Remembering the amino acids is difficult. But frankly, it’s not something that you should skimp on during MCAT prep. Knowing the structures of these 20 critical molecules will serve you well on test day.
Rote memorization may not be the only path forward! As with many subjects in biochemistry, a conceptual framework for the structure of these molecules will help you as you attempt to imprint them into your memory.
First, let’s start with the backbone. Since this portion of the molecule is conserved in each amino acid (save for proline), students generally don’t have as much difficulty with remembering it— but let’s break it down nonetheless.
Aside from the R group, there are two integral attachments to the central (alpha) carbon: the amine and the carboxylic acid. “Amino” is the term for a primary amine attachment (NH2), so that should be clear. The “acid” part of the name isn’t as helpful, because it doesn’t necessarily indicate the presence of a carboxylic acid specifically— but you get the point.
What may not be easy, however, is remembering the side chain structures. Luckily, a good chunk of them can be grouped and simplified using nomenclature!
Let’s start with my favorite example: phenylalanine. Understandably, most students stumble upon this molecule’s name with uncertainty (it’s a pretty weird name, at first glance). How could you possibly glean its structure from this random string of letters?
What if I told you, you’ve already seen this word? Let’s break it down: phenyl-alanine.
If you’ve learned the simple nonpolar amino acids, you know that alanine’s side chain is a methyl group. And if you’ve taken organic chemistry, you know that “phenyl” refers to a benzene ring.
Thus, is it possible that we can deduce the structure of “phenylalanine” by simply attaching a phenyl group to the methyl group of alanine? It turns out we can! (There is, of course, a hydrogen removed from the beta carbon, as the phenyl ring now occupies one of its four bonds).
Taking it one step further, can you guess what “hydroxyphenylalanine” looks like? Spoiler: it’s tyrosine— they changed the name of this molecule just to make it more difficult for students to remember (kidding)!
Let’s look at more examples: asparagine, glutamine, aspartic acid, and glutamic acid, shown below.
First, notice one important thing: asparagine and aspartic acid are identical aside from the presence of an amide vs. a carboxylic acid in the side chains, and the same goes for glutamine and glutamic acid. The similarities in name between N-D and Q-E are not a coincidence— the latter in each pair is produced through hydrolysis of the former’s amide!
As you likely recall from organic chemistry, the suffix “-ic acid” indicates that a molecule contains a carboxylic acid group. Use this to guide you as you differentiate aspartic acid and glutamic acid from asparagine and glutamine.
As for the difference between N-D and Q-E, it is simply a matter of one additional carbon in the side chain of glutamine and glutamic acid. In terms of how you can remember this, the common names unfortunately don’t provide profound chemical insight. However, you can simply remember them in an alphabetical one-two sequence, “aspartic acid/asparagine and glutamic acid/glutamine.” Naturally, the latter will contain the additional carbon.
Finally, we’ll look at 3 aliphatic, nonpolar amino acids: valine, leucine, and isoleucine. Valine, as our starting point here, contains a simple isopropyl group side chain. The only structural difference between valine and leucine is the addition of a carbon before the isopropyl group in leucine. And to get isoleucine, a structural isomer of leucine, we can move a methyl group from the gamma carbon onto the beta carbon— nomenclature once again comes to the rescue!
In this blog post, we only reviewed 10 of the 20 amino acids you’ll need to know for the MCAT, but I hope that you’re able to see the utility in drawing parallels between these structures. In nature, the biosynthetic pathways of many of these molecules are linked and intertwined— their physical forms aren’t isolated from each other, so their mental representations needn’t be either! Use their similarities and differences to build a firm understanding of each amino acid’s structure (and corresponding properties), and I assure you that you’ll thank yourself on test day.
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