The 20 Amino Acids Found in Proteins
The 20 standard amino acids share one core structure but differ in the R group that makes each one chemically unique.
Amino acids are the building blocks of proteins. Each one is built around a central alpha carbon bonded to four groups: an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain) that gives each amino acid its unique chemical identity. The 20 amino acids directly encoded by the genetic code — called the proteinogenic alpha-amino acids — are what you need to recognize cold for the MCAT, by name, structure, and both their three-letter and one-letter codes.
Key Takeaways
All 20 standard amino acids share the same core structure: an alpha carbon bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.
The 20 amino acids split into five side-chain categories: nonpolar/nonaromatic, aromatic, polar uncharged, acidic, and basic.
Know each amino acid's three-letter and one-letter code — these appear directly in MCAT figures and passages.
Glycine is the only achiral amino acid; cysteine is the only amino acid with R (not S) configuration.
Selenocysteine is a 21st, UGA-recoded amino acid — good to recognize, not required for MCAT recall.
The General Structure of an Amino Acid
Every standard amino acid is an alpha amino acid, meaning the amino group is attached to the same carbon that holds the carboxyl group. That alpha carbon is the center of four attachments:
An amino group (–NH₂)
A carboxyl group (–COOH)
A single hydrogen atom
An R group, unique to each amino acid
The R group is the only part that varies from one amino acid to the next, and it's what determines how each amino acid behaves — whether it's hydrophobic or hydrophilic, charged or neutral, reactive or inert.
The 20 Standard Amino Acids, By Category
Nonpolar, Nonaromatic Amino Acids
These have hydrophobic side chains that avoid water and are usually buried in a folded protein's interior.
Amino Acid | 3-Letter | 1-Letter | Notable Feature |
|---|---|---|---|
Glycine | Gly | G | Side chain is a single H atom — smallest amino acid, only achiral one, adds backbone flexibility |
Alanine | Ala | A | Simple methyl side chain |
Valine | Val | V | Branched isopropyl side chain |
Leucine | Leu | L | Branched-chain, packs into hydrophobic protein cores |
Isoleucine | Ile | I | Branched-chain, packs into hydrophobic protein cores |
Proline | Pro | P | Ring structure connects back to its own amino group — rigid, disrupts alpha helices |
Methionine | Met | M | Sulfur-containing thioether; first amino acid incorporated during eukaryotic translation |
Aromatic Amino Acids
These contain ring structures with conjugated pi electrons that absorb UV light, which is why they're useful for detecting proteins via spectrophotometry.
Amino Acid | 3-Letter | 1-Letter | Notable Feature |
|---|---|---|---|
Phenylalanine | Phe | F | Benzyl side chain, strongly hydrophobic |
Tyrosine | Tyr | Y | Aromatic ring plus a hydroxyl group — mildly polar |
Tryptophan | Trp | W | Bulky double-ring structure, more hydrophilic than phenylalanine |
Polar, Uncharged Amino Acids
These form hydrogen bonds and are hydrophilic, but carry no formal charge at physiological pH.
Amino Acid | 3-Letter | 1-Letter | Notable Feature |
|---|---|---|---|
Serine | Ser | S | Short side chain with a hydroxyl group |
Threonine | Thr | T | Hydroxyl group on a slightly larger side chain |
Asparagine | Asn | N | Amide side chain |
Glutamine | Gln | Q | Amide side chain |
Cysteine | Cys | C | Thiol side chain; two cysteines can form a covalent disulfide bond |
Acidic (Negatively Charged) Amino Acids
Deprotonated at physiological pH, giving them a negative charge and strong hydrophilicity.
Amino Acid | 3-Letter | 1-Letter | Notable Feature |
|---|---|---|---|
Aspartic acid / Aspartate | Asp | D | Aspartic acid is the protonated form; aspartate is the deprotonated conjugate base that predominates at physiological pH |
Glutamic acid / Glutamate | Glu | E | Same protonated/deprotonated pattern as Asp, with one extra side-chain carbon |
Basic (Positively Charged) Amino Acids
Protonated at physiological pH, giving them a net positive charge.
