Bioinformatics Advance Access originally published online on February 15, 2005
Bioinformatics 2005 21(10):2534-2536; doi:10.1093/bioinformatics/bti322
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FastContact: rapid estimate of contact and binding free energies
1Department of Computational Biology, University of Pittsburgh Pittsburgh, PA 15261, USA
2Plexxikon Inc. 91 Bolivar Drive, Berkeley, CA 94710, USA
*To whom correspondence should be addressed.
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Summary: Interaction free energies are crucial for analyzing binding propensities in proteins. Although the problem of computing binding free energies remains open, approximate estimates have become very useful for filtering potential binding complexes. We report on the implementation of a fast computational estimate of the binding free energy based on a statistically determined desolvation contact potential and Coulomb electrostatics with a distance-dependent dielectric constant, and validated in the Critical Assessment of PRotein Interactions experiment. The application also reports residue contact free energies that rapidly highlight the hotspots of the interaction.
Availability: The program was written in Fortran. The executable and full documentation is freely available at http://structure.pitt.edu/software/FastContact
Contact: ccamacho{at}pitt.edu
Today, nearly all docking methods use some type of scoring function to differentiate between near-native complexes and non-specific encounter complexes. In the First Critical Assessment of PRedicted Interaction meeting, CAPRI (Janin et al., 2003), computational scoring functions involved free energy-like terms adjusted by free parameters that optimized the discrimination of bound crystal structures (Fernández-Recio et al., 2003; Gray et al., 2003; Ritchie, 2003; Smith and Sternberg, 2003) or more geometrical discriminators, e.g., buried surface area (Gardiner et al., 2003; Krippahl et al., 2003; Law et al., 2003; Schneidman-Duhovny et al., 2003), or hybrids of these two approaches (Ben-Zeev et al., 2003; Chen et al., 2003).
At the same time, in the literature one finds several more sophisticated, and perhaps more accurate, approaches for estimating different free energy contributions—e.g., free energy perturbation (Kollman, 1993), Poisson–Boltzman (Honig and Nicholls, 1995), atomic continuum electrostatic (Schaefer and Karplus, 1996) and generalized-Born solvation (see, e.g., Qiu et al., 1997). However, since protein docking requires filtering or sampling millions of plausible complex structures and these methods are computationally expensive, they are not used for free energy screening.
Finally, a somewhat different approach to screen protein binding interactions has been developed by Camacho et al. (2000). These authors use a free energy scoring function developed independently of the bound crystal structures present in the Protein Data Bank (PDB) (Berman et al., 2000). Namely, the interaction between two proteins is estimated as 
Gbind, where
 | (1) |
Eelec corresponds to the standard intermolecular Coulombic electrostatic potential with a distance-dependent dielectric constant equal to 4r (Pickersgill, 1988). Gdes captures the most essential features of the desolvation free energy in proteins, including hydrophobic interactions, the self-energy change upon desolvating charge on polar atom groups and side-chain entropy loss. Gdes is calculated by an empirical contact potential of the form Gdes = g(r) 
eij, where eij denotes the atomic contact potential (ACP) between atom i of the receptor and j of the ligand. The double sum is taken over all atom pairs and g(r) is 0 for atoms that are more than 7 Å apart, 1 if less than 5 Å apart and in between g(r) is a smooth function varying between these two limits (Zhang et al., 1997b). The ACPs have been defined for a total of 18 atom types, and obtained from a diverse set of close to 90 protein structures by converting frequencies of structural factors into atom–atom contacts. This free energy estimates reasonably well experimental binding affinities from complex crystal structures (Zhang et al., 1997a; b; Kimura et al., 2001). However, filtering decoys with less than optimal side chain packing and structural/charge overlap is not as straightforward. Two problems are the sensitivity of the electrostatic energy to charge overlaps, and the overextended contribution of the desolvation term arising from overlapping contacts that are not at the protein surface. It is worth mentioning that although one could easily remove most overlaps by energy minimization, this is not computationally feasible for a million or so structures. We address these problems by first not allowing two atoms to be any closer than the sum of their van der Waals radii, preventing artificial spikes on the electrostatic term (Vasmatzis et al., 1996; Zhang et al., 1999); and second, we provide an option to always require that at least one of the interacting atoms be exposed to the solvent by at least 1 Å2 in the unbound state (Camacho et al., 1999). The latter is done by computing the solvent accessible surface area of each individual protein using (Lee and Richards 1971) algorithm. It is worth mentioning that the problem of over-counting desolvation contact energies is worst when the receptor–ligand overlap is more than around 300 Å3; for a minimum overlap the range of the contact potential is sufficient to constrain the interactions within the surfaces.
