Outline for Lecture #6
Glucose enters cells by facilitated diffusion = carrier mediated transport in some cell types (adipose & muscle) in presence of insulin. 6. . Current terminology: relationship of early endosomes, late endosomes & lysosomes. Passive mechanisms like diffusion use no energy, while active transport requires GLUT4 is a glucose transporter found in fat and skeletal muscle. Insulin triggers GLUT4 to insert into the membranes of these cells so that glucose can be In fact, this relationship is taken advantage of in certain heart disease medications. Its effect on facilitated diffusion of glucose into fat and muscle cells via modulation characterization of insulin gene and the relationship to diabetes research.
Membrane Transport Mechanisms
See Becker for details if interested. Types of coats are summarized in table More details on structure and function of Golgi later. For LDL receptor, it takes about minutes for one "round trip. Material can remain inside a vesicle or outside cell at all times.
Possible fates of endocytosed material -- Where does vesicle go? Degradation -- vesicle fuses with lysosomes and contents are degraded. Sorting -- not everything in the vesicle may go to the same place. Vesicle may fuse with endosome sorting vesicleand different parts of the endocytosed material may be directed to different destinations. More details on a-c below. Transcytosis requires a receptor for each substance transported. Receptor is not shown on 6C but is clearly shown on diagram of transcytosis 2.
Material binds to receptor and is endocytosed on one surface of the cell. Vesicle moves across cell and material is exocytosed on a different surface. Can be used to move proteins, across a cell, in either direction. See examples above or RP3. Stages of Cycle Numbers match steps on handout 6B. Click here for animation. Receptors bind material 2. Membrane starts to invaginate to form coated vesicle. A single vesicle can contain more than one type of receptor plus ligand. Coated vesicle forms pinching off of vesicle is an energy requiring step 5.
Uncoating occurs relatively quickly uncoating requires energy 6. Vesicle is acidified to become endosome or fuses with pre-existing endosomeand sorting of receptor s and ligand s begins. A single endosome may contain many different receptors and ligands, and different ones are sorted differently. Some examples are given in detail below. The uncoated, acidified vesicle can be called an endosome, early endosome, or a sorting vesicle. Pump is in membrane of vesicle.
Details of sorting and recycling -- the remaining steps -- vary with material endocytosed. More details below for individual cases. In example shown on handout, one half gets the receptor and one half gets the ligand, as is the case for LDL. Other examples will be discussed in class and are outlined in detail below. Pieces of different composition may gradually bud off as internal composition of remainder changes. What Happens to the Different Parts of the Endosome?
Fate of vesicle with material that remains inside the cell -- Vesicle delivers contents to appropriate cell compartment. Some Specific Examples 1. LDL Low density lipoprotein -- receptor recycled, but ligand including protein part degraded.
Many of LDL details may have been included in general case, but are summarized below. Click here for a picture of LDL. Cholesterol is insoluble in blood. Need a way to ferry cholesterol through blood and into cell -- Cholesterol transport requires formation of particle with hydrophilic surface 2 Why a protein apoB? For binding to cell surface receptor LDL receptor.
A protein is needed as ligand to bind to receptor. Receptor, but not protein part of LDL, is recycled. All of LDL including protein stays together; separates from receptor e. Need lysosomes to degrade LDL protein and release cholesterol cholesterol esters in LDL must be split for cholesterol to be used. How does LDL reach lysosomes?
Through fusion of vesicles. Vesicles with substrate fuse with vesicles from Golgi carrying newly made hydrolases to form new lysosomes. More details on how hydrolases pass through the Golgi and are targeted to lysosomes to be discussed later.
Function of LDL uptake -- to supply a nutrient cholesterol. Most of this is FYI. In this course, the term "endosomes" will be used for both early and late endosomes. Term is used differently by different authors. Therefore, early endosomes can mean: Vesicles coming from Golgi carrying newly made proteins more on this later.
More acidic than early endosome. Material not destined for lysosomes has been jettisoned. Formed by maturation of early endosome. More acidic than late endosome. Older terminology found in some texts FYI only: No separate protein ligand required; EGF is a protein -- unlike cholesterol, or Fe see case below.
