Final answer:
The valine residue in Hb S binds to a hydrophobic patch on the hemoglobin, causing its polymerization and the characteristic sickle shape of red blood cells.
Step-by-step explanation:
In Hb S, a valine residue binds to a hydrophobic patch causing polymerization of the hemoglobin. The amino acid substitution from glutamic acid to valine at the sixth position of the beta chain leads to the formation of a sticky patch on the protein surface. This sticky patch facilitates the aggregation of the deoxygenated HbS molecules, which results in the polymerization of hemoglobin and the sickled shape of red blood cells in sickle cell anemia.