Conformational constraints on side chains in protein residues increase their information content

Like all other complex biological systems, proteins exhibit properties not seen in free amino acids (i.e., emergent properties). The present investigation arose from the deduction that proteins should offer a good model to approach the reverse phenomenon, namely top-down constraints experienced by protein residues compared to free amino acids. The crystalline structure of profilin Ib, a contractile protein of Acanthamoeba castellanii, was chosen as the object of study and submitted to 2-ns molecular dynamics simulation. The results revealed strong conformational constraints on the side chain of residues compared to the respective free amino acids. A Shannon entropy (SE) analysis of the conformational behavior of the side chains showed in most cases a strong decrease in the SE of the chi1 and chi2 dihedral angles compared to free amino acids. This is equivalent to stating that conformational constraints on the side chain of residues increase their information content and hence recognition specificity compared to free amino acids. In other words, the vastly increased information content of a protein relative to its free monomers is embedded not only in the tertiary structure of the backbone, but also in the conformational behavior of the side chains. The postulated implication is that both backbone and side chains, by virtue of being conformationally constrained, contribute to the recognition specificity of the protein toward other macromolecules and ligands.