W4L1 Flashcards
Family of Uniporters
12 transmembrane spanning alpha helices
Affinity to glucose based on amino acid composition of these domains
Slight functional differences allow for differential regulation of glucose metabolism
21 types of glucose uniporters
Both the N and C terminals of the helix are in the cytosol, so they are cytosolic
GLUT family comparison
GLUT 1
- ubiquitous
- RBC: RBC have no mitochondria or nuclei, but they still have enzymes that need ATP to function
- High rate of glucose uptake
- Independent of blood [glucose]
- Uptake remains constant as [glucose] increases
- The relative level of GLUT1 found in muscle and fat tissue is relatively low, it is still there but not high amounts. This is bc brain gets first pass on all glucose in the body. Brain weights 3 lbs, and muscle +fat weighs much more
GLUT 2
- liver
- pancreatic beta cells
- GLUT2 is very reactive. If there is no glucose in body, it does not respond. But if there is lots of glucose in body, it becomes more active and takes it up into the cell.
- Glucose uptake increases dramatically as blood [ ] increases
- Lower rate of glucose uptake at low [ ] (compared to Glut1)
GLUT 3
- neurons
- High rate of glucose uptake
- similar to how Glut1 functions
GLUT 4
- Fat and Muscle cells
- Insulin dependent relocation to plasma membrane for function
Pancreatic Beta Cells
High glucose conc in extracellular fluid after a large meal. GLUT2 at cell membrane so glucose enters cell. Remember, GLUT transporters allow glucose to go either in or out of the cell, depending on where the concentration gradient is
Once inside the cell, glucose is phosphorylated to glucose-6-phosphate.
At this point, glucose-6-phosphate cannot be transported by any GLUT transporter. It is locked inside the cell and the cell can use it.
Through glycolysis, it will become pyruvate, go through the mitochondria for oxidative phosphorylation, and produce ATP.
ATP will bind ATP-selective K+ channel, closing the channel. K+ will not get across the membrane from inside to outside now. This destabilizes the membrane potential, depolarizing the membrane.
The depolarization of the membrane will activate voltage-gated Ca2+ channels, allowing Ca2+ from outside of the cell to enter. Cytosolic calcium conc is low; calcium is in ER
Calcium triggers insulin release from inside of cell to outside of cell by exocytosis
Insulin can move through blood stream and go through muscle cells, adipose tissue, activate the insulin receptor, and then the insulin receptor will tell the glucose transporters inside the cells to go to the cell surface
Insulin stimulates GLUT4 translocation to the plasma membrane
In our muscle and fat cells, GLUT4 is found inside cells in vesicles and needs to move to cell surface in order to bring glucose inside the cell
Take GLUT4 transporter and genetically modify it to add extended Green Fluorescent Protein (eGFP). Located on cytoplasmic portion of protein
Myc tag is a small peptide tag, which can be found by myc antibody. Located on extracellular portion of protein.
In experiment, they took live adipocytes. In the absence of insulin, GLUT4 is mostly located inside the cell and myc antibody did not recognize tag because the GLUT4 is not near the cell surface. However, in insulin version of adipocyte, GLUT4 is both inside and at periphery of cell, and myc antibody is recognized at cell surface. Thus, most GLUT4 are inside cell and insulin causes GLUT4 to go to cell surface
Similarities and differences of glucose transporters
Similarities
- Transport glucose
- Can move glucose down the concentration gradient
- Reversible– if [glucose] gets higher in cell, it can transport glucose out
– This is rare bc glucose gets phosphorylated when it gets into the cell and it is not recognized by glucose uniporter
Differences
- Pattern of expression
- Transport
– for e.g., Glut2 increases transport rate as glucose [ ] increases
– Glut1 and Glut3 retain the same transport rate for glucose, regardless of the glucose [ ]
- Glut1-3 are at cell surface but Glut4 needs to be transported to cell surface to work
Current Research
Major challenge currently in research is to determine how post-translational modifications affect Glut uniporter activities
To determine how signals (such as hormones) trigger the activation of the uniporter
- for e.g., Glut4 relocates to a plasma membrane when insulin is present in muscle, thereby increasing glucose uptake
Simple Rehydration Therapy
- intestinal epithelium
Osmotic gradient is created by absorption of glucose and sodium
- When you drink water, you need sodium and glucose to create osmotic pressure for water to follow into cell
During exercise-mediated dehydration, simply drinking water does not help because it would be excreted from the GI tract almost immediately, mostly leaves as sweat
Therefore, drinking glucose and electrolytes is recommended
Glucose Transport in Intestinal Epithelium
Glucose / Na+ transporter is symport or co-transporter
Symporters and Antiporters
Movement of ions and/or macromolecules across a membrane with the aid of another concentration gradient
Form of Secondary Active Transport
Are symporters only involved in glucose uptake?
