Alignment-Independent Comparison of Binding Sites Based on

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Alignment-Independent Comparison of Binding Sites Based on DrugScore Potential Fields Encoded by 3D Zernike Descriptors Britta Nisius and Holger Gohlke* Department of Mathematics and Natural Sciences, Institute of Pharmaceutical and Medicinal Chemistry, Heinrich-Heine University Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany S Supporting Information *

ABSTRACT: Analyzing protein binding sites provides detailed insights into the biological processes proteins are involved in, e.g., into drug−target interactions, and so is of crucial importance in drug discovery. Herein, we present novel alignment-independent binding site descriptors based on DrugScore potential fields. The potential fields are transformed to a set of information-rich descriptors using a series expansion in 3D Zernike polynomials. The resulting Zernike descriptors show a promising performance in detecting similarities among proteins with low pairwise sequence identities that bind identical ligands, as well as within subfamilies of one target class. Furthermore, the Zernike descriptors are robust against structural variations among protein binding sites. Finally, the Zernike descriptors show a high data compression power, and computing similarities between binding sites based on these descriptors is highly efficient. Consequently, the Zernike descriptors are a useful tool for computational binding site analysis, e.g., to predict the function of novel proteins, off-targets for drug candidates, or novel targets for known drugs.

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predicting this location is the first step. Approaches aiming at this scan the surface of the protein for pockets or cavities that most likely are binding sites.11,12 Since many of these methods have been developed in recent years,11 we decided to solely focus on the analysis of already defined binding sites rather than trying to tackle the problem of pocket detection, too. In order to computationally analyze a binding site, its features have to be transformed to numerical values enabling an efficient analysis. To do so, structure-based approaches for computational binding site analysis typically use simplified representations of the amino acids flanking the binding sites, which are encoded as geometric patterns or numerical fingerprints. 13 For a comparison, most of the methods require the binding sites to be optimally aligned, which is often time-consuming. Furthermore, an incorrect alignment of two binding sites can lead to dramatically underestimated similarity scores.14 By contrast, only very few alignment-independent representations are available.14−17 In the present study, we thus aim at developing novel alignment-independent binding site descriptors that can be used for protein binding site comparison as well as for other principal applications in computational binding site analysis, such as druggability prediction.18 While many approaches characterize a binding site based on its shape17,19 or on the amino acids flanking it,20−22 the molecular recognition between a protein and a ligand is governed by interactions that occur within the binding site.

INTRODUCTION Biological processes are based on interactions between proteins and other molecules, e.g., ligands, other proteins, or nucleic acids. Usually, these interactions occur at defined locations, the binding sites. There are several types of binding sites in proteins, e.g., active sites in enzymes, sites for allosteric regulation, or binding interfaces of protein−protein interactions.1 Shape complementarity and physicochemical complementarity are key factors for molecular recognition and interactions.2 Therefore, a binding site’s shape, size, and buriedness, as well as its amino acid composition and, hence, its physicochemical properties, are important properties for its characterization. This complexity makes the analysis of binding sites a demanding problem.3 Detecting biologically relevant similarities between protein binding sites is a main application of computational binding site analysis4 because it allows (I) predicting a drug candidate’s offtarget interactions,3 (II) predicting new targets for known drugs,5−7 or (III) designing selective drugs by analyzing (dis)similarities among members of one target family.8 Likewise, computational approaches for binding site comparison have been applied for protein function-deorphanization9 assuming that proteins with similar biochemical functions should bind similar ligands and, hence, display similar binding site characteristics. Various approaches aiming at the computational identification, analysis, and comparison of protein binding sites have been developed in recent years.10 Unless a cocrystallized ligand readily provides information on the binding site’s location, © 2012 American Chemical Society

