Looking at MCAT Amino Acid Questions

Session 54

Amino acids should be like the air you breathe for the MCAT. You know it so well you just don't think about it. We'll cover some questions today to help you with that. We've discussed it before that every single premed student walking into the test center for the MCAT should be able to draw out all of the amino acids. They should know their one or three-letter abbreviations and everything else.

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[01:50] Proline Question

Question 16: A particular oncogene product has an unusually high number of proline residues. These residues:

  • (A) Aid in the formation of alpha helices
  • (B) Are more likely to be found throughout beta-pleated sheets
  • (C) Aid only in the formation of antiparallel beta-pleated sheets
  • (D) Are more likely to be found in turn regions of the protein

Bryan's Insights:

When it comes to amino acids, Bryan says there are a few rockstars. They are hugely important because of how different they are. Two of which are glycine and proline. Glycine has a side chain with only one hydrogen. It's the only achiral amino acid. So glycine is special in that regard. Proline is special because its side chain connects to its own amino in the amino acid part. It has this weird little looped structure. So one is so tiny and proline has this weird, rigid self attachment structure. As a result, neither glycine or proline tends to play nice when it comes to secondary structure (alpha and beta) particularly proline. Proline is the combo breaker on your alpha and beta structures.

A turn region in the protein is a loose, unstructured region where the amino acids are not locked into an alpha or beta structure. If proline is going to show up, it's going to be like it's breaking the alpha and beta sequence into a more unstructured, turn region. So the right answer here is (D).

[03:47] Proteins in a Membrane-Spanning Domain

Question 26: Which of the following segments of amino acids would be most likely to be found in the membrane-spanning domain of the sodium channel in a nerve axon? (The choices are one-letter abbreviations.)

  • (A) DDR
  • (B) EVE
  • (C) LAD
  • (D) LIV

Bryan's Insights:

Remember that the inside of a plasma membrane is nonpolar. The outside phase and the inside phase are polar because they're facing the water. But within the membrane itself is nonpolar. So we need nonpolar amino acids.

Fortunately, Bryan thinks the test writers here were kind enough to have three of the answer choices include either D or E. Remember that aspartate and glutamate, DNA respectively, are charged amino acids. They have a full-negative charge on them in the body. And you don't want to put a charge in ion inside the middle of a cell membrane. But you want to have nonpolar amino acids that can fit with a nonpolar environment or surrounding here. So cross out all the first three answers here.

Then you say that nonpolar amino acids in a membrane-spanning domain, that's going to be LIV. These are all nonpolar so they would fit for the membrane-spanning domain of any protein.

Bryan goes on to explain the typical areas in the body where you don't want any charges in them. On the insides of a globular protein, you want the polar guys pointing out towards the water. This is in the case of a protein floating around in the cytoplasm or floating around in the blood. So the inside of a globular protein would be nonpolar and the inside of the cell membrane would be nonpolar.

[06:34] Positive and Negative Charges

Question 47: Assuming all other conditions are equal, which of the following amino acids is expected to have the most positive charge at physiological pH?

  • (A) ARG
  • (B) MET
  • (C) ASN
  • (D) ASP

Bryan's Insights:

This is a quick recall question. You've got to know your positive amino acids. Lysine and Arginine are your two positive amino acids at physiological pH. Lysine is not in the answer choice here but Arginine (ARG) is. Histidine is a switch hitter but it generally gets lumped in as a positive under physiological pH. Back to the answer choices, ARG is positive. MET and ASN are neutral, and ASP is negative. So the answer here is (A).

[07:33] Things to Remember About Amino Acids

The last question discussed above is a chemical property category. Every amino acid needs to have a label on it. That label will either read positively charged or negatively charged or polar or nonpolar. One of those four labels applies to every amino acid. What you need for the MCAT is to be able to put a label on it, draw it and know its names and abbreviations.

[08:28] Next Step Test Prep

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