- no
Cotransport (symport) – Na+ goes in and something else goes in afterwards
Exchange (antiport) – Na+ enters and something else leaves cell
Amino Acid Transporters - Amino Acid/ Na+ Symporters
Amino Acid/ Na+ Symporters
- Large family of transporters (> 10 families identified)
- At least 10 members within each family
– Examples:
— Lysine exporter family (basic amino acids)
— Alanine/Glycine symporter (Ala or Gly)
— Branched chain amino acid symporter (2-3 amino acid chains)
- Found in Intestine (absorption) and Kidney (reabsorption)
Antiporters - Ca+2/Na+ Antiporter
Low cytosolic concentrations of calcium is required for maintaining the ability to allow cardiac muscles to contract in response to internal calcium release (more sensitive to change)
- low calcium in cytosol = relax; high calcium in cytosol = muscle contract
A perceived rise in calcium will trigger a contraction
Antiport of 3:1 Na+/Ca+2 is required
- This pumps out the calcium after it has entered the cell to cause muscle contraction
- Na+ enters cell, so Ca2+ can leave cell
When this antiport is activated, the strength of the contraction is reduced because calcium leaves the cell
Ca+2/Na+ Antiporter Steps
- AP enters from adjacent cell
- Voltage-gated Ca2+ channels on cell surface open and Ca2+ enters cell
- Calcium induces calcium release through ryanodine receptor-channels (RyR)
- Increase calcium in sarcoplasmic reticulum causes calcium to enter cytosol. Ca2+ spark.
- Summed Ca2+ spark creates a Ca2+ signal.
- need multiple Ca2+ sparks - Ca2+ ions bind to troponin to start muscle contraction
- Relaxation occurs when Ca2+ unbinds from troponin
- Ca2+ is pumped back into the sarcoplasmic reticulum for storage
- SERCA pump (sarcoplasmic reticulum, endoplasmic reticulum, calcium, ATPase)
- this is one of two methods of causing relaxation in muscle - Ca2+ is exchanged with Na+ by the NCX antiporter
- this is one of two methods of causing relaxation in muscle - Na+ gradient is maintained by the Na+/K+ ATPase
Membrane Transport of Iron
Iron is an essential component in the cells of our body.
Essential to the function of hemoglobin, myoglobin, cytochromes, peroxidase, catalase, etc.
Most of the iron that we normally contain is in
- hemoglobin (~65%)
- liver parenchymal cells (15-30%), mostly as ferritin.
When levels of iron drop, liver releases ferritin stores and iron is transported in the plasma (bound to a carrier protein)
Iron Transport in the Body
- Iron (Fe+3) ingested from the diet, goes into intestine
- ferric iron is found in diet, is insoluble - Fe+2 absorbed by active transport into plasma
- iron can only absorbed as ferrous form - Fe3+ transferrin protein transports ferric iron when it is inside the plasma
- It (Fe3+) is transported to bone marrow. Bone marrow uses Fe to make hemoglobin (Hb) as part or RBC synthesis
- Fe to Heme to Hb to RBC synthesis - RBCs live 120 days in the blood.
- Hb goes to spleen. Macrophages in spleen break up the RBC. Spleen destroys old RBCs and converts Hb to bilirubin
As Hb is converted to bilirubin, iron is removed and recycled back with transferrin for another round.
- Bilirubin and metabolites are excreted in urine (goes to kidney) and feces (goes to liver)
- Liver: metabolizes bilirubin and excretes it in bile, then excretes the bile as bilirubin metabolites in feces
- Kidney: metabolizes bilirubin and excretes it in urine - Liver also stores excess Fe3+ as ferritin
Iron Absorption
Iron absorption is slow, despite a diet high in iron
Iron depletion in the body is ongoing
- Iron stores must be replaced
Iron can build up in the body and cause problems to a variety of issues when the liver stores become saturated.
Therefore total body levels of iron are carefully controlled at the level of iron absorption.
