There are more than 300 variants of human hemoglobin gene. Among these only a few are fatal. Hence, the most important factor to be conserved in a protein for its function is the -
Function in biochemistry revolves around a reversible protein-ligand interaction.
A molecule bound reversibly by a protein is called a ligand'. A ligand binds at a site on the protein called the `binding site'. which is complementary to the ligand in size, charge and hydrophobic or hydrophilic character. As long as the ligand-binding residues are preserved to allow interaction between the ligand and the binding sites the function will essentially be preserved.
Amino acid sequence
Ligand binding residues
Structure
Environment
Ligand binding residues