(alfa1, alfa2, betaS1 and betaS2 chains) Haemoglobin in sickle cells, or drepanocytes (haemoglobin S) differs from haemoglobin A in a single amino acid: glutamate at position 6 on the beta chains is replaced by valine. As a consequence, on the surface of the molecule (usually polar) appears a hydrophobic region, corresponding to valine sidechain; this is usually called a “hydrophobic patch” ( show Val-6).
In deoxygenated haemoglobin (both S and A) there is another small hydrophobic "patch" on the surface of beta chains, due to sidechains from Ala-70 and Leu-88 Highlight hydrophobic patches.
Hydrophobic patches tend to stick to each other, in order to exclude water (hydrophobic effect). In haemoglobin A, the single patch has no serious consequences in this sense, but in deoxygenated haemoglobin S the two different patches make the molecules associate and build a chain (we'll see just a ). As a consequence, haemoglobin S aggregates, polymerises, in the shape of rods or fibers. on the contact point.
These polymerised haemoglobin S fibers may be observed under the electron microscope: figure.
(Although this model serves to illustrate aggregation, the real point of association is another, between Val-6 and a hydrophobic patch located on the surface of alpha chains, not displayed here).