Glycosome

The glycosome is a membrane-enclosed organelle that contains the glycolytic enzymes. The term was first used by Scott and Still in 1968 after they realized that the glycogen in the cell was not static but rather a dynamic molecule. It is found in a few species of protozoa including the Kinetoplastida which include the suborders Trypanosomatida and Bodonina, most notably in the human pathogenic trypanosomes, which can cause sleeping sickness, Chagas's disease, and leishmaniasis. The organelle is bounded by a single membrane and contains a dense proteinaceous matrix. It is believed to have evolved from the peroxisome. This has been verified by work done on Leishmania genetics. The glycosome is currently being researched as a possible target for drug therapies. Glycosomes are unique to kinetoplastids and their sister diplonemids. The term glycosome is also used for glycogen-containing structures found in hepatocytes responsible for storing sugar, but these are not membrane bound organelles. Glycosomes are composed of glycogen and proteins. The proteins are the enzymes that are associated with the metabolism of glycogen. These proteins and glycogen form a complex to make a distinct and separate organelle. The proteins for glycosomes are imported from free cytosolic ribosomes. The proteins imported into the organelle have a specific sequence, a PTS1 ending sequence to make sure they go to the right place. They are similar to alpha-granules in the cytosol of a cell that are filled with glycogen. Glycosomes are typically round-to-oval shape with size varying in each cell. Although glycogen is found in the cytoplasm, that in the glycosome is separate, surrounded by membrane. The membrane is a lipid bilayer. The glycogen that is found within the glycosome is identical to glycogen found freely in the cytosol. Glycosomes can be associated or attached to many different types of organelles. They have been found to be attached to the sarcoplasmic reticulum and its intermediate filaments.