How good are ensembles in improving QSAR models? The case with eCoRIA.

A conceptually new idea in quantitative structure-activity relationships (QSAR) which makes use of ensembles from molecular dynamics (MD) trajectories and information retrieved from enzyme-inhibitor binding thermodynamics is presented in this study. This new methodology, termed ensemble comparative residue interaction analysis (eCoRIA), attempts to overcome the current one chemical-one structure-one parameter value dogma in computational chemistry by modeling the biological activity as a function of molecular descriptors derived from an ensemble of conformers of enzyme-inhibitor complexes. The approach is distinctly different from the standard QSAR methodology which uses a single low-energy conformation or the properties averaged over a set of conformers to correlate with the activity. Each conformational ensemble derived from MD simulations is analyzed for the distribution of the non-bonded interaction energies (steric, electrostatic, and hydrophobic) along with solvation, strain, and entropic energy of the inhibitors with the individual amino acid residues in the receptor and these are correlated to the activity through a QSAR model. The scope of the new method is demonstrated with three diverse enzyme-inhibitor data-sets - glycogen phosphorylase b, human immunodeficiency virus-1 protease and cyclin-dependent kinase 2. The QSAR equations derived from the methodology have revealed all the structure activity relationships previously reported for these classes of molecules as well as uncovered some features that were hitherto unknown and may have a hidden role in driving the ligand-receptor-binding process. Impressive improvements in the predictions of affinity have been achieved compared to other QSAR formalisms namely CoMFA, CoMSIA (receptor-independent QSAR techniques), and CoRIA (a receptor-dependent QSAR technique). eCoRIA could provide an understanding of the thermodynamic properties influencing the ligand-receptor binding over a time scale as sampled by the MD simulation. The advantage of analyzing enzyme-inhibitor interaction energies in a statistical domain is that the noise due to inaccuracies in the potential energy functions can be reduced and mechanistically important interaction terms related to protein-ligand binding specificity can be identified which can assist the medicinal chemists in designing new molecules and biologists in studying the influence of position-specific mutations in the receptor on ligand binding.

Over-expression, purification and isotopic labeling of a tag-less human glucose-dependent insulinotropic polypeptide (hGIP).

Glucose-dependent insulinotropic polypeptide (GIP), a gut peptide released in response to food intake brings about secretion of insulin in a glucose-dependent manner upon binding to its receptor, GIPR. GIP-GIPR has emerged as a new vista for anti-diabetic drug discovery and their interaction is being probed at the atomic level to aid rational drug design. In order to probe this interaction on cells, the current study attempts towards expressing 15N-labeled GIP using classical molecular biology tools. We have developed a methodology to obtain GIP devoid of extra amino acid(s); a prerequisite to the intended interaction study. The synthetic GIP cDNA with a Factor Xa protease site at the N-terminus of GIP was inserted in the vector pET32a(+); the fusion protein thus expressed was eventually cleaved to obtain GIP. After successful Factor Xa cleavage, the cleaved GIP was confirmed by western blot. Subsequently, the (15N)GIP was obtained using the aforementioned procedure and confirmed by MALDI-TOF.

Mapping interactions of gastric inhibitory polypeptide with GIPR N-terminus using NMR and molecular dynamics simulations.

Glucose-dependent insulinotropic polypeptide (gastric inhibitory polypeptide, or GIP), a 42-amino acid incretin hormone, modulates insulin secretion in a glucose-concentration-dependent manner. Its insulinotropic action is highly dependent on glucose concentration that surmounts the hypoglycemia side effects associated with current therapy. In order to develop a GIP-based anti-diabetic therapy, it is essential to establish the 3D structure of the peptide and study its interaction with the GIP receptor (GIPR) in detail. This will give an insight into the GIP-mediated insulin release process. In this article, we report the solution structure of GIP(1-42, human)NH(2) deduced by NMR and the interaction of the peptide with the N-terminus of GIPR using molecular modelling methods. The structure of GIP(1-42, human)NH(2) in H(2)O has been investigated using 2D-NMR (DQF-COSY, TOCSY, NOESY, (1)H-(13)C HSQC) experiments, and its conformation was built by constrained MD simulations with the NMR data as constraints. The peptide in H(2)O exhibits an alpha-helical structure between residues Ser8 and Asn39 with some discontinuity at residues Gln29 to Asp35; the helix is bent at Gln29. This bent gives the peptide an 'L' shape that becomes more pronounced upon binding to the receptor. The interaction of GIP with the N-terminus of GIPR was modelled by allowing GIP to interact with the N-terminus of GIPR under a series of decreasing constraints in a molecular dynamics simulation, culminating with energy minimization without application of any constraints on the system. The canonical ensemble obtained from the simulation was subjected to a detailed energy analysis to identify the peptide-protein interaction patterns at the individual residue level. These interaction energies shed some light on the binding of GIP with the GIPR N-terminus in a quantitative manner.

Exploring the binding of HIV-1 integrase inhibitors by comparative residue interaction analysis (CoRIA).

