CHAPTER 1 PART 4

Current Research (Abstracts)

Lysosomes are membranous sacs of hydrolytic enzymes of many diverse sizes used for controlled intracellular digestion of macromolecules. They contain about 40 different acid hydrolases eg. nucleoses, glucosidases, lipases, phosphotases,and sulfatases, and require an acid environment for optimal performance. The lysosome has a unique membrane that allows the transport proteins to move the final digestive enzymes eg. sugars, amino acids, and nucleotides to the cytosol where they can be excreted or reused. A H+ (hydrogen ion) pump utilizes the energy of ATP hydrolysis, to pump a H+ into the lysosome, therby helping it to maintain the acidic ph. Since the lysosomal membrane proteins are highly glycosylated, they are protected from the lysosomal proteins in the lumens. Lysosomes are found in all eukaryotic cells, and its morphological homogeneity contrasts with the uniform structure of other organelles and its diversity reflects the variety of digestive functions that it carries out. These functions include breakdown of debris (intracellular & extracellular), the destruction of phagocytized microorganisms, and production of nutrients for the cell. Plants have one or more large fluid filled vessicles called vacuoles which can occupy 30-90% of the cells interior space and contain awide varietry of enzymes with diverse functions. One of these functions is to regulate the ph of the cytosol and keep it balanced. Some of these vacuoles serve as a controller of turgor pressure which is the osmotic pressure exerted on the cell wall keeping the plant from wilting. This maintenance is accomplished by changing the osmotic pressure of the cytosol in the vacuole, which in turn is made possible by controlling the breakdown and resynthesis of polymers eg. polyphosphate in the vacuole, and through regulation of various sugars and amino acids across the vacuolar and plasma membranes. Substances stored in the vacuoles vary by plant species. They may contain amino acids for seed germination, anthocyanins for coloration of leaf petals, or noxious toxins to discourage predators from eating them. Lysosomes are a source of convergence wherein digestive enzymes are delivered outward from the ER via the golgi appartatus, and substances to be digested are delivered by pathways according to their source. There are three basic pathways to degradation in the lysosomes. The first is where the endocytosed molecules are diverted into small vessicles called early endosomes. Some of these are selectively retrieved and sent to the plasma membrane while others pass on into the late endosomes. It is here that the two streams of the transport vessicles meet and the materials that are to be digested come in contact with the lysosomal hydrolases which come from the golgi apparatus. The interior of the late endosomes are slightly acidic and it is at this point where the hydrolytic digestion of the endocytosed molecule begins, and where the mature lysosomes are formed from the late endosomes. The second pathway to degradation involves a process known as autophagy. This is the process in which all of the obsolete parts of the cell are disposed of. The third pathway provides large materials to cells that are specialized in phagocytosis. These phagocytes engulf objects to form a phagosome, which in turn forms a lysosome. There is a fourth possibility for a pathway to the lysosomes. and that is through what is known as the KFERQ sequences. In this sequence the K, is for lysine; and Q, is for phenylalanine; E, is for glutamate; R, is for arginine; and Q,is for glutamine. It is these tags, which attached to a protien could serve as a signal to deliver this protein to the lysosomes for degradation. © BOTRESEARCH USA 1998-2008