How many amino acids are there? As I continue my mission to understand the amino acids, this has become an incredibly frustrating question. Everywhere I go, I get a different answer, in part because many sources fail to specify which category of amino acids they’re talking about.
There are 9 amino acids
I got this from a nutrition website. In terms of your diet, there are nine essential amino acids. What’s essential about them? It’s essential that you, as a human being, get them in your diet because the human body cannot make them on its own.
There are 20 amino acids
This seems to be the most common answer. There are twenty standard amino acids. Why are they standard? Because your DNA (and the DNA of all organisms on Earth) directly codes for these specific amino acids, and so these twenty amino acids are incorporated into all the proteins found in your body (and the bodies of every other living thing on Earth).
There are 21 amino acids
The amino acid selenocysteine is also found in some of the proteins in our bodies. It’s just not directly coded by our DNA, so it’s considered non-standard.
There are 23 amino acids
There are two more non-standard amino acids that are only found in prokaryotic organisms (bacteria and archaea). So in total, across all life forms on Earth, there are 23 proteinogenic amino acids, meaning there are 23 amino acids incorporated into proteins.
There are hundreds of amino acids
As important as the 23 proteinogenic amino acids are, there are literally hundreds—maybe thousands—more. The rest are sometimes called non-coding because they’re not coded for by DNA. They’re also called non-proteinogenic because they’re not incorporated into proteins (at least not here on Earth). Sometimes they’re called unnatural, which seems rather unfair. Many of these “unnatural” amino acids serve vital biochemical purposes. Life is made of more than proteins, you know!
Anyway, next Molecular Monday (two weeks from today) we’re finally going to meet an actual amino acid.
Today’s post is part of a special series here on Planet Pailly called Molecular Mondays. Every other Monday, I struggle valiantly to understand and explain some concept in the field of chemistry. Please note: I suck at chemistry, but I’m trying to learn. If I made a mistake, please, please, please let me know so I can get better.
Planet Pailly 
I was never any good at organic chemistry, so no corrections from me. I’m just in learning mode with this series.
The last point about hundreds of amino acids is, I suspect, why most astrobiologists see the probability of our biochemistry being compatible with an alien’s, in terms of eating one another, mating, or whatever, as profoundly low.
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