Biology

The 20 Amino Acids

20cards
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Alanine
A
Ala
Nonpolar, aliphatic
Non-essential
The simplest side chain after glycine — a single methyl group.
Arginine
R
Arg
Basic (positively charged)
Conditionally essential
The most basic of the twenty: its guanidinium group stays positively charged at any physiological pH.
Healthy adults synthesize enough, so some tables list it as non-essential; it becomes essential in infancy, injury and rapid growth.
Asparagine
N
Asn
Polar, uncharged
Non-essential
The amide of aspartate, and the anchor point for N-linked glycosylation.
Aspartic acid
D
Asp
Acidic (negatively charged)
Non-essential
Negatively charged at physiological pH; a frequent catalytic residue in enzyme active sites.
Often called aspartate — the deprotonated form that predominates at physiological pH. The two names refer to the same residue.
Cysteine
C
Cys
Polar, uncharged
Conditionally essential
Two cysteines oxidize into a disulfide bridge — the only covalent cross-link that staples a folded protein together.
Some tables group it with methionine in a separate sulfur-containing class, or call its side chain nonpolar. Essentiality is conditional because it is made from methionine.
Glutamic acid
E
Glu
Acidic (negatively charged)
Non-essential
The brain's principal excitatory neurotransmitter, and the source of umami taste as monosodium glutamate.
Usually called glutamate, the deprotonated form that predominates at physiological pH.

Description

The twenty proteinogenic amino acids, with the structure of each one drawn out. You can learn both the one- and three-letter codes. Each card also carries the side-chain class, the essentiality, and what the residue actually does in a protein.

Textbooks disagree on some classifications; where they do, the card gives details about it.

Author@GentleKoala_5232
Price
Free
Version1
Last updatedJul 30, 2026

Card fields

FieldExample
Name
Arginine
One-letter code
R
Three-letter code
Arg
Side-chain class
Basic (positively charged)
Essentiality
Conditionally essential
Notable feature
The most basic of the twenty: its guanidinium group stays positively charged at any physiological pH.
Where sources differ
Healthy adults synthesize enough, so some tables list it as non-essential; it becomes essential in infancy, injury and rapid growth.
Illustration

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