TBR CBT #3 Question 142

[thomasfx10]

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Can anyone help me out with this question:

142) It is most likely true for an amino acid that migrates to the cathode at a pH value of 6.0 that it has an isoelectric pH:

D. greater than 6.0 and is a basic amino acid.

From what I understand ... If a pH is below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge.

My question is ... I understand the Isoelectric pH would be greater than 6.0 but it states Cathode which should be Positive (+) and would that indicate Acidic and NOT Basic Amino Acid

Their response is below ...

If there is a simplified way to understand this PLEASE let me know ... Thanks!

Detail:
D is the best answer. Because it migrates to the cathode in gel electrophoresis, the compound carries a net positive charge. Recall that anions migrate to the anode, and cations migrate to the cathode. Because the compound is cationic, the pH of the solution must be less than the isoelectric pH of the amino acid. This eliminates choices A and C. In order for the compound to be cationic and have an isoelectric pH greater than 6.0, the amino acid must have a side chain that assumes a positive charge when protonated. Additionally, the pKa of the side chain when averaged with the amino terminal (which usually falls between 9.0 and 9.5) must result in a value greater than 6.0. This can be true only for an amino acid with a basic side chain (like Lys, Arg, or His). Recall that for a basic amino acid, you should average pKa2 and pKa3 to obtain the isoelectric pH. The best answer is D.

 

[BerkReviewTeach]

You have memorized from physics that the cathode of a battery carries a positive charge, which explains why current flows away from the cathode of a battery. But in electrophoresis, the gel is place in an electric field created by the charged plates of a capacitor. The anode plate of a capacitor is connected to the cathode of the battery, so it takes on a (+) charge. This also means that the cathode plate of a capacitor is connected to the anode of the battery, so it takes on a (-) charge. In electric fields such as the one used in gel electrophoresis, cations migrate to the cathode. That is what they mean in that answer explanation.

As far as the amino acid goes, at pH = 6, a basic amino acid (like lysine) will exist as a conjugate acid, thereby carrying a net positive charge.

Hopefully this helps clear it up, because it is often a point of confusion.