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Analysis of Pseudo-Symmetry in Protein Homo-Oligomers
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Symmetry plays a significant role in protein structural assembly and function. This is especially true for large homo-oligomeric protein complexes due to stability and finite control of function. But, symmetry in proteins are not perfect due to unknown reasons and leads to pseudosymmetry. This study focuses on symmetry analysis of homo-oligomers, specifically homo-dimers, homo-trimers and homo-tetramers.
We defined Off Symmetry (OS) to measure the overall symmetry of the protein and Structural Index (SI) to quantify the structural difference and Assembly Index (AI) to quantify the assembly difference between the subunits. In most of the symmetrical homo-trimer and homo-tetramer proteins, Assembly Index contributes more to Off Symmetry and in the case of homo-dimer, Structural index contributes more than the Assembly Index. The main chain atom Carbon-Alpha (CA) is more symmetrical than the first side chain atom Carbon-Beta (CB), suggesting protein mobility may contribute to the pseudosymmetry. In addition, Pearson coefficient correlation between their Off-Symmetry and their respective atoms B-Factor (temperature factor) are calculated. We found that the individual residues of a protein in all the subunits are correlated to their average B-Factor of these residues. The correlation with BFactor is stronger in Structure Index than Assembly Index. All these results suggest that protein dynamics play an important role and therefore a larger off-symmetry may indicate a more mobile and flexible protein complex.