Session 54

Amino acids should be like the air you breathe for the MCAT: You know it so well you don’t even think about it. We’ll cover some questions today to help you with that.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

What Do You Need to Know About Amino Acids for the MCAT?

You should be able to draw the structures of all 20 amino acids for the MCAT. You should also know the one- and three-letter abbreviations. And you should know whether each amino acid is positively or negatively charged.

[01:50] You Need to Know What Makes Proline and Glycine Special for the MCAT

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, there are a few rockstars with unusual structures. You need to know these amino acid structures. They are hugely important because of how different they are. Two of these are glycine and proline. Glycine has a side chain with only one hydrogen. Glycine is the only achiral amino acid. So glycine is special in that regard.

Proline is special because its side chain connects to its own amine group in the amino acid part. So proline has this weird little looped structure. So glycine is so tiny and proline has this weird, rigid self-attachment structure. As a result, neither glycine nor proline tend to play nice when it comes to secondary structure (alpha and beta), especially proline. Proline is the dealbreaker for 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 breaking the alpha and beta sequence into a more unstructured, turn region. So the right answer here is (D).

[Related episode: Common MCAT Prep Mistakes and How to Avoid Them.]

[03:47] You Need to Know Polar and Nonpolar Amino Acids for the MCAT

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, the test writers here were kind enough to have three of the answer choices include either D or E. Remember that aspartate (D) and glutamate (E) are charged amino acids. They have a full negative charge.

And you don’t want to put a charged ion inside the middle of a cell membrane. You want to have nonpolar amino acids that can fit with a nonpolar environment. So cross out all the first three answers here. In contrast, LIV works. These are all nonpolar amino acids, so they would fit for the membrane-spanning domain of any protein.

It helps to know where in the body you want charged amino acids and where you can’t have them. On the insides of a globular protein, you want the polar amino acids 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, along with the inside of cell membranes.

So the answer is (D).

[06:34] You Need to Know Positively and Negatively Charged Amino Acids for the MCAT

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).

[Related post: MD vs DO: What Are the Differences (and Similarities)?]

[07:33] What You Need to Remember About Amino Acids for the MCAT

The last question discussed above was about chemical properties. In your mind, every amino acid needs to have a label on it. That label will either read “positively charged” or “negatively charged.” It will read “polar” or “nonpolar.” These labels apply to every amino acid.

What you need for the MCAT is to be able to put that label on it, draw its structure, and know its names and abbreviations.

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Links and Other Resources

• Check out my book about the MCAT, co-written with Blueprint MCAT (formerly Next Step Test Prep): The Premed Playbook: Guide to the MCAT.
• Related episode: MCAT Foundational Topic: Biochemistry Broken Down.
• Related episode: Looking at MCAT Biochemistry Discrete Questions.
• Need MCAT Prep? Save on tutoring, classes, and full-length practice tests by using promo code “MSHQ” for 10% off Next Step full-length practice tests or “MSHQTOC” for $50 off MCAT tutoring or the Next Step MCAT Course at Blueprint MCAT (formerly Next Step Test Prep)!