Helix movements accommodate interior side-chain mutations

Helix movements accommodate interior side-chain mutations 

The proteins thus adapt to mutations of buried residues by changing their overall structure, which in the globins involves movements of entire a helices relative to each other. The structure of loop regions changes so that the movement of one a helix is not transmitted to the rest of the structure. Only movements that preserve the geometry of the heme pocket are accepted. Mutations that cause such structural shifts are tolerated because many different combinations of side chains can produce well-packed helix-helix interfaces of similar but not identical geometry and because the shifts are coupled so that the geometry of the active site is retained. 

---