Hydrolysis cell surface

In the rhizosphere, microorganisms utilize either organic acids or phytosiderophores to transport iron or produce their own low-molecular-weight metal chelators, called siderophores. There are a wide variety of siderophores in nature and some of them have now been identified and chemically purified (54). Pre.sently, three general mechanisms are recognized for utilization of these compounds by microorganisms. These include a shuttle mechanism in which chelators deliver iron to a reductase on the cell surface, direct uptake of metallated siderophores with destructive hydrolysis of the chelator inside the cell, and direct uptake followed by reductive removal of iron and resecretion of the chelator (for reviews, see Refs. 29 and 54). 

Tepper, A.D., Ruurs, P., Wiedmer, T, Sims, PJ., Borst, J. and van Bhtterswijk, W.J., 2000, Sphingomyelin hydrolysis to ceramide during the execution phase of apoptosis results from phospholipid scrambling and alters cell-surface morphology. J. Cell Biol., 150 F5-7. 

Ren M, Xu G, Jinanbo Z, de Lemos-Chiarandini C, Adesnik M, Sabatini DD. Hydrolysis of GTP on rab 11 is required for the direct delivery of transferrin from the pericentriolar recycling compartment to the cell surface but not from sorting endosomes. Proc Natl Acad Sci USA 1998 95 6187-6192. 

The breakdown of fatty acids in (3-oxidation (see Topic K2) is controlled mainly by the concentration of free fatty acids in the blood, which is, in turn, controlled by the hydrolysis rate of triacylglycerols in adipose tissue by hormone-sensitive triacylglycerol lipase. This enzyme is regulated by phosphorylation and dephosphorylation (Fig. 5) in response to hormonally controlled levels of the intracellular second messenger cAMP (see Topic E5). The catabolic hormones glucagon, epinephrine and norepinephrine bind to receptor proteins on the cell surface and increase the levels of cAMP in adipose cells through activation of adenylate cyclase (see Topic E5). The cAMP allosterically activates... 

Sialidase or NA is the second glycoprotein on the virion surface. One of the most important functions of sialidase is associated with the release of virus progeny from infected host cells to repeat the cycle of infection [6, 50], The enzyme catalyzes the hydrolysis of sialic acid residues from glycoconjugates on the cell surface with either Neu5Ac(a2 3)Gal or Neu5Ac(a2 6)Gal linkages (Gal=galactose) [60, 61]. 

Cholesterol appears to be taken up from plasma lipoproteins by steroidogenic tissues by two receptor-mediated pathways - the LDL pathway and the HDL pathway. Not all tissues of all species can utilise both of these thus, the LDL pathway appears to occur in all species, including man, whereas the HDL pathway occurs mainly in rodents. LDL lipoproteins interact specifically with cell surface-bound receptors, as shown for, e.g., adrenal [9] and ovary [10], after which internalization occurs by endocytosis and hydrolysis of LDLs, plus their cholesterol ester complement by lysosomal action. 

LDL binds specifically to lipoprotein receptors on the cell surface. The resulting complexes become clustered in regions of the plasma membrane called coated pits. Endocytosis follows (see Fig. 13-3). The clathrin coat dissociates from the endocytic vesicles, which may recycle the receptors to the plasma membrane or fuse with lysosomes. The lysosomal proteases and lipases then catalyze the hydrolysis of the LDL-receptor complexes the protein is degraded completely to amino acids, and cholesteryl esters are hydrolyzed to free cholesterol and fatty acid. New LDL receptors are synthesized on the endoplasmic reticulum (ER) membrane and are subsequently reintroduced into the plasma membrane. The cholesterol is incorporated in small amounts into the endoplasmic reticulum membrane or may be stored after esterification as cholesteryl ester in the cytosol this occurs if the supply of cholesterol exceeds its utilization in membranes. Normally, only very small amounts of cholesteryl ester reside inside cells, and the majority of the free cholesterol is in the plasma membrane. 

The release of fatty acids from adipose tissue is regulated by the rate of hydrolysis of triacylglycerol and the rate of esterification of acyl-CoA with glycerol 3-phosphate. The rate of hydrolysis is stimulated by hormones that bind to cell-surface receptors and stimulate adenylate cyclase (which catalyzes the production of cAMP from ATP). Hormone-sensitive lipase (Sec. 13.4) can exist in two forms, one of which exhibits very low activity and a second which is phosphorylated and has high activity. Before hormonal stimulation of adenylate cyclase, the low-activity lipase predominates in the fat cell. Stimulation of protein kinase by an increase in cAMP concentration leads to phosphorylation of the low-activity lipase. An increase in the rate of hydrolysis of triacylglycerol and the release of fatty acids from the fat cell follows. This leads to a greater utilization of fatty acids by tissues such as heart, skeletal muscle, and liver. 

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