Chang G. Ji

Simulation of NMR Data Reveals That Proteins’ Local Structures Are Stabilized by Electronic Polarization

Molecular dynamics simulations of NMR backbone relaxation order parameters have been carried out to investigate the polarization effect on the proteins local structure and dynamics for five benchmark proteins (bovine pancreatic trypsin inhibitor, immunoglobulin-binding domain (B1) of streptococcal protein G, bovine apo-calbindin D9K, human interleukin-4 R88Q mutant, and hen egg white lysozyme). In order to isolate the polarization effect from other interaction effects, our study employed both the standard AMBER force field (AMBER03) and polarized protein-specific charges (PPCs) in the MD simulations. The simulated order parameters, employing both the standard nonpolarizable and polarized force fields, are directly compared with experimental data. Our results show that residue-specific order parameters at some specific loop and turn regions are significantly underestimated by the MD simulations using the standard AMBER force field, indicating hyperflexibility of these local structures. Detailed analysis of the structures and dynamic motions of individual residues reveals that the hyperflexibility of these local structures is largely related to the breaking or weakening of relevant hydrogen bonds. In contrast, the agreement with the experimental results is significantly improved and more stable local structures are observed in the MD simulations using the polarized force field. The comparison between theory and experiment provides convincing evidence that intraprotein hydrogen bonds in these regions are stabilized by electronic polarization, which is critical to the dynamical stability of these local structures in proteins.

Electronic polarization is important in stabilizing the native structures of proteins.

Quantum mechanical computations of proteins based on the molecular fragment approach have been carried out, and polarized protein-specific charges have been derived to provide accurate electrostatic interactions for a benchmark set of proteins. Our study shows that, under the polarized protein-specific force field, the native structure indeed corresponds to the lowest-energy conformation for these proteins. In contrast, when a standard mean-field force field such as AMBER is used, the energies of many decoy structures of proteins could be lower than those of the native structures. Furthermore, MD simulations were carried out and verified that the native structures of these proteins not only are statically more stable but are also dynamically more stable under the polarized protein-specific force field. The present results, together with several recent studies, provide strong evidence that protein polarization is critical to stabilizing the native structures of proteins.

Quantifying the Stabilizing Energy of the Intraprotein Hydrogen Bond Due to Local Mutation

MD simulation of the WW domain of PIN based on a dynamically adjusted polarized protein-specific force field from quantum fragment calculations is carried out in both wild and VAL22ALA mutant states. The result shows that the geometry of the Arg14-TYR23 hydrogen bond is conserved upon mutation of VAL22 to ALA. However, the electrostatic energy of this hydrogen bond in the mutant is found to be 0.6 kcal/mol weaker than in the wild state, in close agreement with the experimentally measured upper limit of 1.2 kcal/mol. Analysis shows that the weakened energy of this hydrogen bond in the mutant is due to its dynamically changed polarization resulting from an altered local electrostatic environment near the hydrogen bond which becomes more exposed to the solvent than in the wild.

Effect of interprotein polarization on protein–protein binding energy

Molecular dynamics simulation in explicit water for the binding of the benchmark barnase‐barstar complex was carried out to investigate the effect polarization of interprotein hydrogen bonds on its binding free energy. Our study is based on the AMBER force field but with polarized atomic charges derived from fragment quantum mechanical calculation for the protein complex. The quantum‐derived atomic charges include the effect of polarization of interprotein hydrogen bonds, which was absent in the standard force fields that were used in previous theoretical calculations of barnase‐barstar binding energy. This study shows that this polarization effect impacts both the static (electronic) and dynamic interprotein electrostatic interactions and significantly lowers the free energy of the barnase‐barstar complex.

Studying the Effect of Site-Specific Hydrophobicity and Polarization on Hydrogen Bond Energy of Protein Using a Polarizable Method

Quantification of backbone hydrogen bond energies in protein folding has remained elusive despite extensive theoretical and experimental investigations over the past 70 years. This is due to difficulties in experimental mutagenesis study as well as the lack of quantitatively reliable methods in theoretical calculation. Recent advance in experiment has enabled accurate measurement of site-specific backbone hydrogen bond energy in protein. In the present work, we developed an accurate and practical polarizable method to study site-specific hydrogen bond energies in the PIN WW domain. Excellent quantitative agreement between our calculated hydrogen bonding energy and recent experimental measurement is obtained. The direct comparison between theory and experiment helps uncover the microscopic mechanism of experimentally observed context dependent hydrogen bond contribution to protein stability in beta-sheet. In particular, our study reveals two effects that act in a cooperative manner to impact the strength of a hydrogen bond. One is the dynamic stability of the hydrogen bond determined by nearby solvent molecules, and the other is the polarization state of the hydrogen bond influenced by local electrostatic environment. The polar character of the hydrogen bond results in strong coupling between hydrophobic and polarization interactions in a cooperative manner. This nonadditive character in hydrogen bonding should help us better understand the microscopic mechanism in protein folding. Our study also investigated the possible structural effect of backbone amide to ester mutation which should be helpful to experimentalists using this technique in mutagenesis study.

