Amino acid

Introduction & Top Questions
Building blocks of proteins
Chirality
Acid-base properties
Standard amino acids
- Group I: Nonpolar amino acids
- Group II: Polar, uncharged amino acids
- Group III: Acidic amino acids
- Group IV: Basic amino acids
Amino acid reactions
- Peptide bond
- Cysteine oxidation
Other functions
Nonstandard amino acids
Analysis of amino acid mixtures
Some common uses
Amino acids and the origin of life on Earth

References & Edit History Related Topics

Images & Videos

Diagram showing two peptide links, each with atoms labeled for oxygen, carbon, nitrogen, and hydrogen. A dotted line represents a hydrogen bond connecting the two peptide links, illustrating chemical bonding between them.

Diagram showing three glycine molecules undergoing a condensation reaction to form a tripeptide chain, with two water molecules released as byproducts.

Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA.

Diagram showing the basic structure and expression of a gene, with DNA made of exons and introns, transcription to RNA, splicing to form mRNA, translation into an amino acid chain, and posttranslational modification to form a protein.

Diagram showing metabolic pathways for amino acid biosynthesis, divided into families: glutamate, aspartate, serine, pyruvate, shikimate, and histidine. Amino acids, underlined to indicate protein components, branch from intermediate compounds, with arrows showing synthesis relationships. Some intermediates and side products are labeled, and related amino acids are grouped by precursor molecules.

Figure 4: Enzyme defects in disorders of the amino acids phenylalanine and tyrosine.

A flowchart illustrating protein and amino acid metabolism, showing how dietary proteins are digested and absorbed into the body's amino acid pool, which is then used for building tissue and plasma proteins, creating amino acid derivatives, energy metabolism, and excretion of waste products like urea.

The chemistry of digestion: How food is broken down

At a Glance

amino acid summary

Related Questions

How can hydrolysis break peptide bonds in proteins?
How do L-amino acids differ from D-amino acids?
How do amino acids contribute to heme, chlorophyll, and the removal of nitrogenous waste?
How do amino acids contribute to neurotransmitters such as serotonin and histamine?
How do beans and rice provide complementary amino acids?
How do cysteine molecules form disulfide bonds?
How do hydrogen bonds between amino acid side chains help stabilize proteins?
How do hydrophobic interactions help a protein fold into its three-dimensional shape?
How does a peptide bond form between two amino acids?
How does an R group make one amino acid different from another?
How does digestion break food proteins into amino acids?
How does histidine help enzymes carry out acid-base reactions?
How does the charge of glycine change in acidic and alkaline solutions?
How is a peptide different from a protein?
How many amino acids are in an oligopeptide?
How many nucleotides are in a codon?
How many standard amino acids are used to build most proteins?
What are the N terminus and C terminus of a peptide chain?
What is the isoelectric point of an amino acid?
Which amino acid is low in legumes such as beans?
Which amino acids can the human body make on its own?
Which amino acids have basic side chains?
Which amino acids have polar uncharged side chains?
Which atoms and groups are lost when a peptide bond forms?
Which foods provide complete proteins with all essential amino acids?
Which nine amino acids are essential in the human diet?
Why are lysine and arginine positively charged at physiological pH?
Why can several different codons specify the same amino acid?
Why do nonpolar amino acid side chains tend to be buried inside globular proteins?
Why is gelatin not a complete protein source?
Why is glycine the only standard amino acid without a chiral alpha carbon?
Why is peptide-bond formation called a condensation reaction?
Why is soybean protein considered a complete plant protein?
What is an amino acid?
What are the 20 amino acid building blocks of proteins?
What is the difference between standard and nonstandard amino acids?
What are some industrial uses of amino acids?
What is an enzyme?
What are enzymes composed of?
What are examples of enzymes?
What factors affect enzyme activity?
What common diseases are caused by a vitamin deficiency?
What nitrogen-containing bases occur in nucleic acids?

Science Chemistry

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External Websites

Biology LibreTexts - Structure and Function - Amino Acids
Healthline - Essential Amino Acids: Definition, Benefits, and Food Sources
National Center for Biotechnology Information - Biochemistry, Amino Acid Synthesis and Degradation
Cleveland Clinic - Amino Acid
WebMD - Foods High in Amino Acids
Purdue University - Chemical Education Division Groups - Amino Acids
Mustansiriyah University - Amino Acid: Structure and Classification
Roger Williams University Open Publishing - Introduction to Molecular and Cell Biology - Amino Acids and Proteins

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

amino acid - Student Encyclopedia (Ages 11 and up)

Amino acid

chemical compound

Written by

Michael K. Reddy

Associate Professor of Biochemistry, University of Wisconsin, Milwaukee.

Michael K. Reddy

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Nov. 11, 2026 •History

Amino acid, any of a group of organic molecules that consist of a basic amino group (―NH₂), an acidic carboxyl group (―COOH), and an organic R group (or side chain) that is unique to each amino acid. The term amino acid is short for α-amino [alpha-amino] carboxylic acid. Each molecule contains a central carbon (C) atom, called the α-carbon, to which both an amino and a carboxyl group are attached. The remaining two bonds of...

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