Proteoglycans are molecules found in connective tissue that make up the extracellular matrix. They are heavily glycosylated glycoproteins with chains of polysaccharides called glycosaminoglycans (GAGs) attached to them. Proteoglycans are negatively charged and can be classified by size. They are synthesized within cells and leave via secretory vesicles. Mucopolysaccharidoses are genetic metabolic disorders caused by the inability to break down proteoglycans, leading to an accumulation in cells and various symptoms.
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 an important part of the extracellular matrix, the material between cells that provides structural support. Unlike other body tissues, the extracellular matrix is the most important part of the connective tissue.
Proteoglycans are heavily glycosylated glycoproteins. This means they are proteins with chains of polysaccharides, a kind of carbohydrate, attached to them. The specific type of polysaccharides attached to proteoglycans are called glycosaminoglycans (GAGs). Proteoglycans are negatively charged due to the presence of sulfates and uronic acids. The GAG chains of a proteoglycan can consist of chondroitin sulfate, dermatan sulfate, heparin sulfate, heparan sulfate, or keratan sulfate.
In addition to the type of GAG they carry, proteoglycans can be classified by size. Big molecules include aggrecan, a major component of cartilage, and versican, which is found in blood vessels and skin. Small molecules found in various connective tissues include decorin, biglycan, fibromodulin, and lumican. Because they are negatively charged, proteoglycans also help attract positive ions, or cations, such as calcium, potassium and sodium. they also bind water and help transport 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 produce proteins from amino acids. The protein is then moved into the rough endoplasmic reticulum (RER). It is glycosylated in the Golgi apparatus, another organelle, in a series of steps.
First, a tetrasaccharide bond on which 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 via secretory vesicles and enters the extracellular matrix.
A group of genetic metabolic disorders known as mucopolysaccharidoses is characterized by the inability to break down proteoglycans due to absent or malfunctioning lysosomal enzymes. These disorders lead to an accumulation of proteoglycans in the cells. Depending on the type of proteoglycan allowed to accumulate, mucopolysaccharidoses can cause symptoms ranging from short stature and hyperactivity to abnormal skeletal growth and mental retardation.
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