Amino Acids Reference Tool

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Amino acids are organic compounds that contain mostly carbon, hydrogen, oxygen, and nitrogen atoms. They also have amine (–NH2) and carboxyl (–COOH) functional groups with side chains (R groups) that differentiate one amino acid from another. Sulfur and other additional elements can be found in some side chains.

Amino acids are the building blocks of proteins. They can be joined by peptide bonds to form polypeptide chains. Depending on the interactions of neighboring amino acids and their side chains, polypeptides fold and form three-dimensional molecules known as proteins. A protein’s shape and its amino acid components will help determine its function.

In addition to their essential role in protein formation, amino acids are important to biosynthesis, neurotransmitter transport, and other biochemical processes.


Amino Acid Categories

Approximately 500 amino acids exist, but only 20 appear in our genetic code. These 20 proteinogenic amino acids are known as standard amino acids.

An amino acid’s polarity is dependent on the structure of its side chains. The molecule will be more polar if the difference in electronegativity between atoms is greater. For example, alkane branches and benzene rings are considered non-polar, but alcohols and amines are polar.

Amino acids are classified by:

  • Charge (positive vs. negative)
  • pH (acidic vs. basic)
  • Solubility (polar vs. non-polar)
  • Core functional group location: alpha (α), beta (β), gamma (γ), or delta (δ)
  • Side chain structure (aliphatic, aromatic, etc.)

Hydrophobicity

Very Hydrophobic Hydrophobic Neutral Hydrophilic
Phenylalanine Tyrosine Threonine Arginine
Isoleucine Cysteine Histidine Lysine
Tryptophan Alanine Glycine Asparagine
Leucine Serine Glutamic Acid
Valine Glutamine Proline
Methionine Aspartic Acid

Based on values at pH 7.

Amino Acids Reference Table

Amino Acid

Abbreviations

pKa, NH2

pKa, COOH

Expand

Alanine Ala          A 9.87 2.35
Alanine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 89.09
Classification Alkyl

Description: Alanine is a small, non-essential amino acid with a relatively non-reactive single methyl group side chain. It’s one of the simplest amino acids and the second most abundant in proteins. It’s encoded by codons that start with GC.

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Arginine Arg          R 9.09 2.18
Arginine
Side Chain Polarity Polar
Charge Positive
Molecular Weight (g/mol) 174.20
Classification Basic

Description: Arginine is a chemically complex amino acid with a 3-carbon aliphatic side chain ending in a guanidino group. It commonly mediates interactions between proteins and negatively charged DNA and RNA backbones.

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Asparagine Asn          N 8.80 2.02
Asparagine
Side Chain Polarity Polar
Charge Neutral
Molecular Weight (g/mol) 132.12
Classification Neutral

Description: Asparagine is an amino acid with a carboxamide side chain. The amide group retains its polarity and is a rich hydrogen bond source, making asparagine a common protein binding site and substrate for glycosyltransferases.

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Aspartic Acid Asp          D 9.60 1.88
Aspartic Acid
Side Chain Polarity Polar
Charge Negative
Molecular Weight (g/mol) 133.10
Classification Acidic

Description: Aspartic acid has a carboxylic acid side chain that reacts with other amino acids, enzymes, and proteins. Its negative charge and large hydrogen-bonding potential enables its participation in many molecular interactions.

Cysteine Cys          C 10.78 1.71
Cysteine
Side Chain Polarity Polar
Charge Neutral
Molecular Weight (g/mol) 121.16
Classification Neutral

Description: Cysteine is a semi-essential proteinogenic amino acid with a thiol side chain that participates in enzymatic reactions. Its sulfhydryl group forms disulfide bonds with other cysteines or molecules with free sulfhydryls.

Glutamic Acid Glu          E 9.67 2.19
Glutamic Acid
Side Chain Polarity Polar
Charge Negative
Molecular Weight (g/mol) 147.13
Classification Acidic

Description: Glutamic acid has one amino and two carboxyl groups that give it a negative charge in virtually all biological systems. This enables glutamic acid to interact with positively charged entities, including metal ions.

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Glutamine Gln          Q 9.13 2.17
Glutamine
Side Chain Polarity Polar
Charge Neutral
Molecular Weight (g/mol) 146.15
Classification Neutral

Description: Glutamine has a side chain similar to that of glutamic acid; one carboxyl group is simply replaced by an amide. It’s important for nitrogen metabolism and needed to help regulate toxic levels of ammonia in the body.

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Glycine Gly          G 9.60 2.34
Glycine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 75.07
Classification Alkyl

Description: Glycine is the simplest amino acid with a single hydrogen atom for a side chain. This makes it ideal for accommodating the tight turns of some polypeptide chains. Glycine also acts as an inhibitory neurotransmitter.

