Biochemistry students often ask themselves one of the common questions; how to memorize structures of all 20 amino acids. However, using some tips, it’s possible to memorize all of the 20 standard amino acids. It’s not that hard for students to do that. You just need to follow some simple tips. First, you need to generalize the side chains of all these 20 amino acids based on their similarities.
Look at the structure of glycine it’s the simplest amino acid with no chiral carbon. I’m sure you will remember the structure of glycine forever. So starting from the glycine, you can actually draw structures of all of the amino acids except for proline. All you need to replace the side chain with other groups.
Replace one H atom in the carbon of glycine with a CH3- group, you will get alanine. Now, replace the one H atom from the -CH3 group with a phenyl group you will get phenylalanine. If you add an -OH group in the phenyl ring of the phenylalanine at a para position you will get tyrosine. See, so easy!
Now come to the alanine again and replace one H atom from the -CH3 group with the -OH group and you will get serine. So simple! Next, replace an H atom from the -CH2 group of the serine with the -CH3 group, and you will get threonine. However, if you replace the O atom of the serine with S, it will give you cysteine. That’s so simple. Up to now, you have managed to remember the structures of seven amino acids.
Now, replace one H atom of the side chain of alanine with a –COOH group you will get aspartic acid. That’s also so simple. Converting the carboxyl group of the aspartic acid into an amide group will give you asparagines. Now, add one extra -CH2 group in the side chain of the aspartic acid you will get glutamic acid. Is not that simple? Again, convert the side-chain carboxyl group of glutamic acid to the amide group and you will get glutamine.
Well if you want to get methionine, replace the side-chain carboxyl group of glutamic acid with the –S-CH3 group. Up to now, you have memorized structures of 12 amino acids and only 8 amino acids left to memorize.
For histidine and tryptophan
Now, you need to remember the structures of an imidazole ring and an indole ring. However, there is no shortcut rule to memorize those rings but you can always play around these two rings to distinguish them and the position of the nitrogen atom in the ring.
For histidine, replace one H atom of the side chain of the alanine with an imidazole ring, you will get histidine. In the same way, if you remove one H atom from the side chain of the alanine with an indole ring, you will get tryptophan. That’s so simple.
In the case of proline, you have to understand what the imino group is. An amino group is one in which a nitrogen atom is connected to two different carbon atoms rather than hydrogen atoms. Proline is the only amino acid that has an imino group. It means that two of the H atoms of its amino group are replaced with two carbon atoms. Other amino acids have an amino group, but proline has an imino group. So, to remember the structure of proline, connect the alpha carbon of the glycine with amino N using a propyl group. It’s so simple. The propyl group attached to amino N and alpha C creates a ring structure; a unique structure of the proline.
Now, come to the alanine again. Now, replace the two H atoms of the side chain of the alanine with two CH3 groups, and you will get valine. If you replace one H atom of the side chain of the alanine with an isopropyl group, you will get leucine. And if you change the position of the terminal methyl group to the beta carbon, you will get isoleucine. Isn’t that simpIt’s It’s really simple. Now, only two amino acids left for which you need to memorize the structure.
For arginine and lysine
Now, only two amino acids left for which you need to memorize the structure and these amino acids are arginine and lysine. Well, they are also similar to each other but again you need to understand the structure of an arginino group. An arginino group looks similar to that of the structure of urea. If you replace the oxygen atom of urea with an NH2+ group you will get an arginino group. It’s so simple.
Now, come to the point. Add a propyl group to the alpha carbon of the glycine and attach an arginino group at the terminal carbon of the propyl group, you will get arginine. In a simple way, the propyl group connects an arginino group with the alpha carbon of the glycine. In the same way, a butyl group connects an amino group to the alpha carbon of glycine to form a lysine. You see, it’s all simple to construct the structure of all 20 amino acids and memorize these structures. Good luck.
Ain’t it suppose to be NH2 in the picture..
Hello, Helen! Thank you for communicating with us. It’s just not clear which amino acid you are talking about. There are two amino acids with amide group in their side chain and they are supposed to be shown having intact the amide group -CONH2. If you are talking about arginine, then again, -NH2 supposed to be the part of the arginine group present in the side chain. At the last, what I can say is that, wether -NH2 stays as -NH2 or as -NH3^+ all depends on the nature of the surrounding atoms. In the case of amides, NH2 group stays as it is even of having basic nature that’s because the -CO- group is electron withdrawing in nature and tends to attract lone pairs of electrons from N atom of -NH2 and making it less available to be accepted by proton. Therefore we represent it as -CONH2 rather than representing as -CONH3^+. I hope your doubts are clear now. Good luck!