> Different proteins can also be related to each other. Even when the sequence similarity between two proteins is low, because of evolutionary pressures, this s

> Different proteins can also be related to each other. Even when the sequence similarity between two proteins is low, because of evolutionary pressures, this similarity tends to be concentrated where it matters, which is the binding site. It’s a small nitpick, but I think that the author actually meant “sequence identity” here, because his statement would make much more sense then. Sequence similarity is physicochemical in nature, and tends to be concentrated, in addition to functionally relevant sites (usually ligand-binding residues, as he mentioned), at key structural regions of the protein such as the hydrophobic core (where a high frequency of similarly hydrophobic residues is expected). This is one of the reasons why proteins from the same family can share highly similar structures while having very low sequence identity, with highly conserved motifs (where the sequence identity is concentrated) taking care of the functionality.