The MCAT Tutor: The Basics of Amino Acids

Posted by Jack the MCAT Slayer on 4/23/14 9:54 AM

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And they said supplements couldn't help you beat the MCAT

I have met two groups of people who are VERY excited about amino acids. The first group is bodybuilders. Bodybuilders are OBSESSED with amino acids, because amino acids make up protein, and protein make up muscles and muscles are what bodybuilders want. Since you are reading this blog looking for an MCAT tutor, I feel it is relatively unlikely that you are a bodybuilder, though you never know.

The second group is medical school applicants. This is because there are 20 amino acids, all with different structures. The first time any prospective medical student comes across a table of all 20 amino acids, the first question is always the same: “Do I have to memorize these?”

The short answer is no. The long answer is, while knowing all of the structures by heart MIGHT be helpful, there is a more basic set of facts that you should become comfortable with first, and which will be much higher yield than memorizing all 20 structures. Remember, the best MCAT tips give you the all the information you need to answer questions and nothing more.

Basics first: an amino acid is the fundamental unit of all proteins. It is made up of a central carbon, bound to an amino group (amino acid), a carboxylic acid group (amino acid), a hydrogen, and a variable R group:


The R groups are where all the fun happens, and by arranging the amino acids in different orders, we get all the amazing properties of proteins which we see in human biology.

Now for the fun stuff:

  1. Amino acids have POSITIVE charges at a low pH and NEGATIVE charges at a HIGH pH.  This is because both the amino group and the carboxyl group are protonated at a low pH, and both are deprotonated at a high pH. At an intermediate pH, the amino group is protonated, and the carboxyl group is deprotonated, making the whole amino acid neutral.
  2. All amino acids are chiral EXCEPT glycine. This is because the R group on glycine is just a hydrogen.
  3. Glycine has the smallest R group and is thus the smallest amino acid. The MCAT loves to test this, so any time you get asked about something in which you might need a small amino acid, think glycine.
  4. Phenylalanine, tyrosine, and tryptophan all have large ring structures in their R groups. Big means bulky! Again, MCAT loves to ask about size.
  5. Alanine, Valine, Leucine, and Isoleucine have hydrocarbon R-groups, and are thus non-polar. This means that they hate water, and will tend to be embedded within membranes or towards the interior of globular proteins.
  6. Histidine, Lysine, and Arginine have BASIC R-groups.
  7. Glutamic acid and aspartic acid are acidic. Duh.
  8. Proline is the only amino acid who’s R group is covalently linked to the amino group of the amino acid. Because of this, it will cause “kinks” in a chain of amino acids.
  9. Cysteine forms disulfide bonds with other cysteine residues. These disulfide bonds are often what holds together multiple polypeptides in quaternary protein structure.

This list is by no means exhaustive. However, it is a GREAT place to start and will get you through 95% of what you need to do great on amino acid questions on the MCAT.

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