Lecture 4: CHO transport and metabolism 2

Question 1

Define the term gluconeogenesis

Answer 1

Gluconeogenesis is the synthesis of new glucose molecules from non-carbohydrate sources (pyruvate, lactate, glycerol, or the amino acids alanine or glutamine).

Question 2

Define the term glycogenolysis

Answer 2

Glycogenolysis is the breakdown of glycogen to glucose-1-phosphate

Question 3

Define the term glycogenesis

Answer 3

the formation of glycogen.

Question 4

What is the importance of glycogen stores in muscles

Answer 4

Storage for local energy production (only used by muscle itself)

Cannot be broken down

Question 5

Describe the pentose phosphate pathway

Answer 5

• Synthesised from glucose
• It is a metabolic pathway parallel to glycolysis.
• It generates NADPH (for reductive biosynthesis) and pentoses (5-carbon sugars) as well as ribose 5-phosphate, a precursor for the synthesis of nucleotides (RNA and DNA)

Question 6

NADPH is required to form which structures?

Answer 6

Cholesterol and fatty acids

Question 7

Draw the glycogenesis pathway

Answer 7

Formation of glycogen from glucose

Question 8

Fill in the blanks

Answer 8

Question 9

Fill in the blanks

Answer 9

Question 10

Draw the glycogenolysis pathway

Answer 10

The breakdown of glycogen

Question 11

Draw the pathway from glucose to glycogen and backwards

Answer 11

Question 12

Fill in the blank enzymes

Answer 12

Question 13

Gluconeogenesis is the synthesis of glucose from a noncarbohydrate source. Name possible non-carbohydrate sources

Answer 13

• Lactate
• Pyruvate
• Glycerol
• Certain amino acids

Question 14

How does the liver make glucose?

Answer 14

Either by breaking down glycogen stores (glycogenolysis) or formed from non-carbohydrate sources (gluconeogenesis)

Question 15

Describe the process of forming glucose from lactate and pyruvate

Answer 15

They are intermediates for glycolysis

The liver can reverse glycolysis, resulting in the formation of glucose from glycolysis intermediates

Question 16

Describe the process of forming glucose from glycerol

Answer 16

The trigylercides are broken down into glycerol and three fatty acids.

The glyerol is used to produce glucose

The fatty acids is used a fuel

Question 17

Describe the process of forming glucose from certain amino acids

Answer 17

This occurs only in extreme starvation

Ketogenesis- formation of ketones

Question 18

When glucose enters a cell, the enzyme ____ (or ____, in the liver) rapidly adds a phosphate to convert it into glucose-6-phosphate.

Answer 18

the enzyme hexokinase (or glucokinase, in the liver)

Question 19

Glycolysis results in the production of two ________ molecules from a single molecule of glucose. In the absence of ________, the end product of glycolysis is ________.

Answer 19

Pyruvate

Oxygen

Lactate

Question 20

A kinase is a type of enzyme that does what?

Answer 20

adds a phosphate molecule to a substrate

Question 21

This conversion step, from glucose to G6P, requires one ____ and essentially traps the glucose in the cell, what does this prevent?

Answer 21

A) ATP

preventing it from passing back through the plasma membrane, thus allowing glycolysis to proceed.

Question 22

The conversion of glucose to G6P is important 2-fold. Explain why?

Answer 22

1. Requires one ATP and essentially traps the glucose in the cell, preventing it from passing back through the plasma membrane, thus allowing glycolysis to proceed.
2. Functions to maintain a concentration gradient with higher glucose levels in the blood than in the tissues. By establishing this concentration gradient, the glucose in the blood will be able to flow from an area of high concentration (the blood) into an area of low concentration (the tissues) to be either used or stored.

Question 23

Where are hexokinase found

Answer 23

Hexokinase is found in nearly every tissue in the body.

Question 24

What are the amino acids that can be converted to glucose

Answer 24

Alanine or glutamine

Question 25

Why does the body create glucose from sources it has just spent a fair amount of effort to break down?

Answer 25

Certain key organs, including the brain, can use only glucose as an energy source; therefore, it is essential that the body maintain a minimum blood glucose concentration.

Question 26

Describe the steps of gluconeogenesis

Answer 26

* Pyruvate is a common starting material for gluconeogenesis. * First, the pyruvate is converted into oxaloacetate. * Oxaloacetate is then converted to phosphoenolpyruvate (PEP) by the enzyme phosphoenolpyruvate carboxykinase (PEPCK). * Phosphoenolpyruvate is converted back through multiple steps to form fructose-1-6-bisphosphate * Fructose-1-6-bisphosphate is converted into fructose 6-phosphate by the enzyme fructose-1,6-bisphosphatase, which is then converted to glucose-6-phosphate by glucose-6-phosphatase.

Question 27

Describe the effects of glucagon and adrenaline on gluconeogenesis?

