Hemoglobin is
a tetrameric protein with two alpha and two beta
subunit chains. The standard form is HbA. In the HbS
form, replacement of the amino acid residue Glu by
Val in the two beta subunits
(dark grey) increases the negative electrical charge where they
contact the alpha subunits [top, left]. Under conditions
of reduced oxygen tension in the blood, the modified tetramer
undergoes a conformational shift that forms stable crystals [top,
right].