Histidine His          H 8.97 1.78
Histidine
Side Chain Polarity Polar
Charge Positive
Molecular Weight (g/mol) 155.16
Classification Basic

Description: Histidine is rare and has a partially protonated aromatic imidazole side chain. It may be neutral or positively charged and can serve as a natural catalyst or prevalent residue at enzyme active sites thanks to its bond potential.

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Isoleucine Ile          I 9.76 2.32
Isoleucine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 131.18
Classification Alkyl

Description: Isoleucine has a branched hydrocarbon side chain arranged in an "L" shape and is considered one of the largest aliphatic amino acids. It’s concentrated in muscle tissue and encoded by the codons AUU, AUC, and AUA.

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Leucine Leu          L 9.60 2.36
Leucine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 131.18
Classification Alkyl

Description: Leucine has an isobutyl group side chain with four fully protonated carbons arranged in a bulky "Y" shape. It’s also the most common amino acid in proteins and one of the largest aliphatic amino acids.

Lysine Lys          K 10.28 8.90
Lysine
Side Chain Polarity Polar
Charge Positive
Molecular Weight (g/mol) 146.19
Classification Basic

Description: Lysine is an essential amino acid with a lysyl side chain. It’s important for proteinogenesis, crosslinking collagen polypeptides, absorbing mineral nutrients, and producing carnitine, key for fatty acid metabolism.

Methionine Met          M 9.21 2.28
Methionine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 149.21
Classification Alkyl

Description: Methionine has a linear side chain and contains a sulfide group. This enables methionine to be a prolific methyl group donor. It also plays a critical role in metabolism, the growth of new blood vessels, and tissue repair.

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Phenylalanine Phe          F 9.24 2.58
Phenylalanine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 165.19
Classification Aromatic

Description: Phenylalanine contains a benzyl group side chain and is considered the simplest and most abundant aromatic amino acid in proteins. It’s a precursor for tyrosine synthesis, dopamine, norepinephrine, epinephrine, and melanin.

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Proline Pro          P 10.60 1.99
Proline
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 115.13
Classification Alkyl

Description: Proline is a proteinogenic amino acid with a cyclic pyrrolidine side chain. It’s non-essential in humans and can be synthesized from glutamic acid. Essential to the immune system, proline is also key to joint and tendon function.

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Serine Ser          S 9.15 2.21
Serine
Side Chain Polarity Polar
Charge Neutral
Molecular Weight (g/mol) 105.09
Classification Neutral

Description: Serine is an amino acid with a hydroxymethyl group side chain. It’s the simplest hydroxyl amino acid and the third most abundant found in proteins. Serine can be synthesized from glycine and several other metabolites.

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Threonine Thr          T 9.12 2.15
Threonine
Side Chain Polarity Polar
Charge Neutral
Molecular Weight (g/mol) 119.12
Classification Neutral

Description: Threonine is an essential amino acid that contains a hydroxyl group, making it polar and uncharged. It shares many properties with serine and is a substrate to kinases and glycosyltransferases. It’s encoded by codons that start with AC.

Tryptophan Trp          W 9.39 2.38
Tryptophan
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 204.23
Classification Aromatic

Description: Tryptophan is both the largest and rarest amino acid found in proteins. Generally found in the hydrophobic core of proteins, tryptophan is often involved in aromatic stacking. Its side chain is dominated by an indole group.

Tyrosine Tyr          Y 9.11 2.20
Tyrosine
Side Chain Polarity Polar
Charge Neutral
Molecular Weight (g/mol) 181.19
Classification Neutral

Description: Tyrosine is an aromatic amino acid similar to phenylalanine with a hydroxyl group instead of a phenyl group in the para position. It has unique chemical properties and serves as a catalyst and substrate for a variety of kinases.

Valine Val          V 9.72 2.29
Valine
Side Chain Polarity Non-Polar
Charge Neutral
Molecular Weight (g/mol) 117.15
Classification Alkyl

Description: Valine is a moderately sized aliphatic amino acid with a side chain consisting of a propyl group attached to the CA at C2. Like the other aliphatic residues, valine is often found in the hydrophobic cores of proteins.

Essential Amino Acids

The human body can produce 10 of the 20 required amino acids. The other half, called essential amino acids, come from your diet and must be consumed regularly.

Produced by the Body (Non-Essential) Supplied by Foods (Essential)
Alanine Glutamine Arginine** Methionine
Asparagine Glycine Histidine Phenylalanine
Aspartic Acid Proline Isoleucine Threonine
Cysteine Serine Leucine Tryptophan
Glutamic Acid Tyrosine* Lysine Valine

*Requires phenylalanine to produce
**Needed by children, but not adults

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