Proteoglycans: Structure and Function

Brittany W., Darla Reed
  • Author
    Brittany W.

    Brittany has a bachelor's degree in biological sciences and biochemistry.

  • Instructor
    Darla Reed

    Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

Understand what proteoglycans are by learning the definitions of proteoglycans. Learn the structural differences between proteoglycan and glycoprotein. Updated: 01/25/2022

Table of Contents


What Are Proteoglycans?

The proteoglycans definition is a group of glycosylated proteins primarily found in the extracellular matrix of most human tissue. The extracellular matrix is often associated with connective tissue, a group of tissue that offers structure, cohesion, and support to the body.

The term glycosylated refers to the addition or attachment of glycosaminoglycans (GAGs) to the core proteoglycan (PG) protein. Glycosaminoglycans are anionic (negatively-charged) polysaccharides. Polysaccharides are long chains of sugar (carbohydrate) molecules. Therefore, a proteoglycan is a string of amino acids (the building blocks of protein) with long chains of sugar molecules intermittently attached via covalent bonds. Covalent bonds are a type of linkage where electrons are shared between the two atoms being linked together.

Due to their structure and location, proteoglycans offer significant structural and functional benefits to the extracellular, and sometimes intracellular, environment.

Location of Proteoglycan

Proteoglycans are found primarily in the extracellular matrix (ECM) of nearly every tissue in the body, including the lungs and cornea. The ECM is an intricate network of molecules that exists to:

  • Provide structural support to nearby tissues and organs
  • Provide mechanical support to nearby tissues and organs
  • Provide nutrients
  • House growth factors (proteins that stimulate the growth of specific tissues) and other bioactive molecules
  • Control major events within the lifespan of a cell (i.e., proliferation, migration, differentiation, and apoptosis)

Proliferation is the rapid reproduction of a cell in order to increase its cell count. Migration is the movement of a cell from one location to another. Differentiation is the evolution of a cell from one cell type to another. Apoptosis is the planned, controlled, and programmed cell death that causes very little disruption to other surrounding cells and tissues.

Proteglycans are primarily found in the extracellular environment (4).

Illustration of the extracellular environment.

Proteoglycans are also found:

  • On the surface of or lying across the cell membrane
  • Among intracellular structures
  • Within joint cartilage

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Proteoglycan Structure

The proteoglycan structure consists of a core strand of amino acids. Proteins are identified by the unique order in which their amino acids are linked together. Glycosaminoglycans are covalently attached to specific amino acid molecules within the core proteoglycan chain. In this way, a proteoglycan can also be referred to as a GAG protein. Monosaccharides are the building blocks of polysaccharides. In this case, glycosaminoglycans are made up of modified monosaccharides such as glucose and galactose.

Due to their long GAG extensions, proteoglycans are relatively large and heavy molecules. They are not very flexible macromolecules (molecules with a large number of atoms), but they maintain enough malleability to absorb shock. Their unique structural makeup is what provides integrity and protection to tissues and organs. The responsiveness of the extracellular environment to mechanical pressure protects vital organs from unnecessary stress and irreparable damage.


A glycosaminoglycan, or GAG, is a long chain of sugar molecules. Sugar molecules are individually referred to as monosaccharides, but they are also grouped by two to form disaccharides. GAGs are usually identified by their disaccharides rather than the individual monosaccharide units.

GAGs contain disaccharides that are made up of two types of sugars each:

  • An amino sugar (N-acetylglucosamine or N-acetylgalactosamine)
  • A sugar acid (glucuronic acid or iduronic acid)

Amino sugars are sugar molecules with an amino group attached. Amino groups are biochemically active portions of a molecule that contain a nitrogen atom. Sugar acids are sugar molecules with a carboxyl group attached. Carboxyl groups contain a carbon atom that is double-bound to an oxygen atom and single-bound to a hydroxyl group (-OH).

The difference between glycosaminoglycans and proteoglycans is that glycosaminoglycans are a component of proteoglycans. GAGs are sugar molecules and PGs are made up of both sugar and amino acid residues. Both molecules are hydrophilic, or water-loving, which makes them suitable for the aqueous (water) environment of the extracellular matrix.

Proteoglycan Function

Proteoglycan function includes:

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Frequently Asked Questions

What is the function of proteoglycans?

Proteoglycans provide structural support to the extracellular matrix as well as surrounding tissues and organs. Their affinity to water also provides cushion, protection, and a source of hydration.

Where are proteoglycans found?

Proteoglycans are primarily found in the extracellular matrix (ECM) of nearly every human tissue, including the lungs and cornea. They are particularly useful in joints and cartilage. Proteoglycans can be found intracellularly, on the surface of the cell membrane, or lying across the cell membrane.

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