Answer 27

Increase gluconeogenesis by: * Decreasing glucokinase activity- less glucose is converted to G6P * Increase Glucose-6-phosphatase activity- more G6P to glucose * Increase PEPCK activity- more oxaloacetate is converted to phosphoenolpyruvate

Question 28

At what level does glucagon and adrenaline stimulate gluconeogenesis? and what is the implication of this?

Answer 28

At the gene expression level Therefore, their effect does not happen immediately and takes hours to have an effect.

Question 29

What tissues in the body are dependent on glucose as their only sources of nutrients? and why?

Answer 29

* Erythrocytes * Does not contain mitochondria, therefore cannot oxidise fuels and therefore its only energy is from glycolysis * Brain matter * As other nutrients, such as fatty acids, cannot cross the blood-brain barrier * Uses many intermediate glucose metabolites

Question 30

Fill in the blank intermediates

Answer 30

Question 31

Fill in the blanks

Answer 31

Question 32

The process of glucose into glycogen is called

Answer 32

Glycogenesis

Question 33

The process of glycogen into glucose is called

Answer 33

Glycogenolysis

Question 34

The process of forming glucose from non-carbohydrate sources (lactate, pyruvate, glycerol and ceratin amino acids) is called

Answer 34

Gluconeogenesis

Question 35

Glucagon ___ blood glucose level

Answer 35

Increase Can cause hyperglycaemia

Question 36

Insulin ___ blood glucose level

Answer 36

decreases Can cause hypoglycaemia

Question 37

What are the 3 important cells in the Islet of Langerhans and what do they do?

Answer 37

**Alpha cells** - Secrete glucagon **Beta cells** - Secrete insulin **Delta cells** - Secrete somatostatin

Question 38

What does delta cells secrete in the Islets of Langerhans

Answer 38

Secretes somatostatin

Question 39

What perecentage of the total mass of the pancreas does the Islet of Langerhans make up?

Answer 39

\<5%

Question 40

Which cells in the Islet of Langerhans secrete glucagon

Answer 40

Alpha cells

Question 41

Which cells in the Islets of Langerhans produce insulin

Answer 41

Beta cells

Question 42

Describe the architecture of the Islets of Langerhans

Answer 42

Alpha, beta and delta cells are distributed throughout the Islets. The Islets are distinct from the exocrine tissue.

Question 43

What are the preceding forms of insulin

Answer 43

Preproinsulin -\> Proinsulin -\> Insulin

Question 44

Describe the process in which preproinsulin becomes proinsulin

Answer 44

The signal sequence gets cut off while the disulfide bonds get formed (S-S bonds)

Question 45

Which GLUT receptor has a low affinity for glucose

Answer 45

GLUT 2

Question 46

Describe how the beta cells secrete insulin

Answer 46

* Insulin is packaged into vesicles which are stored in the beta cells * The beta cells contain GLUT 2 receptors on its surface, which has a low affinity for glucose. Therefore, it needs a high glucose concentration to saturate it (therefore the more glucose the higher its activity) * This, through faciliatative diffusion, moves glucose into the beta cells. * The beta cells contain glucokinase, which phosphorylates glucose into G6P. Glucokinase, similar to GLUT 2, has a low affinity for glucose. Through glycolysis the cell produces ATP, which turns off ATP sensitive potassium channels which in turn causes the depolarisation of the beta cell. This switches on voltage gated calcium channels, which causes the exocytosis of the insulin. * Both GLUT2 and glucokinase having a low affinity for glucose means that the beta cells are able to work at high levels of blood glucose.

Question 47

Describe how the alpha cells secrete glucagon

Answer 47

* Less well characterised * It is secreted in response to low blood glucose levels. * The mechanism of how this happens it not yet known * Maybe there is crosstalking between glucagon and insulin secretions (i.e. crosstalking between alpha and beta cells)

Question 48

Which receptor does insulin bind to

Answer 48

Insulin receptor

Question 49

Why cant insulin diffuse freely through the membrane

Answer 49

Insulin is a peptide hormone therefore cannot diffuse through Must bind to insulin receptors on the cells

Question 50

What tissues contain insulin receptors

Answer 50

Found primarily in the liver, striated muscle and adipocytes

Question 51

Which muscle types are striated?

Answer 51

Cardiac and skeletal muscle

Question 52

Glucagon receptors are what kind of receptors

Answer 52

G protein coupled receptor

Question 53

Describe the mechanism of the glucagon cascade that occurs when glucagon binds to the glucagon receptor

Answer 53

* Glucagon binds to the glucagon receptor, a G protein-coupled receptor, located in the plasma membrane. * The binding causes a conformation change in the receptor, which activates G proteins, a heterotrimeric protein with α, β, and γ subunits. Results in the releasing of the α subunit from the β and γ subunits. * The alpha subunit specifically activates the adenylyl cyclase. * The AC forms cAMP from ATP. * The cAMP activates protein kinase A * This causes downstream effects