Applications of complementarity plot in error detection and structure validation of proteins

The complementarity plot (CP) is based on packing and electrostatics of amino acid residues buried within globular proteins and is a sensitive indicator of the harmony or disharmony of interior residues with regard to short and long range forces sustaining the native fold. As a structure validation tool, it has already been reported to be effective in detecting erroneous side-chain torsions in obsoleted structures. The current study describes the design of several local and global scores based on CP and surveys their utilities in discriminating between obsolete structures and their corresponding upgraded counterparts, detection of wrong rotamer assignments and in identifying packing anomalies. CPs are especially effective in the detection of low-intensity errors (in main-chain geometrical parameters) diffused over the entire polypeptide chain. The methodology is also used to confirm the integral role played by strategic deviations (in main-chain geometrical parameters) in maintaining fold integrity, as reversal to their corresponding ideal values (either unimodal or conformation dependent) lead to large-scale structural distortions. A special feature of this validation tool is to signal unbalanced partial charges within protein interiors. The application of CP in protein homology modeling and protein design is also demonstrated.