Development of an Effective Polarizable Bond Method for Biomolecular Simulation

An effective polarizable bond (EPB) model has been developed for computer simulation of proteins. In this partial polarizable approach, all polar groups of amino acids are treated as polarizable, and the relevant polarizable parameters were determined by fitting to quantum calculated electrostatic properties of these polar groups. Extensive numerical tests on a diverse set of proteins (including 1IEP, 1MWE, 1NLJ, 4COX, 1PGB, 1K4C, 1MHN, 1UBQ, 1IGD) showed that this EPB model is robust in MD simulation and can correctly describe the structure and dynamics of proteins (both soluble and membrane proteins). Comparison of the computed hydrogen bond properties and dynamics of proteins with experimental data and with results obtained from the nonpolarizable force field clearly demonstrated that EPB can produce results in much better agreement with experiment. The averaged deviation of the simulated backbone N-H order parameter of the B3 immunoglobobulin-binding domain of streptococcal protein G from experimental observation is 0.0811 and 0.0332 for Amber99SB and EPB, respectively. This new model inherited the effective character of the classic force field and the fluctuating feature of previous polarizable models. Different from other polarizable models, the polarization cost energy is implicitly included in the present method. As a result, the present method avoids the problem of over polarization and is numerically stable and efficient for dynamics simulation. Finally, compared to the traditional fixed AMBER charge model, the present method only adds about 5% additional computational time and is therefore highly efficient for practical applications.

Interaction Entropy for Computational Alanine Scanning

The theoretical calculation of protein-protein binding free energy is a grand challenge in computational biology. Accurate prediction of critical residues along with their specific and quantitative contributions to protein-protein binding free energy is extremely helpful to reveal binding mechanisms and identify drug-like molecules that alter protein-protein interactions. In this paper, we propose an interaction entropy approach combined with the molecular mechanics/generalized Born surface area (MM/GBSA) method for solvation to compute residue-specific protein-protein binding free energy. In the current approach, the entropic loss in binding free energy of individual residues is explicitly computed from moledular dynamics (MD) simulation by using the interaction entropy method. In this approach the entropic contribution to binding free energy is determined from fluctuation of the interaction in MD simulation. Studies for an extensive set of realistic protein-protein interaction systems showed that by including the entropic contribution, the computed residue-specific binding free energies are in better agreement with the corresponding experimental data.

Glycosylation Modulates Human CD2-CD58 Adhesion via Conformational Adjustment.

Human CD2 is a transmembrane cell surface glycoprotein found on T lymphocytes and natural killer cells and plays important roles in immune recognition. The interaction between human CD2 and its counter receptor CD58 facilitates surface adhesion between helper T lymphocytes and antigen presenting cells as well as between cytolytic effectors and target cells. In this study, the molecular effect of glycosylation of CD2 on the structure and dynamics of the CD2-CD58 adhesion complex were examined via MD simulation to help understand the fundamental mechanism of glycosylation that controls CD2-CD58 adhesion. The present result and detailed analysis revealed that the binding interaction of human CD2-CD58 is dominated by three hot spots that form a binding triangle whose topology is critical for stable binding of CD2-CD58. Our study found that the conformation of human CD2, represented by the topology of this binding triangle, is significantly adjusted and steered by glycosylation toward a particular conformation that energetically stabilizes the CD2-CD58 complex. Thus, the fundamental mechanism of glycosylation of human CD2 is to promote CD2-CD58 binding by conformational adjustment of CD2. The current result and explanation are in excellent agreement with previous experiments and help elucidate the dynamical mechanism of glycosylation of human CD2.

Energetics of protein backbone hydrogen bonds and their local electrostatic environment

MD simulation study of several peptides including a polyalanine, a helix (pdb:2I9M), and a leucine zipper were carried out to investigate hydrogen bond energetics using dynamic polarized protein-specific charge (DPPC) to account for the polarization effect in protein dynamics. Results show that the backbone hydrogen-bond strength is generally correlated with its specific local electrostatic environment, measured by the number of water molecules near the hydrogen bond in the first solvation shell. The correlation coefficient is found to be 0.89, 0.78, and 0.80, respectively, for polyalanine, 2I9M protein, and leucine zipper. In the polyalanine, the energies of the backbone hydrogen bonds are very similar to each other due to their similar local electrostatic environment. The current study helps demonstrate and support the understanding that hydrogen bonds are stronger in a hydrophobic surrounding than in a hydrophilic one. For comparison, the result from simulation using standard force field shows a much weaker correlation between hydrogen bond energy and local electrostatic environment due to the lack of polarization effect in the force field.

Molecular dynamics study of DNA binding by INT‐DBD under a polarized force field

The DNA binding domain of transposon Tn916 integrase (INT‐DBD) binds to DNA target site by positioning the face of a three‐stranded antiparallel β‐sheet within the major groove. As the negatively charged DNA directly interacts with the positively charged residues (such as Arg and Lys) of INT‐DBD, the electrostatic interaction is expected to play an important role in the dynamical stability of the protein–DNA binding complex. In the current work, the combined use of quantum‐based polarized protein‐specific charge (PPC) for protein and polarized nucleic acid‐specific charge (PNC) for DNA were employed in molecular dynamics simulation to study the interaction dynamics between INT‐DBD and DNA. Our study shows that the protein–DNA structure is stabilized by polarization and the calculated protein–DNA binding free energy is in good agreement with the experimental data. Furthermore, our study revealed a positive correlation between the measured binding energy difference in alanine mutation and the occupancy of the corresponding residues hydrogen bond. This correlation relation directly relates the contribution of a specific residue to protein–DNA binding energy to the strength of the hydrogen bond formed between the specific residue and DNA.