The molar mass of hemoglobin is about 65000 g $mo{l}^{-1}$ .  Every hemoglobin contains 4 iron atoms. Thus,

iron content in hemoglobin is 0.35% by mass

 1 mole of hemoglobin contains 56 g iron

  1 mole of hemoglobin contains 224 g iron

 if iron content is increased to 0.56%. molar mass of hemoglobin would be higher than 65000 g $mo{l}^{-1}$