This free energy was the main filter of potential binding sites used by Camacho and Gatchell (2003) in the first CAPRI experiment. These authors produced some of the best predictions at CAPRI1–2 (Méndez et al., 2003), appropriately ranking the native-like models. We have also implemented our method as a fully automated public server named ClusPro (Comeau et al., 2004). ClusPro was the only server validated in the second CAPRI meeting (Gaeta, Italy, 2004), where for 5 (out of 10) targets, native-like structures were submitted. Moreover, for two of the targets, the models predicted using Equation (1) were some of the most accurate among all submissions (Comeau, Vajda and Camacho, Proteins 2005). It is also worth mentioning that, after adding the van der Waals interactions and Equation (1) to the scoring function, the native-like structures submitted after flexible refinement (Camacho, Proteins, 2005). In order to share this utility with the research community, we have implemented this fast scoring function in a program called FastContact. The input of FastContact is as follows:
FastContact RTF receptor.pdb ligand.pdb Num_extra_ligands Contacts SASA The RTF_file defines the united atom composition of each amino acid and it is provided together with the executable. The RTF file includes a list of residue types and their atomic make-up, partial charges and van der Waals radii; the data is consistent with CHARMM19 parameters. The user is free to modify this file provided the format of the data remains the same. The proteins should be in standard CHARMM (Brooks et al., 1983) or CONGEN (Bruccoleri et al., 1997) format with polar hydrogens only. Since in rigid-body docking one often is interested in scoring several ligand conformations against the same receptor, we provide an option that allows computing the binding free energy for as many extra ligands as needed. The program will read from standard input Num_extra_ligands file names of new ligand structures. The main output of the program is directed to the screen and consists of the total electrostatic and desolvation energy. If Contacts 
1, the output details the top 20 residues that have the minimum and maximum contribution to the different free energy components; the residues are renumbered starting with number 1 and the program creates two PDB files, fort.19 and fort.20, with the new numbers. If Contacts = 1 no contact energy information is produced. SASA 1 will check that at least one of the contact residues is at the surface. If this constraint is not deemed necessary then SASA = 1.
Computing the solvent accessible surface area (SASA 1) is the most computationally by expensive step of the algorithm. Using a single Pentium 4 processor, FastContact takes less than 0.1 seconds to compute Gbind for two single domain proteins and SASA = 1, and 3 s if SASA 1. However, once SASA is computed for one receptor and ligand, extra runs using different orientations of the same ligand structure take less than 0.1 s. The maximum number of residues is 1500. If the residue name is not in the RTF file, the program stops; if an atom is not in the RTF then its contribution is made equal to zero and a warning message is spooled to the screen.
The contact information (Contacts 1) is very useful in model refinement of rigid-body docked conformations because in the output list one can read the residues and pair of residues that provide both the most attractive and repulsive free energy. While the former immediately highlights the hot spots of the binding interaction, the latter often suggest side chains that might need to be refined. Also, the residue contact free energies should prove useful in selecting interesting residues for mutagenesis experiments.
FastContact provides a fast estimate of the interaction free energy between two proteins. Because it is based on folding data, the estimate is robust and does not require to be re-parameterized as more complex structures become available. More importantly, as far as we know, it is the only scoring function validated in CAPRI that is made available to the community at large. FastContact can now be combined with the user favorite decoys generator and other scoring functions to further refine predictions of complex structures.
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Acknowledgments |
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We are grateful to Charles DeLisi for his help and support while the authors were at Boston University. C.J.C. also acknowledges the help of S. Vajda and Z. Weng. C.J.C is grateful for the support of the University of Pittsburgh. We are also thankful to Christoph Champ for setting up the link to download. ACP and ACP-based binding energy function was developed by C. Zhang in collaboration with Drs. J. Cornette and G. Vasmatzis while working at Professor Charles DeLisi's laboratory.
Received on October 17, 2004; revised on February 4, 2005; accepted on February 9, 2005
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