The 20 Amino Acids

These 20 molecular building blocks combine in endless ways to create every protein in every living organism on Earth.

The Mathematics of Protein Diversity

Theoretical Combinations

For a protein of length n:

20ⁿ possible sequences

10 amino acids: 20¹⁰ = 10,240,000,000,000 (10 trillion)

100 amino acids: 20¹⁰⁰ = 10¹³⁰ combinations

300 amino acids (average protein): 20³⁰⁰ = 10³⁹⁰ combinations

For perspective: There are only about 10⁸⁰ atoms in the observable universe. The number of possible 300-amino-acid proteins (10³⁹⁰) is unimaginably larger!

Discovered Proteins

~500,000+

Proteins in UniProt (reviewed)

~250 million+

Total protein sequences in databases

~200 million

Structures predicted by AlphaFold

Human proteome: ~20,000 protein-coding genes producing ~100,000+ distinct proteins through alternative splicing and modifications.

Why So Few Proteins Exist?

Evolution

Only proteins that provide survival advantages are selected. Most random sequences would not fold properly or have useful functions.

Folding Constraints

Only ~1 in 10⁷⁷ random sequences can fold into a stable, functional structure. Most sequences are "junk" that would misfold or aggregate.

Protein Families

Proteins evolved from common ancestors. There are only ~2,000 distinct protein fold families, with variations built on these templates.

Amino Acid Categories

Nonpolar (9)

Aromatic (3)

Polar Uncharged (6)

Acidic - Negative (2)

Basic - Positive (3)

G - Gly A - Ala V - Val L - Leu I - Ile P - Pro F - Phe Y - Tyr W - Trp S - Ser T - Thr C - Cys M - Met N - Asn Q - Gln D - Asp E - Glu K - Lys R - Arg H - His

Detailed Amino Acid Profiles

Nonpolar7.2% abundance

Glycine

Gly

Formula:C₂H₅NO₂

MW:75.07 Da

pI:5.97

Hydropathy:-0.4

Side Chain (R group):H (hydrogen)

Smallest amino acid. Highly flexible due to no side chain. Found in collagen (every 3rd residue).

Nonpolar8.3% abundance

Alanine

Ala

Formula:C₃H₇NO₂

MW:89.09 Da

pI:6

Hydropathy:+1.8

Side Chain (R group):-CH₃ (methyl)

Simple, small hydrophobic residue. One of the most common amino acids. Important in energy metabolism.

Nonpolar6.6% abundance

Valine

Val

Formula:C₅H₁₁NO₂

MW:117.15 Da

pI:5.96

Hydropathy:+4.2

Side Chain (R group):-CH(CH₃)₂ (isopropyl)

Branched-chain amino acid (BCAA). Essential amino acid. Important for muscle metabolism.

Nonpolar9.7% abundance

Leucine

Leu

Formula:C₆H₁₃NO₂

MW:131.17 Da

pI:5.98

Hydropathy:+3.8

Side Chain (R group):-CH₂CH(CH₃)₂ (isobutyl)

Branched-chain amino acid. Most abundant essential amino acid. Key regulator of protein synthesis (mTOR).

Nonpolar5.3% abundance

Isoleucine

Ile

Formula:C₆H₁₃NO₂

MW:131.17 Da

pI:6.02

Hydropathy:+4.5

Side Chain (R group):-CH(CH₃)CH₂CH₃ (sec-butyl)

Branched-chain amino acid. Essential. Important for hemoglobin synthesis and blood sugar regulation.

Nonpolar5.1% abundance

Proline

Pro

Formula:C₅H₉NO₂

MW:115.13 Da

pI:6.3

Hydropathy:-1.6

Side Chain (R group):-(CH₂)₃- (cyclic)

Unique cyclic structure. Introduces kinks in protein chains. Abundant in collagen. Disrupts secondary structures.

Aromatic3.9% abundance

Phenylalanine

Phe

Formula:C₉H₁₁NO₂

MW:165.19 Da

pI:5.48

Hydropathy:+2.8

Side Chain (R group):-CH₂-C₆H₅ (benzyl)

Essential aromatic amino acid. Precursor to tyrosine. People with PKU cannot metabolize it properly.

Aromatic2.9% abundance

Tyrosine

Tyr

Formula:C₉H₁₁NO₃

MW:181.19 Da

pI:5.66

Hydropathy:-1.3

Side Chain (R group):-CH₂-C₆H₄-OH (p-hydroxybenzyl)

Precursor to dopamine, epinephrine, thyroid hormones. Can be phosphorylated for cell signaling.

Aromatic1.1% abundance

Tryptophan

Trp

Formula:C₁₁H₁₂N₂O₂

MW:204.23 Da

pI:5.89

Hydropathy:-0.9

Side Chain (R group):-CH₂-indole

Largest amino acid. Essential. Precursor to serotonin and melatonin. Rarest amino acid in proteins.

Polar6.9% abundance

Serine

Ser

Formula:C₃H₇NO₃

MW:105.09 Da

pI:5.68

Hydropathy:-0.8

Side Chain (R group):-CH₂OH (hydroxymethyl)

Polar, can be phosphorylated. Important in enzyme active sites. Precursor to glycine and cysteine.

Polar5.4% abundance

Threonine

Thr

Formula:C₄H₉NO₃

MW:119.12 Da

pI:5.6

Hydropathy:-0.7

Side Chain (R group):-CH(OH)CH₃ (1-hydroxyethyl)

Essential amino acid. Can be phosphorylated. Important for collagen, elastin, and tooth enamel.