Received: May 24, 2012 Published: August 10, 2012 2339

dx.doi.org/10.1021/ci300244y | J. Chem. Inf. Model. 2012, 52, 2339−2347

Journal of Chemical Information and Modeling

Article

Thus, we decided to describe protein binding sites based on their molecular recognition properties encoded in molecular interaction fields (MIFs). MIFs are popular tools for characterizing the ability of a molecule to interact with other molecules.23 The principal idea of MIFs was introduced by Goodford.24 Since then, MIFs have frequently been applied for the analysis of ligands, proteins, and protein binding sites. In the latter case, MIFs have been used for analyzing structural differences within large target families, e.g., proteases,25 protein kinases,26 or nuclear receptors.27 Again, most of these approaches require an alignment of the proteins prior to analysis, although a few alignment-independent approaches for the computational analysis of MIFs have been introduced.15,16 These methods first simplify the MIF by extracting only representative and energetically favorable regions (hot spots). Second, the hot spots are encoded in an alignment-independent way, e.g., by defining pharmacophore patterns15 or by autocorrelation analysis.16 In these two-step approaches, extracting the hot spot regions from the MIF is the most crucial, but also the most difficult step.28 Thus, we intended to develop an approach allowing an alignment-independent encoding of MIFs that does not require an initial extraction of hot spot regions but rather makes use of the complete information of a MIF. To this end, we use the DrugScore29 scoring function for the computation of MIFs. The resulting DrugScore potential fields are then transformed to a set of alignment-independent descriptors via a series expansion in 3D Zernike polynomials.30 So far, 3D Zernike function expansion was mainly used for shape retrieval,31 i.e., for comparing the shapes of proteins,32−34 ligands,35 and protein binding sites.17 However, a series expansion in Zernike polynomials is not limited to binary 3D objects (where, e.g., 1 signals “in” the molecule and 0 “out”). Rather, using these polynomials any 3D object can be encoded as a set of rotationinvariant descriptors and, hence, also MIFs.

Figure 1. Computation of DrugScore potential fields within protein binding sites. To describe the preference of a protein binding site for a particular type of molecular interaction, a cubic grid is placed within the binding site. Then, a probe atom is placed at every grid point, and the strength of the interaction (denoted here by blue spheres of varying sizes) between this probe atom and all amino acids flanking the binding site is computed using the DrugScore scoring function.

Table 1. Probe Atoms and Their Associated Interaction Types

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MATERIALS AND METHODS Characterizing Protein Binding Sites by DrugScore Potential Fields. To characterize binding pockets by MIFs, a cubic grid is placed within the binding site. Then, a probe atom of a particular atom type is placed at every grid point, and the interaction between this probe atom and all amino acids flanking the binding site is quantified by the distancedependent, knowledge-based pair potentials of the DrugScore29 scoring function (Figure 1). We decided to use DrugScore for the MIF computation because it has been shown to be successful in predicting correct binding modes,36−38 thus demonstrating that DrugScore potential fields correctly characterize molecular recognition properties of protein binding sites. Furthermore, based on the formalism of the DrugScore approach, distance-dependent pair potentials for RNA-ligand39 and protein−protein40 interactions have been derived, enabling the computation of MIFs, and subsequently Zernike descriptors, also for these systems. For all computed DrugScore potential fields, a grid spacing of 0.375 Å was used. By using probe atoms of different atom types, different types of interactions within the binding site can be characterized. In this study, we utilized five probe atoms, each one encoding a particular type of protein−ligand interaction (Table 1). When computing DrugScore potential fields, the grid box is usually centered on a ligand placed within the protein structure. If DrugScore potential fields are computed for ligand-free

probe atoma

encoded interaction

C.ar C.3 O.2 N.3 O.3

aromatic interactions aliphatic interactions hydrogen bond acceptor hydrogen bond donor hydrogen bond donor and acceptor

a The types of probe atoms follow the definition of types of ligand atoms used in DrugScore.29

protein structures, the dimensions of the grid box can be defined manually, e.g., after aligning the apo structure to a holo structure of the same protein. Alternatively, one of the many approaches for predicting binding pockets developed recently11 can be applied for defining a pocket. Series Expansion in 3D Zernike Polynomials. Since the cubic, grid-based DrugScore potential fields are inappropriate for a fast and efficient computational analysis of unaligned protein binding sites, we decided to transform the fields to a simpler but still informative set of alignment-independent descriptors. To this end, we utilized a series expansion in 3D Zernike polynomials.30 In general, this technique enables a compact description of any 3D object by a set of object-specific weights. It is based on the idea of generalized Fourier series, where a 3D object given as f(x⃗) is represented as a weighted sum of polynomials Zn,l,m(x⃗) forming a complete orthogonal system of functions (eq 1): ∞

f (x ⃗ ) =

n

l

∑∑ ∑ n = 0 l = 0 m =−l

Ωmnl ·Znlm(x ⃗)

(1)