Amino Acid | 3-Letter | 1-Letter | Notable Feature |
|---|---|---|---|
Lysine | Lys | K | Long side chain ending in a charged amino group |
Arginine | Arg | R | Guanidinium group, resonance-stabilized — one of the strongest bases among the 20 |
Histidine | His | H | Imidazole side chain with a pKa close to physiological pH, useful in enzyme active sites |
Chirality and Configuration Exceptions
Two structural facts are worth memorizing as a pair, because the MCAT likes to test them as exceptions:
All amino acids are chiral except glycine. Glycine's side chain is a single hydrogen atom, so its alpha carbon is bonded to two identical groups (two H atoms), which eliminates the chiral center.
All amino acids have S absolute configuration except cysteine, which is R. Cysteine is still an L-amino acid biologically, but its sulfur-containing side chain (–CH₂SH) outranks the carboxyl group under CIP priority rules, which flips its formal configuration designation from S to R even though its spatial arrangement matches the other L-amino acids.
Selenocysteine: The 21st Amino Acid
Selenocysteine is sometimes called the 21st amino acid. Unlike the standard 20, it isn't assigned its own codon — it's incorporated by recoding an in-frame UGA codon, which normally signals translation to stop. A stem-loop structure in the mRNA called the SECIS element signals the ribosome to insert selenocysteine at that UGA codon instead of terminating translation. It plays a role in a small number of specialized proteins, but you don't need to memorize it for the MCAT.
Common MCAT Mistakes
Forgetting glycine is achiral. Test-takers default to "all 20 amino acids are chiral" — glycine's side chain is just an H atom, so its alpha carbon has two identical substituents and no chiral center.
Missing the cysteine R/S flip. Students assume all L-amino acids are S-configured. Cysteine's sulfur side chain reranks under CIP priority rules, making it R even though it's still the biological L-form.
Confusing aspartic acid/aspartate or glutamic acid/glutamate with different molecules. These are the same side chain in its protonated versus deprotonated state, not two different amino acids — physiological pH favors the deprotonated (charged) form.
Mixing up three-letter and one-letter codes under time pressure. Passages and figures often use one-letter codes only — know both cold rather than translating on the fly during the exam.
MCAT-Style Concept Check
Question: A researcher observes that a particular amino acid's alpha carbon is the only one among the 20 standard amino acids that is not a stereocenter. Which amino acid is this, and why?
A) Cysteine, because its thiol group creates a plane of symmetry
B) Proline, because its side chain cyclizes back to the amino group
C) Glycine, because its side chain is a single hydrogen atom, giving the alpha carbon two identical substituents
D) Alanine, because its side chain is too small to outrank the other three groups
Answer: C
Explanation: A carbon is a stereocenter only when it's bonded to four different groups. Glycine's R group is a single hydrogen atom, so its alpha carbon is bonded to two hydrogens (the R group and the standard alpha-hydrogen) along with the amino and carboxyl groups — two identical substituents eliminate the chiral center. Proline (B) is still chiral despite its ring structure; cysteine (A) and alanine (D) are both chiral, with cysteine notable instead for its R (not S) configuration.
FAQ
How many amino acids are in the human body?
There are 20 standard proteinogenic amino acids directly encoded by the genetic code, plus selenocysteine as a 21st, non-standard amino acid incorporated by recoding a stop codon.
What are the 4 groups attached to the alpha carbon of an amino acid?
An amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain) that differs between amino acids.
Which amino acid is not chiral?
Glycine. Its side chain is a single hydrogen atom, so its alpha carbon is bonded to two identical hydrogens, which removes the chiral center that every other standard amino acid has.
Why is cysteine's configuration labeled R instead of S like the other amino acids?
Cysteine's side chain contains sulfur, which outranks the carboxyl group under CIP priority rules. That reordering flips the formal R/S label even though cysteine is still biologically an L-amino acid like the rest.