Table of Contents
- What Are Proteoglycans?
- Proteoglycan Structure
- Proteoglycan Function
- Proteoglycans vs Glycoproteins
- Lesson Summary
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.
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:
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.
Proteoglycans are also found:
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:
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 includes:
There are a significant number of proteoglycan molecules in the matrix of cartilage. Proteoglycans:
Proteoglycans use their ability to bind water and absorb high compressive loads through water desorption and resorption. Desorption is the unbinding of water from the proteoglycan molecule. Resorption is the rebinding of water to the proteoglycan structure.
Glycoproteins are macromolecules that consist of carbohydrates bound to a central protein molecule. Proteoglycans, by definition, are a specific kind of glycoprotein. However, there are some differences that set proteoglycans apart from other glycoproteins.
The difference between a proteoglycan vs glycoprotein is that glycoproteins usually contain carbohydrate molecules that are short and highly branched. Proteoglycans have carbohydrate chains that are long and unbranched. Glycoproteins do not usually contain repeating sugar molecules, whereas proteoglycans consist of repeated disaccharides with amino and carboxyl groups. Proteoglycans also maintain a higher molecular weight of carbohydrates than most other glycoproteins. Proteoglycans are made up of approximately 95% carbohydrates by molecular weight.
Proteoglycans are a group of glycosylated proteins that are most often found in the extracellular environment of human tissue. A molecule is glycosylated when it has a sugar molecule attached to its core structure. Proteoglycans consist of a core protein that is attached to many glycosaminoglycans. Glycosaminoglycans are negatively-charged, long sugar molecules made up of repeating disaccharides (two-sugar molecules) that make up approximately 95% of the total proteoglycan molecular weight. All glycosaminoglycans have an amino group and a carboxyl group attached to their sugar building blocks. An amino group is a biochemically active portion of a molecule that contains a nitrogen atom. A carboxyl group contains a carbon atom that is double-bound to an oxygen atom and single-bound to a hydroxyl group (-OH). In general, proteoglycans are large and heavy molecules with very long sugar chains. Their negative charge also makes them hydrophilic, or water-loving.
Proteoglycans can be found across the cellular membrane, in the extracellular matrix, and on the surface of cells. Due to their ability to attract water, they often serve as a cushion and hydration source for tissue and organs. They help protect cells from injury by outside forces. They also help control certain major cellular events such as initiating signaling pathways, proliferation (replication), migration (cell movement), and apoptosis (programmed cell death).
To unlock this lesson you must be a Study.com Member.
Create your account
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.
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.
Already a member? Log InBack