Module 8 - Metabolism Regulation

Question 1

Explain the mechanistic basis of why allosteric enzymes exhibit a sigmoidal substrate dependence.

Answer 1

The sigmoidal substrate dependence arises from the combination of two Michaelis-Menten curves, each for the T (less active) and the R (more active) states.

As substrate concentration increases, there is a shift from the T-curve to the R-curve, as more enzymes convert from the T to the R state.

Question 2

Describe the allosteric regulation of PFK-1, including a description of the enzyme quaternary structure, substrate (s), product(s), inhibitor(s) and/or activator(s) involved.

Answer 2

Allosteric inhibitors: ATP, citrate

Allosteric Activators: AMP, ADP, F-2,6-bisP

The enzyme PFK1 has four domains, each with a catalytic site. it has two allosteric sites at which it can lead to the conformational change to the T (less active) or R (more active) state.

Question 3

Compare the major fuel sources, energy stores and metabolism during resting conditions for the following tissues: adipose, brain, liver and skeletal muscle.

Answer 3

Brain:

• Fuel Source: Glucose, Ketone bodies (during starvation)
• Fuel Storage: None
• Rest Metabolism: 60% of GNG glucose consumed

Muscle:

• Fuel Source: Glucose, fatty acids, & ketone bodies -
• Fuel storage: Glycogen
• Rest Metabolism: Fatty acids as major fuel, however, heart muscle prefer ketone bodies.

Adipose:

• Fuel Source: Glucose (used to store and converted to triacylglycerol)
• Fuel storage: >80% of the body’s available energy
• Rest Metabolism: Highly active, insulin activates hormone-sensitive lipase to break down TAG.

Liver:

• Fuel source: glucose, fatty acids, ketone bodies, and amino acids (alpha-keto acids: degradation of amino acids)
• Fuel Store: 1/4 of glycogen body storage
• Rest Metabolism: HIghly active, making glucose via GNG, oxidize fatty acids for energy, forming KB for heart muscle and brain.

Question 4

What is a Type VII glycogen storage disease?

Answer 4

a

Question 5

Explain the basis of the tissue distribution of glucose transporters, in terms of priority tissues.

Answer 5

Red Blood Cell: no mitochondria, relies on anaerobic respiration

Brain: GLUT3 transporter in blood has a low K_t of 1.4 (high glucose uptake)

Question 6

Draw a flow diagram indicating the direct and downstream target of glucagon signal transduction in the liver and how this is involved in the regulation of metabolism during fasting.

Answer 6

Glucagon is synthesized by the α cells of the pancreas when glucose blood concentration is low (fasting). It is recognised by the glucagon receptor (7 TM receptor) in the liver which initiates an enzyme cascade that promotes:

• Gluconeogenesis
• Glycogenolysis

Question 7

Draw a flow diagram indicating the key players and targets involved in the adrenaline signal transduction pathway in the liver and muscle cell.

Answer 7

In the liver, the process switches on gluconeogenesis and glycogenolysis (same as glucagon)

In the muscle, adrenalin switches on glycolysis and glycogenolysis

Question 8

Describe the fright-flight-fight (adrenaline) response in muscle and how adrenaline signaling differs in muscle as compared to liver.

Answer 8

In the liver, the process switches on gluconeogenesis and glycogenolysis (same as glucagon). In the muscle, adrenalin switches on glycolysis and glycogenolysis.

The difference in response came as a result of the following:

1. Liver and muscle express different isoforms of the bifunctional PFK-2/FBPase-2 -> F-2,6-bisP increase (activate) in muscle, decrease in liver
2. M-PK is not phosphorylated (not inhabited) by PKA due to lack of serine residue

Question 9

Describe the bifunctional enzyme PFK-2/FBPase-2 and what happens to it when it is phosphorylated.

Answer 9

It contains two functional domain, a kinase domain and phosphatase domain. When phosphorylated in the regulatory domain, the PFK-2 is inactivated while the FBPase-2 is activated.

Question 10

Describe the formation of the structure of insulin.

Answer 10

1. Preproinsulin has signal sequence which transfers it to ER. the cleaved
2. Proinsulin folds up
3. Oxidized and Disulphide bonds forms
4. Peptide cleaved

Question 11

Draw a flow diagram, describing the signal transduction pathway switched on by Insulin involving PIP3 and how this regulates glycogen metabolism in the liver and GLUT4 transport in muscle and adipose tissue.

Answer 11

Binding of insulin to its receptor leads to the phosphorylation of PIP2 to PIP3, which initiates the following:

1. Activate glycogen synthase (in the liver and muscle)
2. Stimulate movement of GLUT4 transporters to the plasma membrane (muscle and adipose)
3. Produce protein phosphatase which inactivates glycogen phosphorylase

+ Glycogen synthesis - in liver and muscle

- Glycogen breakdown - liver

GLUT4 to membrane - muscle and adipose

It also initiates the phosphorylation of IRS-1, which leads to the activation of Grb2 and the MAPK pathways, leading to increased protein synthesis.