8.01 Biochemistry of Glucose

Question 1

How does glucose enter the cells?

Answer 1

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

Question 2

What are the major glucose transporters of the body and their distributions?

Answer 2

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

Question 3

What is the normal physiological range of glucose in the blood?

Answer 3

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

Question 4

What is the Km of an enzyme?

Answer 4

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)

Question 5

The GLUT3 transporters have low Km values. What is the significance of this?

Answer 5

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)

Question 6

Glut4 transporters have a medium value for Km, what is the implication of this?

Answer 6

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

Question 7

Glut2 have very low levels of activity (high Km), why is this so?

Answer 7

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

Question 8

Describe the general features of the glucose transporters

Answer 8

• 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.

Question 9

In what direction is glucose transport throught the GLUT transporters?

Answer 9

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. )

Question 10

What are the two major hormones that regulate plasma glucose levels?

Answer 10

Glucagon and insulin balances (levels of the two hormones are reciprocally regulated)

Together they maintain the blood glucose concentration.

Question 11

Describe the Glucose tolerance curves for a normal vs. a diabetic individual

Answer 11

Question 12

Describe the rationale behind the glucose tolerance test (including results)

Answer 12

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)

Question 13

What is a major cause of glucose intolerance?

Answer 13

Glucose intolerance is in part attributed to impaired glucose uptake by GLUT4

Question 14

Complete the following table

Answer 14

Question 15

What controls GLUT4 expression on the plasma membrane (and thus glucose uptake into cells)?

Answer 15

Insulin receptor stimulation controls GLUT4 translocation to plasma membrane

Question 16

Describe the mechanism by which insulin acts to increase expression of GLUT4 receptors in the plasma membranes

Answer 16

• 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.

Question 17

Describe the diurnal variation of plasma glucose in normal vs. diabetic individuals

Answer 17

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

Question 18

What is glycolysis? What is the aim?

Answer 18

Glycolysis

A series of reactions that breaks down glucose and forms pyruvate with the production of two molecules of ATP (energy).

Question 19

Describe glycogen metabolism

Answer 19

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.

Question 20

What are the major organs where glycogen metabolism occurs?

Answer 20

• 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)

Question 21

What is Glycogen?

Describe its chemical nature (very basically)

Answer 21

A polymer of glucose

It is a linkage of glucose subunits together using two major types of bonds.

Question 22

Give an explanation of why the liver is so large in terms of glycogen metabolism (glucose storage)

Answer 22

The reason the liver is so big is because it needs to accomodate glycogen (a hydrophilic molecule so requires branching to pack together).

Question 23

Describe the process of glycogenolysis (glycogen breakdown)

Answer 23

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)

Question 24

Is the enzyme phosphorylase (catalyzing glycogenesis) only catalyzing this step in one direction or both?

Answer 24

Phosphorylase catalyses an irreversible reaction thus only in the one direction. It cant catalyse addition of glucose to forming glycogen (glycogen synthesis requires a different pathway and different enzymes)

Question 25

What are the two major steps (and the enzymes) involved in glycogen synthesis?

Answer 25

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

Question 26

What is the rate limiting step in glycogen synthesis?

Answer 26

Step 2, catalyzed by glycogen synthase

Question 27

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?

Answer 27

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.

Question 28

Describe the interaction between glycogen phosphorylase (glycogen breakdown) and glycogen synthase (glycogen synthesis)

Answer 28

They are reciprocally regulated to prevent futile cycles. * Regulation of glycogen phosphorylase occurs mainly through glucagon hormone in the liver or through adrenaline/Ca in the muscles * Glycogen synthase is mainly regulated by insulin * These two hormones are in reciprocal balance with one another.

Question 29

What is the difference between the fate of released glucose-1-phosphate from glycogen breakdown in the muscle compared to the liver?

Answer 29

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.

Question 30

What is glucagon? Where is it synthesised and what is its action?