Since the recognition of HIV-1 integrase as a novel and rational target for HIV therapeutics, remarkable progress has been made in the development of integrase inhibitors. Computational techniques have played a critical role in accelerating research in this area. However, most previous computational studies were based solely on ligand information. In the present work, we describe the application of one of our recently developed receptor-based 3D-quantitative structure activity relationships (QSAR) methods, i.e. comparative residue interaction analysis (CoRIA), in exploring the events involved in ligand-integrase binding. In this methodology, the non-bonded interaction energies (van der Waals and Coulombic) of the inhibitors with individual active site residues of the integrase enzyme are calculated and, along with other thermodynamic descriptors, are correlated with biological activity using chemometric methods. Different combinations of descriptors were used to develop three types of QSAR models, all of which were found to be statistically significant by internal and external validation. This is the first report of such a dedicated receptor-based 3D-QSAR approach being applied to comprehend the integrase-inhibitor recognition process. In addition, the study was performed on 13-different series of inhibitors, thereby exploring the most structurally diverse data set ever used in understanding the inhibition of HIV-1 integrase. The major advantage of this technique is that it can quantitatively extract crucial residues and identify the nature of interactions between the ligand and receptor that modulate activity. The models suggest that Asp64, Thr66, Val77, Asp116, Glu152 and Lys159 are the key residues influencing the binding of ligands with the integrase enzyme, and the majority of these results are in line with earlier studies. The approach facilitates easy lead-to-hit conversion and design of novel inhibitors by optimisation of the interaction of ligands with these specific residues of the integrase enzyme.

Conformational study of fragments of envelope proteins (gp120: 254-274 and gp41: 519-541) of HIV-1 by NMR and MD simulations.

The envelope proteins, gp 120 and gp41 of HIV-1, play a crucial role in receptor (CD4+ lymphocytes) binding and membrane fusion. The fragment 254-274 of gp120 is conserved in all strains of HIV and, as a part of the full gp120 protein, behaves as 'immunosilent', but as an individual fragment it is 'immunoreactive'. When this fragment binds to its receptor, it activates the fusion domain of gp41 allowing viral entry into the host CD4+ cells. The conformation of fragment 254-274 of the gp120 domain and fragment 519-541 of the gp41 domain was studied by NMR and MD simulations. The studies were carried out in three varied media--water, DMSO-d6 and hexafluoroacetone (HFA). The fusogenic nature of the gp41 domain peptide was investigated by 31P NMR experiments with model bilayers prepared from dimyristoyl-L-alpha-phosphatidylcholine (DMPC). The solvent was seen to exert a major effect on the structure of the two peptides. Fragment (254-274) of gp120 in DMSO-d6 had a type I beta-turn around the tetrad Val9-Ser10-Thr11-Gln12 while in HFA a helical structure spanning the region Ile5 to Gln12 was seen with the remaining part of the peptide in a random coil structure. It is possible that the beta-turn may constitute an initiation site for the formation of the helix. In water at pH 4.5, the peptide adopted a beta-sheet. The NMR results for fragment 519-541 of gp41 are conclusive of a beta-sheet structure in DMSO-d6, a conformation which may help in insertion into the membrane, a notion also put forward by others. The 31P NMR studies of DMPC vesicles with this fragment show its fusogenic nature, promoting fusion of unilamellar vesicles to larger agglomerates like multilamellar ones.

Activity and conformation of a cyclic heptapeptide possessing the message sequence His-Phe-Arg-Trp of alpha-melanotropin.

Alpha-melanotropin (alpha-MSH, i.e. alpha-melanocyte stimulating hormone), tridecapeptide (Ac-Ser(1)-Tyr-Ser-Met-G1u(5)-His-Phe-Arg-Trp-Gly(10)-Lys-Pro-Val(13)-NH(2)), has been extensively studied to understand structure-activity relationships. The core sequence (His-Phe-Arg-Trp) is conserved in several species and is considered as the primary active site or "message sequence". Attempts have been made to design conformationally constrained cyclic analogs containing the message sequence to improve the activity. We had earlier reported that the cyclic analog--cyclo[Gly-His-D-Phe-Arg-Trp-Gly], a 18 membered ring system with two fused beta-turn structure, was less active than the corresponding linear peptide. It was suggested that ring size could be an important parameter in the activity of cyclic melanotropic analogs. To investigate the effect of ring size on biological activity, a cyclic heptapeptide, cyclo[Nle(1)-Gly-His-D-Phe-Arg(5)-Trp-Gly(7)], with 21 member ring system was synthesized. This peptide has three orders of magnitude higher biological activity than the cyclic hexapeptide. The conformational study of this cyclic heptapeptide in DMSO-d(6) by NMR and molecular dynamics simulations reveals a structure with two fused beta-turns running across the residues D-Phe(4)-Gly(7) (Type I) and Gly(7)-His(3) (Type II). These findings confirm that stabilization of beta-turns and a relatively larger ring size are essential determinants of activity for cyclic alpha-MSH analogs.

Docking studies reveal a selective binding of D-penicillamine to the transactivator protein of human immunodeficiency virus type 1.

DOCK and Affinity studies were carried out to study the binding of D- and L-penicillamine to the transactivator protein (tat) of human immunodeficiency virus type 1 (HIV-1). These studies reveal a selective binding of D-penicillamine to the cysteine-rich region covering amino acid residues 20-38 of the tat protein. A careful analysis of the components of the binding energy of the D- and L-isomers reveals that the D-isomer has a more favorable van der Waals interaction resulting from an optimal placement of the dimethylthiomethyl side chain in the binding site. This observation matches the experimental data that D-penicillamine is a more potent inhibitor of tat-mediated transactivation than the L-isomer. The docking and experimental data offer an interesting approach to design structural molecules with potential application to block signal functions of the tat protein in HIV-1 pathogenesis.