Function of uptake -- to regulate signaling. EGF is a signaling molecule. Uptake turns off signal and down regulates receptors reduces of cell surface receptors. Receptor not recycled -- Ligand signal molecule and receptor degraded together. Through these activities, insulin has profound effects on both carbohydrate and lipid metabolism, and significant influences on protein and mineral metabolism. Consequently, derangements in insulin signalling have widespread and devastating effects on many organs and tissues.
The Insulin Receptor and Mechanism of Action Like the receptors for other protein hormones, the receptor for insulin is embedded in the plasma membrane. The insulin receptor is composed of two alpha subunits and two beta subunits linked by disulfide bonds.
The alpha chains are entirely extracellular and house insulin binding domains, while the linked beta chains penetrate through the plasma membrane. The insulin receptor is a tyrosine kinase. In other words, it functions as an enzyme that transfers phosphate groups from ATP to tyrosine residues on intracellular target proteins. Binding of insulin to the alpha subunits causes the beta subunits to phosphorylate themselves autophosphorylationthus activating the catalytic activity of the receptor.
The activated receptor then phosphorylates a number of intracellular proteins, which in turn alters their activity, thereby generating a biological response. Several intracellular proteins have been identified as phosphorylation substrates for the insulin receptor, the best-studied of which is insulin receptor substrate 1 or IRS When IRS-1 is activated by phosphorylation, a lot of things happen.
Among other things, IRS-1 serves as a type of docking center for recruitment and activation of other enzymes that ultimately mediate insulin's effects. A more detailed look at these processes is presented in the section on Insulin Signal Transduction. Insulin and Carbohydrate Metabolism Glucose is liberated from dietary carbohydrate such as starch or sucrose by hydrolysis within the small intestineand is then absorbed into the blood.
Elevated concentrations of glucose in blood stimulate release of insulin, and insulin acts on cells thoughout the body to stimulate uptake, utilization and storage of glucose. The effects of insulin on glucose metabolism vary depending on the target tissue. Two important effects are: Insulin facilitates entry of glucose into muscle, adipose and several other tissues. The only mechanism by which cells can take up glucose is by facilitated diffusion through a family of hexose transporters.
In many tissues - muscle being a prime example - the major transporter used for uptake of glucose called GLUT4 is made available in the plasma membrane through the action of insulin.
When insulin concentrations are low, GLUT4 glucose transporters are present in cytoplasmic vesicles, where they are useless for transporting glucose.
Facilitated diffusion - Wikipedia
Binding of insulin to receptors on such cells leads rapidly to fusion of those vesicles with the plasma membrane and insertion of the glucose transporters, thereby giving the cell an ability to efficiently take up glucose. When blood levels of insulin decrease and insulin receptors are no longer occupied, the glucose transporters are recycled back into the cytoplasm. It should be noted here that there are some tissues that do not require insulin for efficient uptake of glucose: This is because these cells don't use GLUT4 for importing glucose, but rather, another transporter that is not insulin-dependent.
Insulin stimulates the liver to store glucose in the form of glycogen. A large fraction of glucose absorbed from the small intestine is immediately taken up by hepatocytes, which convert it into the storage polymer glycogen. Insulin has several effects in liver which stimulate glycogen synthesis.
First, it activates the enzyme hexokinase, which phosphorylates glucose, trapping it within the cell. Coincidently, insulin acts to inhibit the activity of glucosephosphatase. Insulin also activates several of the enzymes that are directly involved in glycogen synthesis, including phosphofructokinase and glycogen synthase.
The net effect is clear: A well-known effect of insulin is to decrease the concentration of glucose in blood, which should make sense considering the mechanisms described above.
Another important consideration is that, as blood glucose concentrations fall, insulin secretion ceases. In the absense of insulin, a bulk of the cells in the body become unable to take up glucose, and begin a switch to using alternative fuels like fatty acids for energy.
Neurons, however, require a constant supply of glucose, which in the short term, is provided from glycogen reserves. When insulin levels in blood fall, glycogen synthesis in the liver diminishes and enzymes responsible for breakdown of glycogen become active.