8.01 Biochemistry of Glucose Flashcards
How does glucose enter the cells?
Glucose is a hydrophillic molecule, it cannot freely enter the cells.
There are various specific glucose transporters and glucose co-transporters that are specifically distributed on different cell types through the body
What are the major glucose transporters of the body and their distributions?
GLUT 1 - erythrocytes, fetal tissue, brain
GLUT 2 - liver, kidney, intestines, pancreatic B cells
GLUT 3 - brain
GLUT 4 - muscle and adipose tissue
GLUT 5 - jejunum
SGLT1 - duodenum, jejunum and renal tubules
What is the normal physiological range of glucose in the blood?
Mean normal is about 5mmol/L
3.9 and 5.6 mmol/L in the fasting state
and shouldn’t exceed 9-10mM post-prandially
What is the Km of an enzyme?
Km = measure of affinity of an enzyme to the substrate (the activity of an enzyme).
The Lower Km, the higher affinity it has (the higher the activity of the enzyme)
The GLUT3 transporters have low Km values. What is the significance of this?
No matter the concentration of glucose in the plasma, the glucose channels of the brain is always working at maximal velocities Vmax
(because brain cells require glucose at all times to sustain electrical activity)
Glut4 transporters have a medium value for Km, what is the implication of this?
In muscles, glucose uptake is only occuring in times of plenty (thus it fluctuates with the plasma concentration) to increase uptake when there is a lot of glucose and stores it as glycogen
Glut2 have very low levels of activity (high Km), why is this so?
Liver, Glut2 activity is always low due to anatomical arrangement of the blood vessels from intestines to liver cells
The portal vein takes blood directly from the gut to hepatocytes, and thus the plasma has high levels of glucose (15-20mM). Therefore, the transporters only require low response to the enzyme enabling it to work at its optimal level
Describe the general features of the glucose transporters
- Uniporters specific for hexoses
- Energy-independent facilitative transport of glucose
- Glucose diffusion depends on the glucose gradient: high extracellular glucose concentration favours entry of glucose into cells via GLUT.
In what direction is glucose transport throught the GLUT transporters?
Transports glucose in BOTH directions depending on the needs of the plasma concentrations (need uptake or excrete glucose to maintain plasma concentrations)
(depends on inward-open outward closed or vice versa conformations - A portion of the molecule is always open to enable ligand binding or ligand release. )
What are the two major hormones that regulate plasma glucose levels?
Glucagon and insulin balances (levels of the two hormones are reciprocally regulated)
Together they maintain the blood glucose concentration.
Describe the Glucose tolerance curves for a normal vs. a diabetic individual
Describe the rationale behind the glucose tolerance test (including results)
Measuring blood glucose levels before and after intake of glucose to determine the efficacy of insulin in causing glucose to be removed from the plasma
- Normal people, there is only a slight increase in plasma glucose after intake.
- In diabetic subjects, glucose concentrations continue to rise and have a delayed onset on return to normal levels - this is because there is problems with their ability to clear it from the plasma (insulin activity)
What is a major cause of glucose intolerance?
Glucose intolerance is in part attributed to impaired glucose uptake by GLUT4
Complete the following table
What controls GLUT4 expression on the plasma membrane (and thus glucose uptake into cells)?
Insulin receptor stimulation controls GLUT4 translocation to plasma membrane
Describe the mechanism by which insulin acts to increase expression of GLUT4 receptors in the plasma membranes
- Insulin binds to a receptor on the cell membranes
- This activates receptor tyrosine kinase activity causes signaling events
- The key players including Akt (serine and threonine) phosphorylase proteins
- These lead to recruitment and translocation of GLUT 4 to the plasma membrane.
Describe the diurnal variation of plasma glucose in normal vs. diabetic individuals
There are small increases in plasma glucose levels post-prandially (after a meal) in a normal person which rapidly (within 2hrs or less) decrease back to baseline.
- Glucose levels are in constant fluctutation and at high levels even between meals
- Glucose concentrations kept at a constant level
What is glycolysis? What is the aim?
Glycolysis
A series of reactions that breaks down glucose and forms pyruvate with the production of two molecules of ATP (energy).
Describe glycogen metabolism
The process where glycogen is a readily mobilized into the storage form of glucose.
It is a very large, branched polymer of glucose residues that can be broken down to yield glucose molecules (glycogenolysis) when energy is needed.
What are the major organs where glycogen metabolism occurs?
