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These changes nudge the neighboring chains into a different shape, making them bind oxygen more easily.
Thus, it is difficult to add the first oxygen molecule, but binding the second, third and fourth oxygen molecules gets progressively easier and easier.
This shifts the position of an entire alpha helix, shown here in orange below the heme.
This motion is propagated throughout the protein chain and on to the other chains, ultimately causing the large rocking motion of the two subunits shown in blue.
To avoid this problem, novel hemoglobin molecules have been designed where two of the four chains are physically linked together, as shown in PDB entry , shown in the picture here.
However, the need for matching blood type, the short life of stored blood, and the possibility of contamination are still major concerns.
An understanding of how hemoglobin works, based on decades of biochemical study and many crystallographic structures, has prompted a search for blood substitutes and artificial blood.
Nitric oxide affects the walls of blood vessels, causing them to relax. Recent studies have shown that nitric oxide can bind to specific cysteine residues in hemoglobin and also to the irons in the heme groups, as shown in PDB entry .
Thus, hemoglobin contributes to the regulation of blood pressure by distributing nitric oxide through blood.