Hemoglobin is a
tetrameric
protein
with two alpha and two beta subunit chains. The
standard form is HbA. In the HbS form, replacement
of
glu by val in the two beta
subunits (dark grey) 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]. Crystal formation result in sickle-shaped
red blood cells [below, left].
