How to memorize the 20 amino acids (names, codes and side chains)

The 20 amino acids, grouped

Proteins are built from 20 common amino acids, and what makes each one different is its side chain. They come up in biology, biochemistry, the MCAT and the chemistry unit that opens anatomy and physiology. Group them by side chain first: five small groups are far easier to hold than one list of twenty.

Textbooks don’t all draw the lines in the same place. OpenStax’s organic chemistry book, for example, splits only the acidic (aspartic and glutamic acid) and basic (arginine, histidine and lysine) ones from the other fifteen, which it calls neutral.

Some books move glycine, tyrosine or tryptophan between the polar and nonpolar groups. Learn whichever grouping your course uses; the deck at the end follows the table above.

Four sentences that spell the groups

Each group has a mnemonic whose first letters are the one-letter codes, so learning the sentence drills the codes at the same time:

In the last one, the two acids come first (D, E) and the three bases after (K, R, H).

The three-letter codes are mostly the first three letters

Sixteen of the 20 three-letter codes are just the first three letters of the name. The four exceptions:

• Asn asparagine and Gln glutamine: the n marks the amide. The first three letters, Asp and Glu, belong to the acids, aspartate and glutamate.
• Ile isoleucine: I-L-E, not “Iso”.
• Trp tryptophan: T-R-P, skipping the y.

The one-letter codes, and why the odd ones are odd

The one-letter code was set by an international nomenclature committee (IUPAC-IUB), and it published its reasons. Knowing them makes the code stick.

Six simply use their first letter because no other amino acid shares it: C cysteine, H histidine, I isoleucine, M methionine, S serine, V valine.

Five share a first letter, and it went to the simplest or most common one: A alanine, G glycine, L leucine, P proline, T threonine.

The rest got letters chosen to be memorable:

The nine essential amino acids

Nine amino acids are essential: the body can’t make them, so they must come from food. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Mnemonic: PVT TIM HiLL is phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine, lysine.

It uses the names’ first letters, not the codes, so watch three of them: P is phenylalanine (code F), the second T is tryptophan (code W) and the second L is lysine (code K).

An older version, PVT TIM HaLL, adds arginine, which is only conditionally essential, needed from food during illness or stress.

The mix-ups that cost points

• Glutamate versus glutamine, aspartate versus asparagine. The acids (Glu E, Asp D) carry a negative charge; their amides (Gln Q, Asn N) are polar but uncharged.
• Lysine is K, leucine is L. Leucine got L because it’s the more common of the two.
• Isoleucine and leucine are isomers: same atoms, different arrangement, and both nonpolar and essential.
• Histidine’s charge. It’s grouped with the basic amino acids, but its side chain has a pKa near 6, so at the body’s pH most histidine side chains are uncharged.
• Tyrosine versus tryptophan. Tyrosine (Y) has an OH on its ring and isn’t essential; tryptophan (W) has a double ring and is.
• Cysteine versus methionine. Both contain sulfur, but only cysteine forms disulfide bonds.

A week that makes it stick

The longer you need to remember something, the wider your review gaps should be (Cepeda et al., 2006), so spread this over days rather than one night:

If your course also wants structures, add them after the names and codes are solid: a structure is much easier to learn once its name already means something.

More lists worth memorizing

The same approach works for any fixed list your class tests: group it, find the pattern, then let spaced flashcards do the rest. See how to memorize the 12 cranial nerves, the polyatomic ions and the unit circle, each with a free deck.