07-Receptors

07-Receptors - BSCI330 Cell biology and physiology Fall...

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BSCI330 – Cell biology and physiology Fall 2009 LAB MANUAL – Lab Exercise #7 Carpenter et al. 2009. BSCI330 Laboratory Manual. University of Maryland, College Park. 1 EXERCISE 7: MEMBRANE RECEPTORS The plasma membrane of a eukaryotic cell typically consists of 50% protein, 40% lipid, and 2– 10% carbohydrate (by mass). The lipids form a continuous double layer (phospholipid bilayer) 6–10 nm in width that acts as an effective hydrophobic barrier between the intracellular and extracellular aqueous mediums and delineates the border of the cell. To effectively maintain homeostasis, the membrane must be selectively permeable with the ability to sense and interact in an adaptive manner with the external environment. Most of these functional properties of the cell membrane are carried out by the membrane proteins. Transmembrane proteins extend across the bilayer and are exposed to both the intracellular and extracellular environment. These are the most common type of membrane protein. Other proteins are more loosely attached to either the inner or outer membrane surface and are referred to as peripheral proteins. Many membrane proteins (as well as lipids) have attached oligosaccharide chains on the portion of the molecule that faces the extracellular medium. The collective sugar residues of the glycoproteins and glycolipids form a complex network of cell surface carbohydrate known as the glycocalyx, which has important functions in cell attachment and recognition processes. Cell-surface receptors are an important class of membrane proteins that allow the cell to sense the external environment and are involved in chemical signaling between cells. For example, the glycoprotein hormone insulin is produced by the beta endocrine cells of the pancreas in response to elevated plasma glucose concentrations. The loss of these cells and the consequent lack of insulin is the principal cause of diabetes mellitus. Insulin stimulates a variety of processes in many cells including an increase in glucose transport and protein synthesis. The cells that respond to insulin contain insulin receptors on their plasma membranes, and the insulin receptor is a transmembrane glycoprotein. The binding of the ligand (insulin) to the receptor is a highly specific interaction, analogous to the binding of an enzyme to its substrate. As a result of the ligand binding, the receptor is activated and undergoes a conformational change which is transmitted through the membrane to the effector domain of the receptor protein located on the cytoplasmic face of the membrane. The effector domain of the insulin receptor is an enzyme (a tyrosine kinase) that is catalytically active only in the presence of insulin. The activated receptor initiates an intracellular cascade of events within the cell, which ultimately leads to an increase in glucose membrane transport and protein synthesis.
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07-Receptors - BSCI330 Cell biology and physiology Fall...

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