3.1 Frequency distribution of hotspot and anchor residues
In all, calculations were made for a total of 5774 interface residues. To our knowledge, this is one of the comprehensive study on hotspot residues in PPI and PPepI. Out of 5774 residues, 3732 residues amounting to 64.6% of the total dataset belong to hotspot categories having ∆∆G ≥ 2 kcal/mol. This is suggestive of the fact that nature has remarkably optimized a great majority (~65%) of the interface residues in protein-protein complexes during evolution. This finding is also in contrast to earlier notion that complex interface comprises only few hot spot residues either isolated or in clusters [51]. The frequency distribution is given in Table 2. Fig. 1 illustrates the histogram of hotspots in PPI and PPepI categories. Altogether both charged and polar residues contribute about 60% of hotspots. In the histogram, the frequency curves drawn for both categories are in perfect sync with a very minor difference at Gln, indicating the similar overall tendency observed in PPI and PPepI. Arg, Tyr, Leu, Lys and Gln, are the preferred hotspot residues at the PPI interfaces with Arg alone accounting for over 10% in the frequency distribution. Met, His, Trp, Gly and Cys are the least preferred hotspot residues with Cys presence is mere 0.1 %. In contrast, Tyr, Leu, Arg and Ile are the most favoured hotspot residues in PPepI category and Cys, Gly, Gln, Met and Trp are the least preferred ones. Examining the trend, the PPI dataset is characterized by the dominance of charged and polar residues followed by hydrophobic residues, whereas PPepI dataset, the polar and hydrophobic followed by charged residues predominantly occupy the frequency distribution. The fact that negatively charged residues are not the ones among preferred hotspot residues suggest that the electrostatic complementarity is not a predominant factor in PPI and PPepI as well.
In PPI dataset, 249 anchor residues were recognized (37 weak, 118 moderate and 87 strong hotspots types; Supplementary. Table 1). In PPepI category, 92 anchor residues were identified (12 weak, 46 moderate and 34 strong types; Supplementary. Table 2). Anchor residues mostly occur for PPI dataset in moderate and strong types. The anchor residues comprise of 8.3% of hot spot residues and about 5.3% of total residues investigated in PPI and PPepI category. Anchor residues demonstrate similar trend with Arg, Leu, Tyr Gln, Phe as the most preferred anchor residue for PPI. For PPepI hydrophobic residue predominantly occupy at the interface - Leu, Ile, Val, Phe, Tyr & Arg (Fig.1).