1.6.3 A faulty shape
An example of the effect of a single change in the amino acid sequence of a protein is provided by haemoglobin, the protein in the red blood cells that binds oxygen as it is transported from the lungs to the cells elsewhere in the body. The sixth amino acid in the haemoglobin chain starting from the amino end, is normally glutamate, but in some individuals, valine appears there instead, profoundly altering the shape of the haemoglobin molecule.
The R groups of glutamate and valine are shown below.
Why do you think the shape of the haemoglobin molecule is affected so much when valine replaces glutamate?
The R group of glutamate carries a negative charge (—COO−, carboxylate group), which interacts with other charged groups and is important in determining the overall shape of the haemoglobin molecule. The R group of valine has no charge and so is unable to take part in such interactions.
Haemoglobin molecules with valine at position 6 fold up into the wrong shape. The red blood cells containing this type of haemoglobin assume a curved ‘sickle’ shape under certain conditions, as shown in Figure 9a, instead of the normal disc shape (see Figure 3). This altered shape is distinctive of the condition known as ‘sickle cell disease’. The characteristics of the disease are shown in Figure 9b. This case is just one example of how the correct amino acid sequence of a protein is fundamental to the way that the protein functions.