Proteoglycans are a type of molecule found in the connective tissue of the body. Connective tissue is fibrous tissue that provides support for other body structures. Proteoglycans make up a major part of the extracellular matrix, the material between cells that provides structural support. Unlike in other body tissues, the extracellular matrix is the most important part of connective tissue.
Proteoglycans are heavily glycosylated glycoproteins. This means that they are proteins with chains of polysaccharides, a kind of carbohydrate, attached. The specific type of polysaccharides attached to proteoglycans are called glycosaminoglycans (GAGs). Proteoglycans are negatively charged because of the presence of sulfates and uronic acids. The GAG chains of a proteoglycan may be made of chondroitin sulfate, dermatan sulfate, heparin sulfate, heparan sulfate, or keratan sulfate.
In addition to the type of GAG they carry, proteoglycans can be categorized by size. Large molecules include aggrecan, an important component of cartilage, and versican, which is found in the blood vessels and skin. Small molecules present in various connective tissues include decorin, biglycan, fibromodulin, and lumican. Because they are negatively charged, proteoglycans also help to attract positive ions, or cations, such as calcium, potassium, and sodium. they also bind water, and aid in the transport of water and other molecules through the extracellular matrix.
All components of a proteoglycan are synthesized within cells. The protein portion is synthesized by ribosomes, which make proteins out of amino acids. The protein is then moved to the rough endoplasmic reticulum (RER). It is glycosylated in the Golgi apparatus, another organelle, in a number of steps.
First, a link tetrasaccharide on which the polysaccharides can grow is attached to the protein. Then, the sugars are added one by one. When the proteoglycan is complete, it leaves the cell through secretory vesicles, and enters the extracellular matrix.
A group of genetic metabolic disorders known as mucopolysaccharidoses are characterized by the inability to break down proteoglycans due to absent or malfunctioning lysosomal enzymes. These disorders lead to a proteoglycan buildup in the cells. Depending upon the type of proteoglycan allowed to build up, mucopolysaccharidoses can cause symptoms ranging from short stature and hyperactivity to abnormal skeletal growth and mental retardation.