Question 21.2: Amino Acid Nomenclature Give the three-letter and one-letter...

Amino Acid Nomenclature
Give the three-letter and one-letter abbreviations for the following amino acids:
(a) Glycine                       (b) Glutamic acid
(c) Glutamine                 (d) Lysine
(e) Tryptophan               (f) Phenylalanine
(g) Leucine                      (h) Aspartic Acid
(i) Alanine                        (j) Arginine

Strategy
Three-Letter Abbreviations
The three-letter abbreviations (TLA) are straightforward. In most cases, it is just the first three letters of the amino acid. However, there are some exceptions. What do we do about aspartic acid and asparagine? They would be the same. In this case, aspartic acid gets its normal TLA—Asp. To distinguish, asparagine is labeled Asn. The same is true for glutamic acid and glutamine—Glu and Gln.

Isoleucine and tryptophan are the only other outliers. This is likely because their TLAs would otherwise be iso and try, which could be confused with other words. Thus, they are given the modified TLAs, Ile and Trp.

One-Letter Abbreviations
In a perfect world for biochemistry students, the twenty common amino acids would all start with a different letter, making remembering the one-letter abbreviations (OLA) a piece of cake. This works for 11 of the amino acids, but not the other 9. If two amino acids start with the same letter, which one gets its proper OLA? It will be the one that has the simpler sidechain. For example, there are four amino acids that begin with A: alanine, arginine, aspartic acid, and asparagine. Alanine has the simplest sidechain, so it has the OLA of A. There are three that begin with G, glutamine, glycine, and glutamic acid. Again, glycine has the far simplest sidechain, so its OLA is G.

For the rest of them, you either have to memorize them by brute force or use some sort of memory trick. There are some tricks that have been thought up to help. They may seem silly, in some cases, but once you hear them, they cannot be unheard and you will remember them. For example, let’s look at arginine. The A was already taken for alanine, so arginine needs a different OLA. The one chosen is R, which is the second letter. Think of saying arginine like a pirate—“aaaarginine,” and you will have it. How about tryptophan? The T was already taken with threonine. You can’t use the second letter, as that would be R, which is taken with arginine. Its OLA is W. Say tryptophan like Tweety Bird would say it—“tWyptophan,” and there you have it. Let’s look at aspartic acid. The A is already taken, as is its second letter, S. Aspartic acid’s OLA is D. When you say aspartic acid quickly, the T is usually softened to more of a D sound anyway—“asparDic acid.” Once you remember that one, then remember that glutamic acid is very similar to aspartic acid, just with one more carbon in the side chain. Glutamic acid’s OLA is E (one more than D). Phenylalanine’s OLA is phonetic, F, for the sound of its Ph.

Any that you cannot think of a memory trick for will just have to be remembered.