Which amino acid has the lowest isoelectric focus point?
a. Lys
b. Arg
c. Gln
d. Asp
Solution: Isoelectric focus point or pI, is the pH at which a protein has no net charge and does not move in an electric field. Isoelectric focus point basically tells us how acidic or basic a protein is. The more acidic the protein is, the lower it's pI is. Therefore, looking at the answer choice we must first determine whether these amino acids are acidic, basic or neutral. Lysine is a basic amino acid and therefore should have a high pI. Arginine is also basic and should also have a high pI. Glutamine is a polar neutral amino acid, and thus can be eliminated. Aspartate is an acidic amino acid and must have a low pI. Therefore, choice D is the correct answer.
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