Primary structure of proteins is made of thousands of amino acid residues, yet only a very small region seems to play a role in most of its function. This region is often called “binding site,” “active site,” or “catalytic site” depending on the context and the type of function associated with it. An amino acid residue in protein is not only its compositional alphabet but also serves as a minimal unit of mutations and source of its stability and functional diversity. It is amazing how changes as small as one or a few key residues can bring about qualitative changes in the cellular functions of organisms, and hence it becomes essential to identify these residues and understand their role in structure and function, as well as the mechanism of their action. A lot of experimental and computational research in biology is devoted to the identification of these so‐called functional residues. Yet, the process of finding functional residues remains incomplete in terms of both experimental discovery as well as predictive strategies. In this special issue, we invite papers reporting results from experimental as well as computational studies, which help us in identifying functional residues in proteins. Potential topics include, but are not limited to:

• Functional residues in single proteins, such as enzymes, and protein‐protein, protein nucleic acid, and protein ligand complexes
• Effect of mutations on structure and function of single proteins, their multimers, and large assemblies
• Cooperativity between residues and discovery of sequence and structural motifs
• Evolutionary dynamics, conservation, and functional diversity of amino acid residues
• Amino acid alphabet, information, and entropy in protein sequences and structures
• Thermodynamics and calorimetric studies on functional residues, for example, binding sites and active sites
• Prediction of binding sites, active sites, or residues which are important for function and structure

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