Amino Acids has always been an important topic in biochemistry and metabolic diseases in medical school. Be it medical school exams where classification of amino acids is a typical long question or be it PG entrance exams like NEET-PG, AIIMS, USMLE where direct one-liners are repeatedly asked, year after year.
What makes matters tough is that there are 20 Amino Acids found in humans, and all share different groups and properties. It is difficult to remember the properties and classification groups, simply because there is so much to learn.
Thankfully, we have mnemonics and memory aids to our rescue.
In this article, we will be enlisting all the standard mnemonics to help you remember amino acids and their properties.
We will also enlist the common facts that are asked on this topic in NEET-PG and USMLE.
Once you’ve learnt all the mnemomics, do check out the short quiz and test yourself.
Firstly, to understand the basics, you need to know that Amino acids are molecules containing:
- An amino group (-NH2)
- A carboxylic acid group (-COOH)
- A side chain that varies between different amino acids
Note that the amino group is Basic and -COOH is acidic.
If that is confusing, CAAB is the mnemonic you need to remember (Pronouned CAB)
Carboxyl Acidic – Amino Basic.
Essential and Non-essential Amino Acids
The most commonly asked question in USMLE and NEET on this topic goes something like this
All are essential amino acides except :-
Now it is easy to answer if you remember the mnemonic AV HILL MPTT
A – Arginine
V – Valine
H – Histidine
I – Isoleucine
L – Leucine
L – Lysine
M – Methionine
P – Phenylalanine
T – Threonine
T – Tryptophan
Note that of these 10 – AH! ( Pronouned ah!) ie Arginine and Histidine are Semi-essential (as they can be synthesized in adults, but not in chilldren). The 8 others enlisted above are purely essential – as these can not be synthesized by the body and need to be supplied in the diet.
Remember the 10 essential amino acids by the mnemonic AV HILL MPTT and it is easy to remember that the other 10 are non-essential.
Amino acids can be classified into four groups based on polarity
1) Polar with Positive -R group (Basic)
2) Polar with Negative -R group (Acidic)
3) Polar with Neutral -R group
4) Non Polar
Common question on this topic is Which of these is a Polar Amino Acid?
Use these mnemonics to remember the classification.
Remember BASIC HAL – Basic AA ie Polar with Positive -R group includes Histidine, Arginine and Lysine.
Acidic AA are easy to rememeber as Aspartate is also called Aspartic acid and glutamate is also called glutamic acid.
The third group ie Polar with Neutral -R group is a bit tricky to remember.
Use STY (Serine, Threonine, Tyrosine), CNQ (Cysteine, Aspargine, Glutamine) to remember the polar AA with neutral -R group.
All other amino acids not enlisted here are non-polar.
Structural Classification of Amino acids
1. Aliphatic side chains: GAVLI
2. With OH group: STT (Pronoune OH S*TT)
3. Acidic: AAGG
4. Sulphur containing: CM
5. Basic: HAL
6. Aromatic: PTT
Glucogenic and Ketogenic Amino acids
Amino acids are either glucogenic or ketogenic or both.
14 amino acids are glucogenic ie can be converted to glucose.
Ketogenic amino acids are Leucine and Lysine (Mnemonic Keto LL). They can be converted to ketone bodies.
Four amino acids are both glucogenic and ketogenic.
These are Isoleucine and other 3 aromatic amino acids (Tyrosine, Tryptophan, Phenylalanine). So remember that Iso-Aromatic (Isoleucine + 3 aromatic AA) are both ketogenic and glucogenic.
One-liners on Amino Acids
- Smallest and simplest amino acid
- Responsible for flexibility of protein
- Optically inactive
- Lacks chirality (handedness)
- Glycine with arginine and methionine (GAM + Ornithine) synthesize creatinine.
- Glycine (with succinyl CoA) is used for Heme synthesis.
- Most stable amino acid at physiologic pH
- Can serve as best buffer at pH 7
- Can protonate and deprotonate at neutral pH
- Precursor of histamine
- Storage and transport form of ammonia
- Removal of ammonia from brain
- Precursor of purines and pyrimidines
Phenylalanine and Tyrosine
- Phenylalanine is precursor of tyrosine
- Tyrosine is a precursor of:
- Can be synthesized in body from methionine (both contain sulphur)
- Responsible for reducing action of glutathione
- Form S-adenosyl methionine (SAM) which is a major methyl group donor in body
- Antioxidant (reducing property is because of cysteine sulfhydryl group) – detoxify H2O2 by glutathione peroxidase
- Carrier in transport of certain amino acids across membrane in kidney
- Conjugation reaction
- Precursor of niacin and serotonin (which form melatonin)
- 60 mg tryptophan form 1 mg niacin
- Also known as alpha-amino beta-3 indole propionic acid
- Most basic amino acid
- Precursor of nitric oxide
- Transport form of ammonia from muscle
High-Yield facts on Amino Acids
- All amino acids found in proteins exist in L-form
- Amino acids have 2 isomers ie D and L forms. As each amino acid has one chiral carbon.
Remember the exceptions : Glycine does not have any chiral carbon, so no optical activity and no isomers.
Isoleucine and Threonine have 2 chiral carbons each, so four optical isomers each.
- Maximum absorption of light is shown by Tryptophan.
Note that only aromatic AA absorb light.
- Largest amino acid- Tryptophan
Smallest amino acid- Glycine
- Most acidic amino acid- Aspartate
Most basic amino acid – Arginine
- Most polar amino acid – Glutamine
Most non-polar amino acid – Phenylalanine
- Amino acid with Imino group – Proline
Amino acid with Indole group – Tryptophan
Amino acid with Guanidium group – Arginine
- 21st amino acid is Selenocysteine.
It is coded by codon UGA (usually it is a stop codon).
- 22nd amino acid is Pyrolysine
It is coded by UAG (usually it is also a stop codon).
(Ok, here is a word on stop codons – UAG, UGA and UAA are the three stop codons. but, UAG sometimes codes for Selenocysteine and UGA sometimes codes for pyrolysine. So UAA is regarded as the only true stop codon.)
That is all we have for you on mnemonics and high yield info on amino acids.
We will post a short quiz on Amino acids soon, based on the mnemonics shared here.
Do let us know in the comments if you have better ways to remember this highly volatile stuff!