- Liver
- Kidney cortex
- Muscle cells
- (Note: also occurs in the brain astrocytes for the maintenance and survival of neurons and in cancer cells for their own survival)
What is Glycogen?
Describe its chemical nature (very basically)
A polymer of glucose
It is a linkage of glucose subunits together using two major types of bonds.
Describe the process of glycogenolysis (glycogen breakdown)
This is when glycogen stores are broken down to form free glucose that is able to be used for energy by the cells.
It is a single step reaction that is catalyzed by the enzyme phosphorylase that brings in an inorganic phosphate (this is the rate limiting step)
What are the two major steps (and the enzymes) involved in glycogen synthesis?
Activation of glucose by the addition of UDP from UTP to form UDP-glucose (required for incorporation into the chain). This is catalyzed by UDP-glucose pyrophosphorylase
Transfer of the activated glucose moiety to the glycogen chain catalyzed by the enzyme Glycogen Synthase
What is the rate limiting step in glycogen synthesis?
Step 2, catalyzed by glycogen synthase
The step catalyzed by UDP-glucose pyrophosphorylase is a reversible step. How is there a net production of UDP-glucose if this is the case?
Once the UTP (triphosphate) reacts with the glucose-1-phosphate, it transfers the alpha phosphate to glucose and leaves behind the beta and gamma phosphates of the UTP and together these form pyrophosphate (PPi) that readily undergoes hydrolysis pulling the reaction in the forwards direction.
What is the difference between the fate of released glucose-1-phosphate from glycogen breakdown in the muscle compared to the liver?
Once you gave G-1-phosphate from glycogen breakdown, it goes into glycolysis.
In muscles it generates energy for muscle contraction OR it goes down the PPP.
In the liver, glycolysis generates Acetyl-CoA; a precursor for fatty acid synthesis.
What is the consequence of glucagon binding to its GPCR?
- Activation of the secondary messenger system causes dissociation of the G protein
- This leads to activation of adenylate cyclase enzyme by the alpha subunit catalyzing the formation of cAMP from ATP.
- Once formed the cAMP binds to Protein kinase A and activates it
- PKA phosphorylates phosphoylase kinase (Activates it and locking it in active conformation)
- Phosphorylase Kinase is able to activate Glycogen phosphorylase for glycogen breakdown
What are the main limitations to the glycogen breakdown pathway?
The amount of ATP generated by glycolysis is much smaller than that can be generated by oxidative phosphorylation (electron transport chain) and glycogen thus glycogen needs to be mobilized quickly in large amounts.
NAD+ is required for the conversaion of glyceraldehyde-3-phosphate into 1,3-BPG for energy production (ATP). The supply of NAD+ in cytoplasm is limited.
How is the limited supply of NAD+ in the cytosol replenished?
What is a major disadvantage of this process and the means by which the body overcomes it?
NAD+ is replenished through the step catalysed by lactate dehydrogenase in which pyruvate is converted to lactate; NADH is used to oxidise pyruvate into lactate [regenerating NAD+ in the process; allows glycolysis to proceed]
However process results in accumulation of lactate – need to relieve muscle cells of this acid production by transporting lactate into the liver where it undergoes gluconeogenesis.
What is the Cori cycle?
The Cori cycle (Lactic acid cycle) is the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized
What is the Alanine Cycle?
Alanine cycle involves degeneration of alanine (by breakdown of muscles or pyruvate from the glycolysis cycle), which travels to the liver and functions as a precursor for gluconeogenesis.
Note: the alanine cycle also takes up Nitrogen – used to produce urea [involved in detox for liver urea cycle]
Describe the changes in the levels of:
- Glucose
- Insulin
- Glucagon
In the plasma before, during and after a meal
(Draw a graph to aid explaination)
What is the net effect of glucagon stimulation on the liver?
Increase in glucose concentration in hepatocytes to be released into the plasma by GLUT2 to maintain blood glucose
What are the major mechanisms of glucagon mediated production of glucose?
- Inhibits glycogen synthase. Glycogen converted to glucose-1-P, which is converted to G6P – glucose-6-phosphatase converts G6P into glucose which leaves hepatocytes via GLUT2.
-
Gluconeogenesis is promoted
- Stimulation of PEPCK (phosphoenolpyruvate carboxykinase); rate limiting enzyme of gluconeogenesis and also have activation of FBPase [fructose-1,6-bisphosphatase].
- Inhibition of glycolysis. For glycolysis, PFK1 is the rate limiting enzyme – inhibited by glucagon [also inhibits pyruvate kinase]
- Increased glycogenolysis