Polar1.4% abundance

Cysteine

Cys

Formula:C₃H₇NO₂S

MW:121.16 Da

pI:5.07

Hydropathy:+2.5

Side Chain (R group):-CH₂SH (thiol)

Contains sulfur. Forms disulfide bonds critical for protein structure. Important antioxidant (glutathione).

Nonpolar2.4% abundance

Methionine

Met

Formula:C₅H₁₁NO₂S

MW:149.21 Da

pI:5.74

Hydropathy:+1.9

Side Chain (R group):-CH₂CH₂SCH₃ (methylthioethyl)

Essential. Start codon (AUG) codes for Met. Important for methylation reactions. Contains sulfur.

Polar4.0% abundance

Asparagine

Asn

Formula:C₄H₈N₂O₃

MW:132.12 Da

pI:5.41

Hydropathy:-3.5

Side Chain (R group):-CH₂CONH₂ (carbamoylmethyl)

Amide of aspartic acid. Common site for N-linked glycosylation. Important for protein folding.

Polar3.9% abundance

Glutamine

Gln

Formula:C₅H₁₀N₂O₃

MW:146.15 Da

pI:5.65

Hydropathy:-3.5

Side Chain (R group):-CH₂CH₂CONH₂

Most abundant amino acid in blood. Important nitrogen carrier. Fuel for rapidly dividing cells.

Acidic5.3% abundance

Aspartic Acid

Asp

Formula:C₄H₇NO₄

MW:133.1 Da

pI:2.77

Hydropathy:-3.5

Side Chain (R group):-CH₂COOH (carboxymethyl)

Negatively charged at pH 7. Important in enzyme active sites. Precursor to other amino acids.

Acidic6.2% abundance

Glutamic Acid

Glu

Formula:C₅H₉NO₄

MW:147.13 Da

pI:3.22

Hydropathy:-3.5

Side Chain (R group):-CH₂CH₂COOH

Negatively charged. Major excitatory neurotransmitter. Used as flavor enhancer (MSG).

Basic5.7% abundance

Lysine

Lys

Formula:C₆H₁₄N₂O₂

MW:146.19 Da

pI:9.74

Hydropathy:-3.9

Side Chain (R group):-(CH₂)₄NH₂ (aminobutyl)

Essential. Positively charged. Important for collagen crosslinking. Can be acetylated/methylated (histones).

Basic5.7% abundance

Arginine

Arg

Formula:C₆H₁₄N₄O₂

MW:174.2 Da

pI:10.76

Hydropathy:-4.5

Side Chain (R group):-(CH₂)₃NHC(=NH)NH₂ (guanidino)

Positively charged. Precursor to nitric oxide. Important in immune function and wound healing.

Basic2.3% abundance

Histidine

His

Formula:C₆H₉N₃O₂

MW:155.16 Da

pI:7.59

Hydropathy:-3.2

Side Chain (R group):-CH₂-imidazole

pKa near physiological pH - acts as buffer. Essential for infants. Found in hemoglobin active site.

Essential vs Non-Essential

Essential Amino Acids (9)

Must be obtained from diet - body cannot synthesize them.

HisIleLeuLysMetPheThrTrpVal

Remember: "PVT TIM HaLL" - Phe, Val, Thr, Trp, Ile, Met, His, Arg (conditionally), Leu, Lys

Non-Essential Amino Acids (11)

Body can synthesize these from other molecules.

AlaArgAsnAspCysGlnGluGlyProSerTyr

Note: Arginine, Cysteine, Glutamine, Tyrosine, Glycine, and Proline are "conditionally essential" during illness or stress.

The Genetic Code

Each amino acid is encoded by one or more three-nucleotide sequences (codons) in mRNA. This redundancy helps protect against mutations.

Amino Acid	1-Letter	3-Letter	Codons	# Codons
Alanine	A	Ala	GCU, GCC, GCA, GCG	4
Arginine	R	Arg	CGU, CGC, CGA, CGG, AGA, AGG	6
Asparagine	N	Asn	AAU, AAC	2
Aspartic Acid	D	Asp	GAU, GAC	2
Cysteine	C	Cys	UGU, UGC	2
Glutamic Acid	E	Glu	GAA, GAG	2
Glutamine	Q	Gln	CAA, CAG	2
Glycine	G	Gly	GGU, GGC, GGA, GGG	4
Histidine	H	His	CAU, CAC	2
Isoleucine	I	Ile	AUU, AUC, AUA	3
Leucine	L	Leu	UUA, UUG, CUU, CUC, CUA, CUG	6
Lysine	K	Lys	AAA, AAG	2
Methionine	M	Met	AUG (Start)	1
Phenylalanine	F	Phe	UUU, UUC	2
Proline	P	Pro	CCU, CCC, CCA, CCG	4
Serine	S	Ser	UCU, UCC, UCA, UCG, AGU, AGC	6
Threonine	T	Thr	ACU, ACC, ACA, ACG	4
Tryptophan	W	Trp	UGG	1
Tyrosine	Y	Tyr	UAU, UAC	2
Valine	V	Val	GUU, GUC, GUA, GUG	4
Stop Codons	*	Stop	UAA, UAG, UGA	3

Total: 64 codons encoding 20 amino acids + 3 stop signals = genetic code redundancy

Explore Proteins Made From These Amino Acids

See how these 20 building blocks combine to create the proteins that power life.

View Common Proteins Learn Protein Structure