So it turns out that scientists have isolated over 700 different molecules that count as amino acids in nature. They go and find living creatures and things, then they isolate them and realize, wow, this is an amino acid. There are 700 of those. But don't worry, we're not going to have to learn 700 in this section. We're really just going to be focusing on the 20 most common amino acids. It's really only going to be these 20. The other 680 that are possible show up in other types of macromolecules but not really in eukaryotic proteins, which would be like proteins of multicellular organisms like us, okay? So, what are the common traits of these 20 amino acids? Remember, I said earlier, they differ only in terms of that R side chain. But if you just had to eyeball it before knowing, before memorizing what your amino acids are, if you just had to look at one and say, is it standard or is it nonstandard, meaning rare, is it outside of this group of 20, how would you be able to tell the difference?

Well, in general, all of the 20 amino acids share these two characteristics. One is that they are alpha amino acids. We already discussed this, right? This is all about alpha amino acids, but just realize that it's possible to have beta, gamma, delta amino acids as well. They still count in the group of 700 but they don't count in the group of 20 common amino acids. Secondly, and this comes from the concept of chirality in organic chemistry, they're going to have an L configuration, which in the rules, the R and S rules of organic chemistry, means that they're going to have an S configuration in organic chemistry at the chiral center. So if there's a chiral center present in the amino acid, that chiral center has to be in an S configuration, okay? In biology, we call it L because it's just an old name given. And throughout this course, we're going to talk about them being L amino acids.

So let's go ahead and look at this example and identify which amino acids we believe to be rare. Now, you wouldn't be able to memorize all 20 amino acids by doing this but you might be able to pick out some amino acids that are not part of the 20.

So let's look at number 1. Let's just do this together. Is this an alpha amino acid that is part of the group of 20? No, it's not. Why? Because it does have an amine group and it does have an acid group. But what is it missing? Notice that the amine is not in the alpha position. This is the alpha carbon and that should have the nitrogen coming off it but it doesn't. Instead, we're coming off all the way off of the beta, gamma. This is on the gamma carbon. And it turns out that this is a very common amino acid in the human body but it's not found in proteins. This is actually a neurotransmitter in the brain called GABA. And the name GABA stands for gamma-Aminobutyric acid. So it's an amino acid but it's not found in proteins so we're not going to have to memorize that one. That is what's called a nonstandard or rare amino acid.