Though selenomethionine and methionine have the same structures, except the replacement of sulphur by selenium in selenomethionine (figure attached), the average mass of selenium (78.96) is more than twice of sulphur (32.07). Since the electronegativity is almost same for both the atoms (2.4 and 2.5) will there be any change in the function of the protein by the replacement of a heavy atom like selenium by sulphur?
Also, why a pdb file represents selenomethionine as hetero atom eventhough it is an amino acid?