In sickle-cell haemoglobin (HbS), a glutamic acid on each of the two beta chains is replaced by…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In sickle-cell haemoglobin (HbS), a glutamic acid on each of the two beta chains is replaced by valine. A laboratory runs normal haemoglobin (HbA) and HbS side by side in an electric field, at a pH of 8.6 where both are negatively charged.
- Which of the two carries the smaller negative charge, and why?
- Which of the two moves further towards the positive electrode?
- Explain, in terms of folding, why HbS molecules join in fibres when oxygen is low.
Need a reminder? The key ideaShow
Folding is driven by the hydrophobic effect and held by hydrogen bonds, salt bridges and disulphide bridges. Heat, pH, urea and thiols denature a protein, but its peptide bonds stay whole. The isoelectric point, pI, is the pH of zero net charge: the average of the two pK values on either side of the neutral form.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The charge. Glutamic acid is negative at pH 8.6. Valine is neutral. HbS has lost two negative charges, one on each beta chain, so it carries the smaller negative charge.
Step 2. The movement. The pull on a charge in a field grows with the charge. HbA is more negative, so it moves further towards the positive electrode.
Step 3. The fibres. Valine is hydrophobic. At the surface of the molecule it makes a sticky patch. When oxygen is low, a hydrophobic pocket on a neighbouring molecule fits that patch, and the molecules join in long fibres.
Answer: HbS has the smaller negative charge, HbA moves further, and the surface valine causes the fibres.
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