To enable 3D function expansion in Cartesian coordinates, Canterakis30 introduced the 3D Zernike polynomials (eq 2) 2340



Article

k

Znlm(x ⃗) = ( ∑ qklϑ |x |⃗ 2ϑ ) ·elm(x ⃗) ϑ= 0

Table 2. Set of Five Diverse Proteins for Evaluating the Descriptive Power of the DrugScore-Based Zernike Descriptors

(2)

for 2k = n − l. eml (x⃗) are harmonic polynomials, and qϑkl are coefficients ensuring the orthonormality of the Zernike polynomials within the unit sphere. A detailed derivation and description of these Zernike polynomials can be found in the publication by Canterakis.30 The object-dependent coefficients Ωmnl, which are called Zernike moments, can be computed via eq 3: Ωmnl =

3 4π

∫|x|⃗ ≤1 f (x ⃗)·Znlm(x )⃗ dx ⃗

(3)

l

∑

(Ωmnl)2 (4)

m =−l

While any 3D object can be exactly described by a sum of infinitely many weighted Zernike polynomials according to eq 1, in practice, the series expansion is only performed for a limited number of Zernike polynomials (eq 5): N

f (x ⃗ ) ≈

n

l

∑∑ ∑ n = 0 l = 0 m =−l

Ωmnl ·Znlm(x ⃗)

abbreviation

PDB ID

cyclin-dependent kinase 2 carbonic anhydrase 2 HIV-1 protease adenosine A2a receptor cyclooxygenase 2

CDK CA HIV A2A COX

1b38 1oq5 3o9a 3qak 6cox

having a pairwise sequence identity 10 Å, whereas the largest distance of corresponding side chains in two thrombin structures is 0.92 is obtained for the Zernike descriptors, showing the alignment independence of these descriptors. The deviation from a perfect correlation can be explained by the fact that DrugScore potential fields are computed on a discrete, cubic grid within the binding pocket. Thus, different orientations of the same protein yield slightly different DrugScore potential fields; these differences are then carried forward to the Zernike descriptors. Detecting Similarities among Diverse Proteins Binding Identical Ligands. To evaluate the performance of the DrugScore-based Zernike descriptors in detecting similarities among diverse proteins that bind identical ligands, we utilized the Hoffmann data set. Each protein was once used as a query, and AUC values were computed based on the ranking of all

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RESULTS AND DISCUSSION Reconstructing DrugScore Potential Fields. To initially evaluate the descriptive power of the DrugScore-based Zernike descriptors, we used five diverse proteins (Table 2) and performed series expansions for increasing maximal expansion orders. We selected an aromatic carbon atom as an exemplary probe atom and computed Zernike moments for the corresponding DrugScore potential fields using eq 3. Then, these Zernike moments were applied to compute reconstructed potential fields (eq 5). Finally, the reconstructed potential fields were compared to the original potential fields using Pearson’s correlation coefficient as a measure of reconstruction quality. The higher the correlation between the original and reconstructed potential fields, the more information about the DrugScore potential fields is encoded in the Zernike moments and corresponding Zernike descriptors. A maximal expansion order of 20 already yields a reconstruction quality of r > 0.9 (Figure 2). Furthermore, maximal expansion orders larger than 25 do not yield a significant improvement in reconstruction quality. Since similar results were obtained for other probe atoms too (data not shown), we decided to use N = 25 for the remainder of this study. In Figure 3, original and reconstructed DrugScore potential fields for two exemplary proteins and an aromatic carbon probe atom are shown. The original potential fields consist of more than 50,000 grid points. By expanding the potential fields in terms of Zernike polynomials, this information is encoded by only 3276 Zernike moments from which the reconstructed 2342



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Figure 3. Comparing original and reconstructed DrugScore potential fields. DrugScore potential fields were computed for a probe atom of type aromatic carbon and two exemplary proteins (A, 1oq5; B, 6cox). Then, Zernike moments for a maximum expansion order of 25 were computed. The resulting Zernike moments were utilized to compute reconstructed potential fields. The original potential fields are shown in blue; the reconstructed ones are shown in red. All potential fields were contoured at a value of −20,000 DrugScore units.

descriptors varies among the different subsets: 7 out of 10 cases show an average AUC value >0.7 and 2 out of 10 cases

Alignment-Independent Comparison of Binding Sites Based on

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