Answer 30

* Peptide hormone (29 AAs, Mr = 3,483) * Synthesized in the alpha cells of the pancreas * Secreted into the blood when [glucose] blood drops \< 4.5mM 
in order to cause an increase in plasma glucose concentration (by increasing glycogenolysis and gluconeogenesis)

Question 31

What is the effect of glucagon on the liver?

Answer 31

Glucagon activates **glycogen phosphorylase** in liver (the breakdown of glycogen to release glucose)

Question 32

Describe the glucagon receptor

Answer 32

Glucagon receptor is a 7 transmembrane protein (GPCR), when glucagon binds to it it activates G proteins secondary messenger systems to act

Question 33

What is the consequence of glucagon binding to its GPCR?

Answer 33

* 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

Question 34

What is gluconeogenesis?

Answer 34

A metabolic pathway that results in the **generation of glucose** from **non-carbohydrate carbon substrates**

Question 35

Why is there a need for gluconeogenesis if there is a storage system (glycogen) that we can draw from?

Answer 35

Glycogen stores (hepatocytes mainly) can sustain blood glucose concentration for 24 hours; after 24 hours, if you don’t eat, most liver glycogen is depleted. Other pathways needed to maintain blood glucose concentrations to allow for adequate brain function.

Question 36

What are the substrates for gluconeogenesis?

Answer 36

Non-carbohydrate carbon substrates: * Alanine (obtained from muscles) * Lactate (from other tissues and transported to the liver) * Glycerol

Question 37

During rigorous muscular activity, TCA (krebs) cycle and electron transport chain cannot supply enough ATP because of the limitation of O2 supply. What is the main means of the body to overcome this?

Answer 37

**Anaerobic glycolysis** in the cytosol (away from the mitochondria) Degrading glycogen via glycogen phosphorylase to make G1P and then G6P and eventually ATP.

Question 38

What are the main limitations to the glycogen breakdown pathway?

Answer 38

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.

Question 39

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?

Answer 39

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**.

Question 40

What is the Cori cycle?

Answer 40

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

Question 41

What occurs to the muscles in periods of prolonged fasting or starvation and why?

Answer 41

During periods of prolonged fasting or starvation, muscles break down, muscle proteins used as precursors/AA which are converted into alanine and used for gluconeogenesis.

Question 42

What is the Alanine Cycle?

Answer 42

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]

Question 43

Is regulation of glycolysis and gluconeogenesis the same in the liver vs. the muscles?

Answer 43

NO. **The function of hepatic**: * Glycolysis is to generate acetyl-CoA for fatty acid synthesis * Gluconeogenesis is to generate glucose to maintain glucose concentrations **The function of muscle:** * Glycolysis is to generate glucose and thus ATP for muscle contraction (there is no generation of free fatty acids) * There is no gluconeogenesis in the muscles

Question 44

Describe the role of the pancreas in all of this

Answer 44

The pancreas can sense the blood [glucose] and adjust its secretion of glucagon and insulin accordingly.

Question 45

Does the muscle cell express glucagon receptors? Explain

Answer 45

There is no gluconeogenesis or depletion of glycogen stores in the muscles and hence there are no glucagon receptors on muscles

Question 46

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)

Answer 46

Question 47

Describe the effect of insulin on liver vs. muscle cell GLUT transporters

Answer 47

Insulin has no effect of transport (GLUT 2) in liver whereas insulin stimulates high amounts of GLUT4 expression in muscles as it signals an optimal time to store glucose as glycogen

Question 48

What is the net effect of glucagon stimulation on the liver?

Answer 48

Increase in glucose concentration in hepatocytes to be released into the plasma by GLUT2 to maintain blood glucose

Question 49

What are the major mechanisms of glucagon mediated production of glucose?

Answer 49

* **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**

Question 50

How is insulin involved in plasma glucose concentration regulation?

Answer 50

Insulin is secreted in times where glucose concentrations are high and thus needs to be stored as glycogen and fatty acids and inhibit the breakdown of glycogen * Inhibits glycogen breakdown * Stimulates glycogen synthesis