text: user4476: Anyone knows of an example of a protein that, without coming from a recent duplication event, underwent a mutation(s) that caused it to have a novel interaction with a new ligand, substrate, other protein or molecule?
  By novel I imply that it doesn't "go back" to an ancestral interaction or doesn’t develops an interaction that it's already performed by a close relative. 
  I assume that these mutations are extremely rare, and that a duplicate is almost always necessary to have them fixed in a lineage.
  
  user3848: HA Sialic acid receptor binding in influenza virus determining host specificity, viz. Bird flu vs human flu. 
  
original_text: 4476: <p>Anyone knows of an example of a protein that, without coming from a recent duplication event, underwent a mutation(s) that caused it to have a novel interaction with a new ligand, substrate, other protein or molecule?
  By novel I imply that it doesn't "go back" to an ancestral interaction or doesn’t develops an interaction that it's already performed by a close relative. 
  I assume that these mutations are extremely rare, and that a duplicate is almost always necessary to have them fixed in a lineage.</p>
   3848: <p>HA Sialic acid receptor binding in influenza virus determining host specificity, viz. Bird flu vs human flu. </p>
  