Amino Acid Three Letter Code Explained: Definition, Examples, Practice & Video Lessons (2025)

In this video, we're going to focus on the amino acid three-letter codes for all 20 of the standard amino acids. For convenience sake, amino acids can actually be abbreviated in two different ways. And that's a great thing because, otherwise, my wrist would really, really hurt from writing out the long names of all of these amino acids. The first way that we can abbreviate amino acids is by using a three-letter amino acid code or three-letter amino acid abbreviations. The second way that we can abbreviate amino acids is by using a one-letter amino acid code or one-letter amino acid abbreviation. Each of the 20 amino acids has their own three-letter code and their own one-letter code. There are a total of 20 three-letter codes and a total of 20 one-letter codes. Combined, there are a total of 40 abbreviations. That's a lot of amino acid abbreviations. But it's worth taking the time to memorize all of those amino acid abbreviations because those abbreviations are actually commonly used throughout the entire scientific world by all biochemists. And even though there are a lot of abbreviations, they were all designed to be pretty easily memorized. That's a great thing for us because our professors expect us to memorize all of these abbreviations. Moving forward, I'm going to give you guys a lot of helpful tricks and hints to better and more easily memorize all of these abbreviations. In this video, we're first going to focus on the three-letter amino acid code or the three-letter amino acid abbreviations. Then, later in our course, in a different video, we'll talk about the one-letter amino acid codes. Moving on with the three-letter amino acid code, it's important to recognize that the vast majority of the three-letter abbreviations are super easy for us to memorize. That's because most of the three-letter abbreviations are simply the first three letters of the amino acid itself. This applies to 16 out of 20 of the amino acid three-letter abbreviations. Because these three-letter abbreviations are so easy because they're literally just the first three letters of the amino acid, we don't need to focus on amino acid abbreviations that are harder to memorize. It turns out that there are four amino acids with three-letter abbreviations that are slightly altered. These three-letter abbreviations are not the first three letters of the amino acid itself. These three-letter abbreviations are slightly altered to help identify the amino acids phonetically or by the sound of speech. Those four amino acids with phonetic three-letter abbreviations are asparagine, glutamine, tryptophan, and isoleucine. These four amino acids have the hard three-letter abbreviations that we want to focus most of our time memorizing. All of the other 16 abbreviations, 16 three-letter, amino acid abbreviations, we don't need to spend most of our time memorizing those. We want to focus most of our time memorizing these four. What you'll notice is that asparagine's three-letter code is ASN, and it does not match the first three letters of asparagine. The N here in asparagine is used to help identify it phonetically or by the sound of speech, "aspar gene". Similarly, the three-letter abbreviation does not match the first three letters of the amino acid for glutamine, tryptophan, and isoleucine. That is, again, something that's unique only to these four amino acid three-letter abbreviations, because all the other ones are the first three letters of the amino acid. I came up with this mnemonic that helps me memorize the four amino acids with three-letter abbreviations of phonetic origin. But before we get to this mnemonic, I first want to point out that some of you guys may have noticed that "a spare gene" sounds a lot like a spare guess, and that's done intentionally because the amino acid asparagine was first isolated from asparagus. Asparagine was named after asparagus. Whenever I see asparagine, I think about asparagus. In our mnemonic, to help us memorize these four amino acids of phonetic origin, three-letter abbreviations, is "just a spare gene chilling on three tropical islands". The three here helps me remember that this mnemonic is specifically for the phonetic three-letter codes. Whenever I see asparagine, I think about asparagus chilling on three tropical islands where "tropical" is spelled phonetically here with TRP, the three-letter code of tryptophan, and "islands" is also spelled phonetically with the three-letter code of isoleucine. In our image, you'll see we have all this asparagus chilling on these beach chairs. You can see an asparagus over here, and he has a sign that says, "Hello, my name is Asparagine" because Asparagine is literally chilling on these three tropical islands. If you can remember the mnemonic, asparagine chilling on three tropical islands, then you'll be able to remember the four amino acids with phonetic three-letter abbreviations. These are the four that you really want to commit to memory. Moving forward, down below, we have a chart of all 20 of the amino acids organized alphabetically with their three-letter symbols or their three-letter codes. This chart is color-coded where we have blue amino acids and pink amino acids. The blue amino acids are all of the amino acids with three-letter abbreviations that are super easy to memorize because the three-letter abbreviations are literally the first three letters of the amino acid. For example, looking at alanine, notice that its three-letter code is ALA, and the first three letters of alanine are also ALA. This applies to all of the amino acids that are colored in light blue on this chart. That's 16 out of the 20 amino acids that have abbreviations, three-letter abbreviations, that are really easy to memorize. The ones that we want to focus most of our time memorizing are highlighted in pink throughout our chart. Remember, these are the amino acids that have three-letter abbreviations of phonetic origin. These include asparagine, glutamine, isoleucine, and tryptophan. We want to focus most of our time memorizing the ones in pink because once we memorize those four, then memorizing all of the three-letter amino acid codes or symbols will be super easy for us. The mnemonic that helps us memorize these four amino acids of three-letter abbreviations of phonetic origin is "just a spare gene chilling on three tropical islands". Moving forward, we'll be able to apply a lot of these concepts in our practice videos. I'll see you guys there.