.

Amino Acid Sequence↗

PubMed2004

A model for the mediation of processivity of DNA-targeting proteins by nonspecific binding: dependence on DNA length and presence of obstacles.

A physical and mathematical model is presented to explain processivity of proteins on DNA. In this model, a DNA-targeting protein such as a restriction enzyme can diffuse to the DNA surface and nonspecifically bind to it. Once on the DNA surface it will either move along the DNA or equilibrate with the surrounding region. Owing to the nonspecific binding, the search for a specific site on the DNA occurs in a reduced dimensionality, and the protein appears processive when moving from one specific site to another. The simplest version of this nonspecific-binding-facilitated diffusion model is solved and the results quantitatively explain experimentally observed dependence of the processivity ratio on the intervening DNA length between two specific sites.

Binding Sites↗

PubMed2004

Enhancement of association rates by nonspecific binding to DNA and cell membranes.

A comprehensive analytic theory is developed for the kinetics of reversible association with specific sites on DNA and receptors on cell membranes in the presence of nonspecific binding to the target surfaces. Nonspecific binding is treated as a short-range attractive potential, which is more fundamental and realistic than the surface sliding model. The presence of a surface potential around the target enhances the rate of specific association and for reversible reactions leads to deviations from single exponential relaxation.

Binding Sites↗

PubMed2004

Polymer models of protein stability, folding, and interactions.

The unfolded state and flexible linkers in the folded structure play essential roles in protein stability and folding and protein-protein interactions. Intrinsic to these roles is the fact that unfolded proteins and flexible linkers sample many different conformations. Polymer models may capture this and complement experiments in elucidating the contributions of the unfolded state and flexible linkers. Here I review what can be predicted from these models and how well these predictions match experiments. For example, Gaussian chain models give quantitatively reasonable predictions of the effects of residual charge-charge interactions in the unfolded state and qualitatively reasonable results for the effects of spatial confinement and macromolecular crowding on protein stability. A wormlike chain model has met with success in quantifying the effects of flexible linkers in binding affinity enhancement and in regulatory switches. In future developments, more realistic models may emerge from molecular dynamics simulations, and these models will guide experiments to advance our understanding of the unfolded state and flexible linkers.

Animals↗

PubMed2004

Loops, linkages, rings, catenanes, cages, and crowders: entropy-based strategies for stabilizing proteins.

A protein molecule exists in either a compact folded state or a variable and open unfolded state. Since the unfolded state is favored by chain entropy, restricting its entropy is an attractive mechanism for shifting the equilibrium toward the folded state. A number of entropy-based strategies have been engineered or used by natural proteins to increase the folding stability: (a) shortening of loop lengths, (b) covalent linkage of dimeric proteins, (c) backbone cyclization, (d) catenation, (e) spatial confinement, and (f) macromolecular crowding. Theoretical analyses demonstrate the importance of accounting for consequences on the folded as well as the unfolded state and provide guidance for further exploitation of these stabilization strategies.

Animals↗

PubMed2004

Improving the understanding of human genetic diseases through predictions of protein structures and protein-protein interaction sites.

Over 1,500 human disease genes have been identified, of which only a small fraction have experimental structural information on the protein products. To better understand the mechanisms of these hereditary diseases, we undertook a systematic study to predict the structures of disease proteins and characterize their interactions with other proteins. This study was facilitated by two tools developed previously: COBLATH, a structure-prediction method that exploits the complementarity of PSI-Blast and sequence-structure threading and PPISP, a method that predicts the residues involved in protein-protein interactions. In this initial study of human disease proteins, we were able to build structural models for 60 proteins involved in human diseases. For a number of proteins, new structural domains were identified. In the case of ABCD1, a protein responsible for adrenoleukodystrophy, the disease mutation P484R was positioned at the homodimer interface. This positioning is consistent with experimental observation that the P484R mutation impairs ABCD1 self-interaction and suggests that that the disease mechanism of this mutation lies in the impaired ABCD1 dimerization. This initial study illustrates the value of the predicted structure models and may serve as an example for expanded studies of other disease proteins.

ATP Binding Cassette Transporter, Subfamily D, Mem↗